These waves are called microseisms. They are minute vibrations endlessly rippling through the earth's crust in response to the interplay of myriad forces, both natural and man-made. They are so faint that it is unlikely anyone in Alexandria other than Kammer was conscious that the earth was shaking last New Year's morning.

Earth waves are to the seismograph what light waves are to the telescope. Just as the telescope reveals cosmic details too dim or far away for the eye to see, the seismograph enables scientists to catch glimpses of the earth's internal structure. And as the telescope reports new comets and exploding stars, so, too, the seismograph provides a running account of unusual activities: sudden spurts in the growth of new mountains, readjustments of internal forces in response to surface erosion, the collapse of subterranean caverns, the interplay of stresses set up by tides and shifting air masses. Thus the seismograph is an instrument for investigating from a safe distance some of the most awesome spectacles staged on earth, as well as the only known tool for studying the planet's interior beyond a depth of six or seven miles.

It is regrettable that amateurs thus far have not been much attracted by seismology. Thousands of telescopes have been built by laymen, but the number of amateur seismographs in existence is very small. Professional seismologists have appealed repeatedly for amateur cooperation. To make a really detailed plot of the earth's interior, something still to be achieved, science must assemble observations from a closely spaced network of seismological stations.

Our present knowledge of what lies beneath the earth's surface is vague and indistinct. In general, it is believed that the granite crust extends down about 30 miles. Then, like the progressively thicker rings of an onion, comes a layer of basalt and glassy rock to a depth of about 420 miles, and below that to some 1,800 miles a layer or "mantle," probably composed of metallic oxides. The core, roughly 4,100 miles in diameter, is believed to be liquid, but its exact consistency is unknown. Some theories call for convection currents in this plastic mass, supplying the forces that build mountains. But these are only informed guesses and must remain so until seismology assembles enough facts to bring into clear focus the picture of what lies such a relatively short distance beneath our feet.

Kammer believes that three mistaken impressions have discouraged amateurs from taking up seismology. First, most laymen think that nothing much happens to a seismograph until an earthquake comes along; and judging by accounts published in newspapers, quakes seem to be few and far between. Second, laymen look upon seismograph records as a rather dull succession of squiggles which earthquakes convert into a hopeless scramble. Finally, the instrument strikes most laymen as a formidable piece of machinery. The few on public display feature massive frameworks anchored to concrete piers set in bedrock. The amplifying levers pivot on sapphire jewels. Finely wrought clocks drive precision drums carrying cylinders of smoked paper that look messy to handle. Certainly such an instrument lies beyond the pocketbooks and home workshop capabilities of most amateurs.

Kammer admits that seismology owes most of its interest to earthquakes. Their waves do a lot of bouncing around on the layers inside the planet before registering on the instrument. That is why earthquake waves carry so much information about the places they have been. Contrary to lay notions, however, earthquakes are not rare. One occurs in some part of the earth on an average of every two hours and 27 minutes. Hence the analysis of quakes alone can become a full-time avocation.

But as shown by Kammer's chart of the microseism on New Year's Day, the seismograph is by no means idle during intervals between earthquakes. The surface of the earth quivers endlessly in response to waves that rush at miles-per-second speeds in all directions. "A television set gives you far less action for your money," says Kammer. "With a little experience the records become not only easy to read but exciting. You can tell at a glance whether a quake originated on the other side of the globe or only a few thousand miles away, whether it was a disturbance at the surface or a readjustment of deep forces. What seem meaningless wiggles become to the experienced observer images of a coastal region sinking beneath the sea, a distant storm center, the birth of a volcano-or merely a tell-tale trace that follows your wife as she walks from the kitchen to the living room."

