D. Todd — Open- Air Telescope. 9 



tube. The weight of objective and cell at one end, and of 

 observing-carriage, the observers and accessory apparatus at 

 the other, can be accurately estimated. 



It will be apparent that the alt-azimuth type of mounting 

 follows as a necessary consequence of this evolution ; and the 

 altitude motion gives no trouble whatever in either design or 

 practical construction : it is only when we reach the azimuth 

 motion that mechanical and engineering difficulties arise, 

 though they are not wholly insuperable ones. 



As we go downward from the telescope to the ground, our 

 troubles increase ; and they become a maximum when we 

 reach the plane of junction between earth and mounting. Let 

 us now consider the method of dealing with this problem. 



Apparently a simple one, it admits of several solutions at 

 first sight practical, but which would actually prove infeasible 

 on trial. The most obvious plan is the one adopted in Sir 

 "William Herschel's mounting. Modern ball-bearings provide a 

 vast improvement on the Herschelian construction ; but the 

 weakest member of the whole scheme is the horizontal, circular 

 track, which must be at least 50 feet in diameter. To true up 

 this track to a perfect, jointless horizontal plane, and maintain 

 it so, is a practical mechanical impossibility : at least I have not 

 found any mechanical engineer who would undertake either to 

 do it or to suggest a certain way of accomplishing it. Easy 

 enough it is to think such an ideal steel track into existence, but 

 to build one that will stay so is quite another affair. Mounting 

 and adjusting the rollers in a circular race offer no great 

 difficulties ; in fact, Mr. Henry Hess of the Hess-Bright Manu- 

 facturing Company of Philadelphia, and his corps of able engi- 

 neers, have worked out the bearing details of at least two 

 feasible designs. In these the rollers are attached to the hori- 

 zontal base-member supporting the two upright pyramidal 

 towers on which rest the Y's of the horizontal altitude axis. 



The simplest and least expensive of the three Hess-Bright 

 designs for rotating motion of the azimuth-base would utilize 

 50 commercial car-wheels with ball-bearing mounts, and travel- 

 ling on a 90-pound rail track about T6 feet in diameter. But 

 the best design and the easiest working one is an adaptation 

 of the " Grubb live-ring " with coned rollers of large diameter, 

 their axles turning in ball-bearings and the live-ring travelling 

 with half the speed of the plate itself. 



Another plan which presents some advantages would involve 

 the inversion of this design, the rotatory track attached to the 

 underside of the base plate being trued off on its lower side. 

 The supporting wheels would then be themselves stationary, 

 and adjustment vertically as well as horizontally and radially 

 would be facilitated. 



