THE DESCRIPTION of a perpetual saltwater fountain in this department last June by Seelye Martin of the University of Washington set Octave Levenspiel of Oregon State University thinking about the possibilities of exploiting the principle of osmosis. He proposes to use it to draw fresh water from the ocean. Levenspiel writes:

"The idea of bringing salt water to the surface from ocean depths by employing the chimney effect of a vertical pipe is interesting. What if it were possible to bring up fresh water from the depths of the sea-perpetual fountains of it to drink, wash in and make deserts bloom? Such an achievement would make the dreams of the alchemists look pale.

"The apparatus with which I propose to desalinate the seas resembles the perpetual saltwater fountain that has been successfully demonstrated but differs from it in one essential respect. Whereas the saltwater fountain is based solely on the chimney effect, my device includes the principle of osmosis.

"To demonstrate osmosis, close the end of a glass tube with a semipermeable diaphragm such as a sheet of cellophane or parchment. The tube can be about a foot long and of any convenient diameter. Partly fill the tube with salt water of any concentration. Insert the tube vertically in a container of fresh water until the level of the brine is even with the surface of the surrounding fresh water. By osmosis fresh water will spontaneously diffuse through the membrane, thus creating an upward-flowing column of increasingly dilute brine that will eventually spill over the top of the tube.

"The spontaneous flow of fresh water through the diaphragm can be stopped by exerting sufficient pressure on the brine. The effect necessitates redesigning the experimental apparatus slightly. A length of pipe is divided at the middle by a semipermeable diaphragm. Fresh water is on one side of the diaphragm and salt water on the other side [see illustration at left]. If the pressures are equal on both sides, fresh water will diffuse through the diaphragm and dilute the brine, as demonstrated in the first experiment.

"To stop the flow one must impose a pressure of roughly 22 atmospheres (about 320 pounds per square inch) on the salt side if the salt side consists of seawater. That pressure would be equivalent to what is exerted by a column of water about 700 feet high. To reverse the flow and produce fresh water from salt water one must apply a pressure in excess of 22 atmospheres on the salt side.

"Keeping these pressure effects in mind, consider the merits of my proposed freshwater fountain. We close the bottom of a long pipe with a semipermeable membrane, fill the pipe with fresh water and lower it into the ocean. Since the density of seawater is about 3 percent greater than that of fresh water, the pressure on the outside of the pipe will be higher than it is on the inside. The difference in pressure increases with the depth to which the pipe is lowered. A depth can be reached where the pressure difference becomes 22 atmospheres. If the pipe is then lowered a bit more, the pressure on the outside will exceed the pressure on the inside by more than 22 atmospheres, a condition suggesting that fresh water will perpetually diffuse into the pipe at the bottom and flow out at the top. We might even consider the idea of letting the output of the fountain flow through a waterwheel for generating electric power as a by-product!

"Spoilsports may contend that we must push the pipe pretty far down (about five miles), that no real membrane has been developed to date that will stand these pressures and that the temperature, density and salinity of the oceans change with depth. Complications of this kind do not alter the main argument, which relies on gravitational work to separate water from salt, rather than on heat, phase change or other traditional desalination techniques." I leave Levenspiel's proposal with readers of this department for appraisal. I shall have more to say about it in a subsequent issue.

RODNEY A. WOLF of Highland Park, III., uses the acceleration of gravity as the basis of a clock that approaches the ultimate in simplification. He merely drops a weight and interprets the distance of its fall as a measure of time. Wolf built the clock for determining the energy of projectiles that are launched by a slingshot, but it can be applied in experiments of many kinds. He describes his project as follows:

"One day while I was shooting stones straight up into the air with a slingshot and trying to make them land nearby I realized that I could quite easily measure the energy of the projectiles. In theory a projectile spends equal time going up and coming down, and it strikes the ground at the same velocity with which it left the slingshot. The time a projectile spends in flight can be measured with a stopwatch.

"The projectile is in free fall during half of the interval. The maximum height from which the projectile falls is equal to half the product of the acceleration of gravity multiplied by the square of the time. The potential energy of the projectile at the highest point in its trajectory is equal to the product of the mass of the projectile multiplied by its maximum height and by the acceleration of gravity.

"By means of these formulas, a stopwatch and a laboratory balance I determined the potential energy of a number of projectiles. The precision of the method was good. All my measurements were consistent. I learned later, however, that my results were in error by as much as 50 percent. I had not taken the resistance of the air into account.

"To minimize the influence of air resistance I redesigned the experiment. Projectiles were shot at high velocity along a relatively short, horizontal trajectory. The time of flight was correspondingly reduced. The stratagem minimized the effects of air resistance but introduced the need for an accurate clock capable of measuring time in small fractions of a second.

