Todd — Untried Form of Mounting for a Telescope. 463 



An electric motor, or gas engine electrically controlled, will be 

 found well adapted to this delicate service ; but in order that 

 the movement may be perfectly steady, care must be taken to 

 provide a considerable excess of the power actually necessary 

 to set the sphere in motion. 



I have given much attention to the question of slow motions. 

 Several different schemes are possible, and all admit of auto- 

 matic control and action through the multiplex cable. Should 

 it be desired to maintain the utmost delicacy of adjustment in 

 the field, for spectroscopic or micrometric work or long-expo- 

 sure photography, it may be found best to provide a sliding 

 collimator-plate, to which the tail-piece is rigidly attached. 

 Corresponding and appropriate collimation of the objective 

 would not offer any serious mechanical difficulties. 



No one familiar with observatory construction will fail to 

 notice that this projected type of instrument combines tele- 

 scope, observing-chair and dome all in one. I have made an 

 attempt to estimate the cost of such an astronomical establish- 

 ment, but the largest present element of uncertainty pertains 

 to the sphere itself. 



As the sphere will not need the ordinary running-gear, we 

 cannot be far wrong in estimating double the cost of a hemi- 

 spherical dome. This will make, 



Approximately $1 75,000 



Allow for a 5-foot objective 75,000 



For the masonry and cement basin 5,000 



For the clock-work and motions 1 0,000 



For the tube and eyepiece accessories 10,000 



$275,000 



I need hardly add that some of the unique advantages of 

 M. Loewy's equatorial coude are available with this type of 

 mounting ; for by sealing the internal tube, the sphere or a 

 part of it can be kept at a comfortable temperature by electric 

 heaters. And if such a telescope were installed on a high 

 mountain, the barometric pressure of sea-level might be 

 mechanically maintained within a compartment of the sphere, 

 housing over the adjustable platform. By connecting with 

 exterior rooms through closed passage-ways, this might readily 

 be so contrived as to safeguard the observer against those 

 serious effects of mat des montagnes, which hitherto have pre- 

 cluded the permanent astronomical occupation of mountain 

 elevations much in excess of 8,000 feet. 



Amherst, Mass. 