Kammer says that if he had to use one of the old museum-type seismographs his enthusiasm for seismology would probably dwindle. "I don't think I could work up much zest for a steam-driven bench saw today, either." Fortunately in our electronic age all engines have shrunk in size and cost, including the seismograph. The instrument Kammer has designed for amateurs sits on a plate only 20 inches long by 10 inches wide [see Roger Hayward's drawing above]. It consists of three units: a seismometer, an amplifier and a recorder. A horizontal beam, pivoted at one end, carries a coil of insulated copper wire at the opposite end which swings between the poles of a permanent magnet. This pickup or seismic coil serves as the bob of the seismometer's pendulum. Flexible leads connect the coil with a line leading to the amplifier, which, if desired, may be located a mile or more away from the seismometer. The amplifier drives an electrical registering pen. Kammer uses a standard 5-0-5 milliampere Esterline-Angus recorder which he purchased on the war-surplus market.

In addition to permitting the seismometer to be located at a remote point favorable to it, the design has other advantages over old-type seismographs. Because it contains no delicate parts, the unit may be moved without the risk of disturbing its adjustment. The equilibrium position of the seismic coil is not critical, nor, for that matter, is the position of any of the parts with the exception of the Cardan-type hinges. These must be in good alignment or unpredictable forces will be set up when the pendulum is near its zero position. The builder is free, therefore, to modify the design and "make do" with such parts as his junk box or the surplus market affords. Kammer's pickup coil is scramble-wound with 50,000 turns of number 42 B-S gauge enameled copper wire, but both the number of turns and wire size may be varied. The coil swings between the poles of a war-surplus magnetron magnet. An electromagnet would serve as well. The size of the air gap between the poles and the thickness of the coil is unimportant as long as adequate clearance is maintained.

Kammer says that it is desirable to increase the voltage output from the seismometer by increasing the number of turns in the seismic coil until critical damping can be obtained by connecting a resistance of several megohms across it. The increased signal level permits the use of fewer stages in the amplifier. The unit is fitted with a cover to shield it from stray electrical and magnetic effects. The circuit is so arranged that a small battery can be switched momentarily across the seismic coil, thereby driving it as a motor and deflecting it from its normal position of equilibrium. Upon disconnecting the battery, the coil is again connected across the input to the amplifier. In this way the free period of the undamped seismometer oscillation as well as the proper value of damping resistance can be determined without disturbing the cover.

Perhaps the chief advantage of Kammer's instrument is the control it gives the observer over the character of the record. Unlike instruments that employ optical levers and record on photosensitive papers, the electronic seismograph's records can be observed continuously as they are being written. Faint, closely spaced microseisms can be enlarged at a twist of a knob to show as much detail as desired, while large-scale disturbances can be reduced, thus preventing the pen from swinging off the record sheet.

As presently designed, the instrument has only one questionable feature. It is a "velocity" type seismograph; the electrical output of the seismic coil varies in proportion to the velocity of the pendulum rather than its displacement. Professional seismologists are interested in the shape of the displacement wave-the amount of the earth's excursion on either side of the point of equilibrium-rather than in the velocity of the excursion. Velocity graphs can be converted to displacement by performing a single integration, easily accomplished with one of the commercially available integrating devices or by feeding the output of the amplifier into an electronic integrating circuit of the type recently developed for electronic analogue computers. These circuits are simple to build and the parts do not cost much. But Kammer says it is surprising how much you can learn from a straight velocity graph. He does not use an integrator.

The most remarkable of the three units is the amplifier, which responds to frequencies extending from three cycles per second down to one cycle in twenty seconds. Kammer deliberately reduced the response of the amplifier to frequencies above three vibrations per second in order to suppress the effects of manmade disturbances such as motor traffic.

Most long-period amplifying devices are troubled by a gradual drift of the zero or neutral recording position. Kammer's amplifier [see diagram at left] obtains long-term zero stability by means of capacity-resistance coupling between the stages. The five-megohm grid leak (damping resistor) across the first of two special 10-microfarad condensers gives the unit a 50 second time constant. Hence it is possible to observe typical seismograms containing vibrations up to a period of 30 seconds. Gradual changes in the characteristics of vacuum tubes or battery voltages do not occur rapidly enough, according to Kammer, to develop bothersome potentials across the grid resistors. The two 10-microfarad condensers are the only critical parts in the amplifier. They must be of the best quality and have at least 1,000 megohms leakage resistance. Condensers of this type are currently priced at about $10 each. (They are available from Condenser Products Company, Chicago.)