"Usually one must pay a high price for an accurate timing device, but in this case I looked around for cheaper ways of timing accurately. Falling objects accelerate at the rate of 980 centimeters per second per second. With this fact in mind I made a clock by fastening a piece of ticker tape to a weight. When the weight is dropped from the edge of a table, it pulls the strip of paper across the top of the table. Intervals can be timed by dropping the weight at the beginning of an event and placing a mark on the moving tape at the end of the event. The duration of the interval can be calculated by knowing the distance the tape moved during the event as indicated by the mark.

"Several accessory gadgets were improvised to drop the weight and mark the tape automatically [see illustration at left]. A slingshot of the rubber-band type was mounted to the end of the table, along with a meterstick. Shots of reasonably uniform velocity could be made by stretching the rubber to a predetermined length, as indicated by the meterstick. The weight that pulls the tape is normally supported near the end of the table by an electromagnet, which is energized by a storage battery. The circuit of the battery includes a switch that is opened by the slingshot. The paper tape slides under a marking stylus that is normally supported above the tape by a second electromagnet. The circuit of this electromagnet includes a switch that is opened when a projectile strikes the distant target, releasing the stylus, which prints a dot on the tape.

"The elapsed time in seconds is equal to the square root of the distance in centimeters, as measured from the beginning of the tape to the dot made by the stylus, divided by 490. The distance between the slingshot and the target is known: 20 feet in the case of my experiments. The velocity of the projectile, in feet per second, is equal to 20 divided by the time of flight in seconds. The kinetic energy imparted to the projectile by the slingshot is equal to half the product of the mass of the projectile multiplied by the velocity. The energy, in joules, is equal to , where m is the mass of the projectile in grams and v is the velocity of the projectile in feet per second. The constant is a conversion factor.

"The electromagnets can be made at home by wrapping about 1,200 turns of No. 22-gage magnet wire on iron bolts 3/8 inch in diameter [see illustration at right]. The ends of the spool can be disks of cardboard. The bolts should be wrapped with a layer of electrical tape before the winding is applied. The weight of my clock is a hefty iron bolt about five inches long that I found near some railroad tracks. It weighs about half a pound. To protect the floor I catch the falling bolt in a bucket filled with rags. I made a hook of coat-hanger wire and taped it to the bolt near the top. The hook is useful for supporting the weight close to its normal position temporarily before power is applied to the electromagnet. The magnets tend to overheat if power is applied continuously.

"The stylus that marks the tape can be made of the same iron that is used for the core of the magnets. Any soft iron rod about 3/8 inch in diameter is satisfactory. I drilled a hole about 1/16 inch in diameter transversely through the stylus near the top. A rubber band was passed through the hole, stretched and tacked to the supporting bracket. The band exerts downward force and helps to accelerate the stylus when it is released by the electromagnet. A penny was cemented to the base of the apparatus directly under the stylus to serve as an anvil.

"A disk of heavy carbon paper is sandwiched between the pointed end of the stylus and the paper tape. The disk is loosely attached to the supporting base bracket by a thumbtack through the center of the disk. The carbon paper rotates freely on the tack as the tape slides by, presenting a fresh surface of carbon after each mark is made. The tape is guided across the tabletop by loose-fitting staples made of coat-hanger wire.

"The switch that is operated by the slingshot consists of a pair of small brass plates that can be cemented with epoxy to a block of wood [see illustration at left]. The switch is closed by placing a strip of brass across the contacts. An alligator clip is screwed to the center of the brass strip. The switch is operated by a length of fishing line, one end of which is tied to the pouch of the slingshot. The line runs from the slingshot through a saw kerf in a bracket to the alligator clip, where the end is clamped between the jaws of the clip.

"The length of the line is adjusted to be snug when the brass strip is in position on its mating contacts and the rubber bands of the slingshot are fully relaxed. When the slingshot is cocked, the line is slack. The projectile attains maximum velocity at the moment when the rubber bands have fully relaxed. Simultaneously the moving pouch of the slingshot draws the line tight and pulls the brass strip away from its contacts, starting the clock. The alligator clip wedges against the saw kerf and drops free as the moving pouch pulls the line from the jaws of the clip. The end of the line continues through the kerf. I learned to wear a shirt with long sleeves when operating the apparatus because the line can cut a bare arm.

"The target assembly is an 18-inch square of corrugated cardboard glued to a stick about three feet long. A transverse hole in the stick about six inches from the cardboard slips over a horizontal axle that supports the assembly and also enables the target to swing freely [see illustration at right]. On the free end of the stick is a small brass plate that normally makes contact with a similar plate cemented to a fixed bracket. These plates are the contacts of the target switch and form part of the circuit that includes the electromagnet controlling the stylus. The impact of a projectile rotates the target assembly, separating the contacts of the switch.

"The velocity of projectiles launched by a slingshot ranges from about 100 to 300 feet per second. The clock can measure intervals of about a hundredth of a second with good accuracy. The distance between the target and the point where the slingshot releases the projectile was accordingly made about 20 feet. On the average the ticker tape moves about 11 centimeters during the flight of a slingshot projectile.

