86 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



ing of heavy wagons produced almost as great, though intermittent, 

 tremor, so that it appeared that very good piers would be necessary 

 even if no stirring was contemplated. 



LONG FOCUS HORIZONTAL TELESCOPE 



A mirror of 50 centimeters aperture and 40 meters focus was 

 ordered from the J. A. Brashear Company, and it was decided to 

 instal this as a horizontal telescope lying in a north and south direc- 

 tion, to be fed by a coelostat or siderostat as seemed most desirable. 



It was deemed necessary to provide heavy and deep-founded piers 

 for the coelostat and image-forming mirror, and although it was not 

 indispensable for the work immediately in view that the pier where 

 the image was formed should be as free from tremor as the others, 

 it was deemed best to make the three piers alike, to provide for possi- 

 ble contingencies. For each pier a pit was dug twelve feet square 

 and ten feet deep, and the four sides of the pit were supported by 

 walls of pebble and cement one foot thick. At the bottom of the 

 cubical cavity remaining was filled in twenty-four inches of sand, and 

 on this a twenty four-inch base of pebble and cement with a clear 

 space of six inches all round between it and the walls of the pit. On 

 this base was erected to the surface of the ground a hollow brick pier, 

 with eighteen-inch walls on four sides, and a thirteen-inch wall 

 across the center. A cap-stone eight feet long, seven feet wide, and 

 seven inches thick completed each pier, except that for the coelostat 

 (finally decided upon) a superstructure of brick was provided. 



THE CCELOSTAT 



The coelostat consists essentially, as is well known, of a plane mir- 

 ror fixed parallel to a polar axis which turns uniformly at the rate 

 of one rotation in forty-eight hours. An instrument of this kind 

 was described and illustrated in an article by Von Littrow^ in 1863, 

 but he ascribes the principle to August, who appears to have discov- 

 ered it about thirty years earlier. It is surprising that so simple and 

 excellent a device did not come into general use sooner, but though 

 rediscovered and employed for a time about 1880 by Mr. Langley at 

 Allegheny, and very likely also by others, it has been only within the 

 last five years that the coelostat has become generally known and used. 

 Its chief merit, besides simplicity and consequent accuracy of driving, 

 is the fact that the whole field of view remains fixed, whereas in 

 other forms of siderostats and heliostats one point only of the re- 



'■ Weill, her., xlviii, ii, pp. 337-348, 1863. 