The output stages consist essentially of a bridge circuit similar to those used in vacuum-tube voltmeters. The first tube of the bridge acts both as amplifier and phase inverter-note the one-megohm resistor between the plate of the first tube and the grid of the second-to drive the final tube's grid. This increases the unit's sensitivity. Current to drive the recorder is taken from across the cathodes of the bridge tubes. If desired, voltage can be tapped from across the plates to operate a cathode-ray oscilloscope or other voltage-actuated device. Kammer made his gain control of fixed resistors that diminish by half-value steps from grid to ground, each tapped by a rotary switch. The positive action of the switch generates less noise than the potentiometer which has been substituted for simplification in drawing. The .1-microfarad capacitors, connected between the plates and ground, limit the upper-frequency response of the unit. Without them, nearby vibrations would unduly thicken the trace.

A well-regulated power supply may be substituted for battery operation. The heaters of the tubes are connected in series directly across the high-voltage supply through a dropping resistor which limits the current to 150 milliamperes. Plate and screen voltages for the pentode stages are derived from batteries, even though a power supply is used. Batteries improve the stability of the unit and, since the current drain is small, they will last their rated life.

The arrangement of the amplifier parts is not critical, though Kammer suggests some protection from vibration to eliminate microphonic disturbances originating in the vacuum tubes. In using this amplifier it is important, he warns, to be aware of phase shift introduced by the coupling networks when signals from several seismometers are being correlated. When studying microseisms from simultaneous recordings of three instruments-the so-called "tripartite" arrangement-signals being compared have the same period and hence experience the same amount of phase shift if the amplifiers are nearly identical. This condition, Kammer explains, is easily achieved and can be tested by connecting the inputs of all amplifiers in parallel, driving them from one signal source and comparing the records on separate recorders.

Although this remarkable amplifier was designed to operate from a velocity-type seismometer, it should perform equally well when driven by other long period voltage sources. It takes little imagination to conceive of seismometers that would produce voltages proportionate to either pendulum displacement or acceleration. Beyond the field of seismometry, the amplifier should find wide application in stellar photometry and in other studies which require the recording of very low frequencies.

ANOTHER unusual amateur seismograph is operated by Elmer Rexin, maintenance superintendent of the Nunn-Bush Shoe Co. in Milwaukee. It is as permanently based as Kammer's instrument is mobile. It is a well-water oscillation seismograph. Although Rexin's instrument is not without precedent, it is, so far as we know, the only one of its type now being operated by an amateur.

Heavy earthquake shocks often cause water to squirt from the ground in some regions. Small mounds of sand, resembling the craters of volcanoes, frequently mark the spots where underground water has erupted under pressures generated by the quake. The flow of artesian wells and the general level of water in wells in the vicinity are disturbed by these quakes. Many wells have been equipped with apparatus for recording water level, as well as hydrostatic pressure, and some of the resulting records have shown typical earthquake waves.

We are indebted both to Mr. Rexin and to Earthquake Notes, journal of the Eastern Section of the Seismological Society of America, for the following account of the Milwaukee well:

"In 1925 the Nunn-Bush Shoe Company added to its water supply by drilling a well to a depth of 380 feet and clearing it to 400 feet with a shot. The well was used until 1945, when gas contaminated the water. The casing was then plugged. In 1946 the United States Geodetic Survey received permission to install a water-level recorder in the hole. Shortly after the instrument went into operation, groups of closely spaced, vertical lines appeared on the charts.

"These lines aroused my curiosity," Rexin writes, "and I consulted the head geologist of the Geodetic Survey of Wisconsin about them. We suspected that earthquakes might cause them. He requested reports from Washington, D. C., and when the cards arrived about three weeks later our guess was confirmed. The reported quakes coincided with the records made by the well. With our curiosity thus satisfied, we dismissed the subject and I paid no further attention to the well.