"I normally load the clock with a strip of tape about a foot long. A pencil line is drawn across the tape to mark its initial position with respect to the stylus. The line should be drawn exactly under the point of the stylus, but the magnet is in the way. For this reason I put the mark an inch ahead of the stylus and subtract the inch when 1 measure the distance between the mark and the timing dot made by the stylus.

"The apparatus can be used for timing projectiles of higher velocity by increasing the range enough to maintain about the same interval of flight. The optimum range in feet is approximately equal to the velocity of the projectile multiplied by .15. The probable velocity can be estimated. The muzzle velocity of a .22caliber pellet pump gun might be 550 feet per second and that of a .22-caliber rifle 1,300 feet per second. Appropriate ranges for timing these projectiles would be 80 and 200 feet respectively. As I have mentioned, the velocity of the projectile is equal to the quotient of the range (in feet) divided by the time of flight.

"When the slingshot is released, everything happens so fast that the experimenter cannot observe the action in detail. You must trust the machine. Just make sure that the target has been hit. I have sometimes missed the target and hit the table, with a resulting jar that caused the stylus to drop. These measurements are discarded.

"Often a string of dots appears on the tape. They are caused by bouncing of the stylus. The first dot in the string is the one that counts. Circle it with a pencil and cross out the others. Tape can be used more than once because dots made during earlier shots can be identified by the circles.

"Having determined the velocities and energies of numerous projectiles, I measured the efficiency of several slingshots: the ratio of energy carried away by the projectile divided by the energy expended in stretching the rubber bands, expressed as a percentage. Input energy was determined by hooking a spring scale to the pouch of the slingshot and reading the force at a series of points as the rubber was increasingly stretched. It turns out that slingshots of the highest efficiency are not necessarily the easiest ones to use. Part of the energy used to cock the slingshot is transformed into heat by the rubber when the pouch is released. This loss increases disproportionately with increased stretching of the rubber and with the velocity at which the rubber snaps back. The efficiency increases with the mass of the projectile because the greater inertia causes the rubber bands to contract more slowly. These experiments suggest only a few of the effects that can be investigated with the simple clock. The apparatus can be modified easily for experimenting with the ballistics of projectiles, measuring acceleration and so on."

ALTHOUGH amateur astronomers make excellent pictures of the moon and the brighter planets, their photographs of nebulas and unresolved portions of the Milky Way usually show some evidence of underexposure. The prints are characterized by low contrast: a narrow range of grays in which prominent details of the objects are submerged. The low surface brightness of these objects lies somewhat beyond the grasp of the small telescope and the sensitivity of ordinary, uncooled photographic emulsions. Even so, an underexposed negative may contain much latent detail that can be made visible by the use of appropriate development and printing techniques. Charles L. Townsend of Oxnard, Calif., has recently been experimenting with a procedure for enhancing differences in intensity between bright and faint areas of underexposed negatives. His procedure more than doubles the detail disclosed by normal processing. He writes:

"My technique for enhancing intensity differences in underexposed negatives is based on repeated copying. I use 35 millimeter Kodak Plus-X film and develop it with Kodak HC-110B developer according to the manufacturer's recommended procedure. From the developed negative I make a projection print in the form of a positive transparency, using Du Pont Type-S Orthochromatic Litho-Negative material. When making the exposure, I stop down the lens of the enlarger as far as possible. The positive transparency is then printed by contact on a second sheet of the Litho-Negative material. This step yields a negative transparency of identical proportions but of substantially higher contrast. The resulting negative transparency is used for making a contact print on Luminos F-3 Bromide paper. The result is a positive print that has high contrast and displays much photographic detail that is lost during normal processing.

"I use Printol paper developer for processing both the transparencies and the final print. With two exceptions I develop the transparencies and prints according to the recommended procedures of the National Research and Chemical Company and the Eastman Kodak Company. When I make up the stop-bath solution for the transparencies I depart from the recommended procedure by using one part of Kodak indicator stop-bath working solution to 2 parts of water. The developed transparencies are processed in Kodak fixing solution. The fixing is stopped promptly when the transparencies lose their milky appearance (as viewed with a red safelight). I normally rinse the fixed transparencies in a bath of Kodak Clearing Agent to minimize washing time.

"Most of my negatives are made using an f/2.8, 35-millimeter-focal-length Lentar lens stopped down to f/3.5 or f/4. The Lentar lens is mounted in a Praktica camera body, and this combination is attached to the tube of an equatorially mounted f/6.3 refractor guide telescope with a focal length of 500 millimeters. A small, frequency-controlled, synchronous motor drives the assembly in right ascension. A manually controlled tangent arm is used for making small changes in declination."

Bibliography

PRINCIPLES OF PHYSICAL CHEMISTRY: AN INTRODUCTION TO THEIR USE IN THE BIOLOGICAL SCIENCES. Wallace S. Brey, Jr. Appleton-Century-Crofts, Inc., 1958.

OUTER SPACE PHOTOGRAPHY FOR THE AMATEUR. Henry E. Paul. AmPhoto, 1960.

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