J. V. McAdam, of Hastings-on-Hudson, New York, drew us a diagram of his drive, and this reproduced on this page, accompanied by some compact notes which will be of value to other workers. He writes:

"G. E. motor, synchronous, K-4256047, 110 V., 60-cycle phonograph motor 1800 r.p.m. with 2-to-46 reducing worm gear built in, making (M) turntable speed 76.26087 r p.m.

1 sidereal day = 1 solar day less 3 m. 56.555 sec. = 1436.057 solar minutes.

1436.057 x 76.26087 = 112387.0702 revolutions of M per sidereal day.

Using three worm gear reductions of 91:1, 65:1, and 19:1 gives a reduction ratio of 112385:1-a difference of 2 revolutions out of 112387 per sidereal day, or about 1-1/2 seconds. This it much closer than the power house cycle can be controlled.

The above worm reductions are not, however, standard. They may be obtained by using a combination of "change gears" and worm gears listed in gear catalogs and carried in stock as follows:

HAROLD A. LOWER, 1032 Pennsylvania St., San Diego, California, the author of the chapter on "Celestial Photography" in the new edition of the book "Amateur Telescope Making," sends in a close-up photograph of the clock drive on his 12-inch Cassegrainian (ATM p. 448) and writes:

"The speed regulation at the generating station is not all that could be desired. I have found that there are two systems in use for providing synchronize time: In one system the governors are controlled automatically by a master clock, and regulation is quite accurate. In the other system, the operator has to adjust the governor by hand when ever he notices that the synchronized clock is running away from the master clock. This means that the speed may be suddenly change by as much as one cycle, causing the synchronized clock to run fast or slow until the difference between the two clocks is made up. Our variable ratio friction drive' permits regulation of the speed at the telescope, and has worked out fairly well. (By "our," Lower refers to his father and himself, who co operate-- Ed.) It also enables one to adjust the telescope to sidereal time without special gears which are usually costly. The gears used in our drive are all stock gears, which are easily obtainable. The only machine work which was required was in the making of the friction drive. The driving wheel is Bakelite. The driven disk is cast iron and must be accurately made. The surface of the disk should be ground and polished. A turned surface is too rough and will wear the Bakelite driving wheel quite rapidly. The motor is a 1/20 h.p. synchronous, and would have power enough for a 60-inch telescope. It is moved axially, that is, along the radius of the driven disk, by means of a screw, to provide the speed control."

DR. SERGE A. KORFF, an astronomer at the Mount Wilson Observatory, writes:

"I have just built a small polar axis, the design of which might be of possible interest to amateur astronomers. On the tops of two upright two-by-fours are mounted bearings, which in turn carry the shaft of the polar axis. This shaft runs through the center of another piece of two-by-four (horizontal in the picture). The shaft is driven, through a set of worm drive reduction gears, by a phonograph motor. The speed control is visible on the upper left.

"The phonograph governor will allow the main drive shaft to rotate at one revolution per second; and consequently the gears were chosen to give a speed reduction of 3600 x 24:1. The governor speed is adjusted until the axis follows the diurnal rotation satisfactorily. Latitude adjustment is accomplished by raising the rear of the unit to the required amount.

"This polar axis was designed primarily to carry a small camera for eclipse cinematography but, by a small change in the speed of the driving motor, it can be adjusted for stellar, lunar, or planetary photography. It was especially intended to be portable. I am now planning to use it to carry cosmic ray counters, to assist in the study of the origin of these rays. By halving the speed of the drive, and mounting a mirror on the two-by-four, the arrangement may be converted into a heliostat."

DR. E. N. RYDER of Croton Falls, New York has quite accurately graduated a setting circle as follows: He obtained from one of the manufacturers of gears an iron spur gear of 9-inch pitch diameter, having 180 teeth, for about three dollars, and mounted this horizontally on a stand or table so that it would rotate on a vertical stub shaft. On top of the gear he attached a circular wooden table to hold the annular flat ring of sheet metal which was to be marked. Above this, on a separate support, he mounted the marking tool which was made from an old file ground t a point. This ran in a guide, in order t obtain marks of the desired length. The guide was provided with toothed steps, in order to permit making long marks of different lengths. To rotate the gear accurately he dug out of a junk pile an old screw which served (well enough) as a worm and, by rotating this worm through any desired part of a circle the trick was turned.

A HIGH-GRADE wooden tube of the Herschel type, similar to the one shown in the March number, except that it is hexagonal instead of octagonal, ca be made with a small, fine-toothed circular saw, as shown in the accompanying drawings. These are taken from some which have been sent us by Charles W. Ingalls 368 North Hill Avenue, Pasadena, California, with whom we have been acquainted since about 4 A.M. on January 16, 1888. Make the triangles of the guide by very precisely copying in wood a 30-60-90-degree standard triangle, he advises. For the staves use about 3/4-inch stock. Groove the edges 3/16-inch wide and 1/4-inch deep and set the slip-tongues in white lead. In the second drawing two positions of the guide are combined in a single sketch, for convenience. That is, in order to obtain the other aspect, turn the magazine sidewise.

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