"Then, on the afternoon of May 7 1947, something happened that was to make a radical change in the way I use my off hours. An official of the company rushed into the maintenance shop at 8:38 and told me that the whole boiler room was shaking and that he had heard a deep, rumbling noise. Within seconds the chief engineer rushed in with the same story. Although I had felt or heard nothing, I wondered if the disturbance could be caused by an earthquake. So the three of us went down to the well, and sure enough there were the lines- big ones! I telephoned our newspaper office and the reporter tried to convince me that the shock came from a big explosion on the southwest side of the city. But while I was speaking with him another reporter yelled that Father Joseph Francis Carroll, chairman of the department of physics at Marquette University, had just reported a local earthquake. From that day forward I have been an earthquake enthusiast.

"The next day I met Father Carroll and told him about the well. He was interested immediately and with his encouragement I decided to equip the well with an improved recorder, one that would make an enlarged chart of the waves. In this new device [see drawing on right] the motion of the water is transferred from the well to the record sheet by means of a float and a counterweighted line which passes over a pulley above the well. This pulley is belted, in turn, to a pair of matched pulleys that carry an endless belt to which the pen is attached. The oscillation of the matched pulleys causes the pen to move back and forth in a straight line. The record sheet is carried beneath the pen by a motor-driven drum. The drum makes one revolution in about four and a half hours.

"The new recorder worked even better than we expected. The first markings to appear were not earthquake waves but extremely long curves, two every 24 hours, in the shape of perfect sine waves. Previously Father Carroll and I had discussed the tides of Lake Michigan, and I now consulted the library of Milwaukee's meteorologist. An early report by the U.S. Army Corps of Engineers showed that in spring the combined lunar and solar tides would amount to about two inches. When I made a check of the well against the predicted tides, and took measurements of them at the beach, all three corresponded.

"Then I noticed that other curves were appearing on the graph. I suspected that variations in barometric pressure might be causing some of them. A microbarograph borrowed from the Johnson Service Company of Milwaukee proved that this was a good guess.

"On May 29, 1947, the water rose and fell six tenths of a foot over a period of six hours, in addition to changes accounted for by the tide and barometric pressure. The following day I learned that a tidal wave had swept in on this side of the Lake with a great loss of property. So I had to add tidal waves to the growing list of events that disturb the well.

"Earthquake waves were being impressed on the record right along, of course, and it was necessary to unscramble these other curves in order to interpret the quake records. While learning to do this I first observed a curious effect. As areas of high barometric pressure moved out over the Lake, the water level in the well would rise instead of going down. By this time I had learned to distinguish the small seiches caused by abrupt changes in barometric pressure during severe lightning storms. The seiches, tides, tidal waves and changes in barometric pressure at the Lake all appeared to operate in reverse of the well. When the water in the Lake rose, that in the well fell, and vice versa. Father Carroll and I discussed this with Father James B. Macelwane, head of the Geophysics Technology Institute of St. Louis University. We reached the conclusion that the well is connected with the Lake by a natural tunnel of some sort. Hence the effect we observed is the normal functioning of a U tube- with the well forming one arm of the U and the Lake the other.

"The sensitivity of the well to earthquakes is probably accounted for by a water-filled fault that connects with the well at the 400-foot level, but until now we have not devised an experimental method for checking this theory. If our guess is correct, the action would be much like that of a syringe, with the fault serving as the rubber bulb. Pressure created by quakes would compress and expand the fault, thus forcing water into the well and sucking it out again. As a seismograph, the well is quite sensitive, and I have little trouble distinguishing primary and secondary waves of even low-magnitude quakes.

"At present I am interested in setting up apparatus for measuring short-period pressures that may be created in the well. The apparatus will consist of a waterproof microphone which will drive an electrical recorder through a high-gain amplifier. I once sank a microphone to a depth of 160 feet and detected a regular pattern of pressure waves, but the seal broke where the lead entered the water-tight container, and you can -guess what the record showed after that."

ONE TEST of the quality of a telescope is to photograph the moon with it and note how much the photograph can be enlarged without loss of sharpness. The amateur telescope maker Henry Paul has made a photograph of the moon at the focus of his 10-inch, f/9 reflector, where the image was eight tenths of an inch in diameter, which he was able to enlarge 20 times, giving an image of the moon 16 inches in diameter, before loss of sharpness became apparent. Usually a picture begins to be fuzzy with anything over a fivefold enlargement. Thus Paul beat par by four times.

The photograph is enlarged five times. In the photograph the moon is eight days old, or near its first quarter. The edge of darkness crosses the lofty Apennines, the finest range of mountains on the moon. Near their eastern (right-hand) end, isolated in black shadows as the moon rotates westward, is a tiny cusp of light. This is the eastern rim of the crater Eratosthenes peeping out of the night at first dawn. (On the moon the sun rises in the west.) At left on the next page is another photograph, taken one day later, which shows the entire Eratosthenes crater, 37 miles in diameter. The outer ring of the crater rises about a mile above the surrounding plain. The crater floor is more than a mile below the level of the plain. The central mountain rises to the level of the plain.

It is easy to see the shape of a lunar crater as long as its parts cast shadows, but when the sun is high in the sky and they are illuminated on both sides, no shadows are cast. Even as large a crater as Eratosthenes can then so nearly vanish that an observer who has watched it and drawn it again and again at the telescope eyepiece may have difficulty in finding it. The right-hand illustration on the next page reveals the same area as its left-hand companion, photographed at lunar noon, sun directly overhead, full moon from the earth. Without the aid of the companion illustration it would be far from easy to find this crater amidst the camouflage that surrounds it. Other craters put on similar disappearing acts each month.

The commonest method of becoming familiar with the moon's topography is to keep observing along the dark edge, because there the shadows easily interpret the relief. By doing this throughout the month, night after night as the day-night borderland creeps to the east then retreats to the west, the whole visible face of the moon is surveyed. Here many telescope users stop, and thus miss seeing changes on the moon that were long ago minutely described by the American selenographer W. H. Pickering and others. Let us ferret out some of these changes.

When the left-hand photograph was taken, the sun's altitude in the lunar sky was approximately equal to that on earth at 6:20 a.m. Close comparison with the right-hand illustration yields practically no correspondence between the light and dark areas. The reason is that the dark areas in the first are the shadows of objects, while those of the other are not, for the sun is near the noon angle and shines down on all sides of all prominences. The dark areas seen in the right-hand photograph appear monthly in and around the crater between the 9 a.m. and 2 p.m. positions of the sun and keep shifting their position. What these patches are is unknown; they are not related to the moon's topography. The bright areas consist partly of streaks best seen in the illustration through nearly closed eyelashes, and rounded plats where the streaks meet. These streaks and plats shift in position each month, in approximately the same cycle. Most interesting is the fact that the behavior of the dark and bright areas varies from month to month in minor respects that are unpredictable.

Pickering found it easier to observe Eratosthenes with a small telescope than Mars with a large one; the apparent diameter of Mars at its nearest is but half that of Eratosthenes, and Eratosthenes contains far more detail than Mars. To show that the major changes on Eratosthenes can be observed with a six-inch telescope anywhere in the U. S., he made drawings of it with only a three-inch telescope and a magnification of 90 from his home on the high plateau of the tropical isle of Jamaica, British West Indies, where the seeing is good. He systematized the study of the monthly changes by charting the eight fields in Eratosthenes as shown in the drawing (left). The following is his summary of the more marked changes that may be expected to be seen each month. The times given are not for a single day but simply indicate the angle at which the sun is shining on the moon at the time of observation. At 6:30 a.m. the summit of the central peak becomes visible.

At 6:40 a "canal" (streak of brightness) joining the SC and SE fields may appear.

At 7:50 fog begins to form within the crater, increasing till 10 a.m.

At 7:55 canals appear in the SE field.

At 8:00 the eastern side of the NW field darkens.

At 8:20 the northern part of the bird-shaped NE field begins to fade and the southern part to extend.

At 8:40 two dark spots in the E field unite.

At 9:20 the central field begins to narrow at the northern end [see drawings above left, corresponding respectively with SC and C fields at 8:30, 9:30 and 10:30 a.m.]. The northern field crosses the crater rim from the outside and begins extending inside the crater.

At 9:40 the SE field crosses the rim of the crater.

At 10:00 the SC field darkens and a canal sometimes begins joining the SC and SE fields, while the southern part of the NE field begins to fade.

At 10:40 the EC field darkens; at 1:30 it is conspicuous, and at 5:00 it is lost in the shadows.

At 10:50 the E field joins the NE, which soon begins to shrink.

At 11:00 the two arms of the SE field begin to curve inward.

At 1:00 the SC field becomes notched at the south (upper) end and at 3:20 it fades out.

At 2:00 the NE field begins to fade.

These are only the major changes. There are so many minor ones that the principal difficulty of the selenographer is to sketch them as fast as they occur.

The "canals" are streaks of brightness a mile or two in width-about one 200th the width of the Martian canals as seen at about 200 times the moon's distance. Pickering called these lunar streaks canals because they resemble the streaks on Mars that are called canals, though no water has ever been seen in them. The plats at their intersections resemble the lakes of Mars and, like some of them, shift in position.

The fogs mentioned by Pickering occur in craterlets usually less than one mile in diameter. These emit a brilliant white circular glow after being warmed by the sun. They remain conspicuous until sunset. Other selenographers have seen fogs in craterlets within Eratosthenes. The British selenographer Patrick A. Moore calls them mists. He says their existence "cannot be questioned, as the evidence is overwhelming; various craters have at various times been seen mist-filled."

Pickering argued that because the markings on the moon and on Mars are neither shade nor shadow they must be either changeable surface discolorations or something growing or something moving over the surface-"mineral, vegetable or animal." He found that the "vegetation" in general is gray, like sagebrush; in places near the moon's equator it is purplish black, like lichens. Eratosthenes is one of a number of oases in the desert waste of the moon's surface. The "vegetation" is often associated with minute craterlets within large craters. Its growth and decline must be very rapid.

After Pickering published his findings in Popular Astronomy (November, 1919; August-September, 1921; May, 1922; February, 1924; May, 1924; August-September, 1924) critics dismissed the alleged changes as due merely to the shifting of shadows and the changing angle of illumination. Pickering replied: "If so, why do they always appear at full moon? How explain dark markings that advance toward the setting sun?" He denied that he had said that terrestrial vegetation would be possible on the moon. The lunar "vegetation" must be very different from that of the earth and we know nothing about what it is like. The same would be true, he said, if the dark areas were swarms of ant-sized insects migrating.

After Pickering had published his articles, the Italian selenographer Mentori Maggini spent several days discussing the moon and Mars with him and later made drawings of Eratosthenes with a 13-inch refractor in Sicily. He verified nearly all the details of Pickering's drawings He, too, saw the canal-like streaks and noted they were continuously visible, unlike those of Mars, which peep out for no longer than one fiftieth of a second or at most a second. He said they resembled the lines of Mars and asked, "Does this similarity lead us to believe that we have to do with the same phenomenon, an optical one?"

Maggini's optical interpretation of the moon's changes was this: "The glimpsed details, or invisible ones, are gathered together and integrated into linear sensations or into spots. From the point of view of the optical theory these lines and spots are the means by which the eye of the observer, who always wishes to see something, succeeds in representing fleeting detail. In the region of Eratosthenes it has seemed to me that a great number of lines have their origin in the contrast between two bright areas; when two bright regions form, one can see between them a dark line. And the optical theory explains the displacements of the fields and canals by a change in the maximum distribution of elementary spots scattered over a region."

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.