SOLAR RESEARCH AT MOUNT WILSON, CALIFORNIA. 1 73 



It also seemed advisable to stretch the canvas fly at a much greater 

 distance from the tube and to provide means of exit at the top for 

 any heated air which might be found under the fly. As soon as the 

 tube and fly had been rearranged in this manner a great improvement 

 was immediately noticed. The definition of the image became much 

 better and the deterioration observed in the previous instance was 

 no longer seen. The air in the tube remained cool, whereas before 

 it had become greatly heated. 



These experiments would seem to throw some light on the ques- 

 tion of designing suitable tubes and shelters for telescopes used in 

 a horizontal, or nearly 'horizontal, position. It seems likely that if 

 the coelostat and the instruments used with it could be mounted on 

 piers at a height of 70 feet or more above the ground, it would 

 be unnecessary to use any tube, particularly if the ground below the 

 path of the beam were shielded from the sun by a light canvas cover, 

 stretched at a height of several feet above the surface and suitably 

 ventilated. Of course, the practical difiiculties in such a construc- 

 tion are very considerable, on account of the great cost and the lack 

 of stability of high piers. For the Snow telescope it therefore 

 seemed advisable to design a special form of house, in the hope of 

 securing good definition with a solar beam at a moderate height 

 above the ground. Experiments made with the 15-inch coelostat 

 seem to show that this latter instrument is too near the ground 

 for the best results, although it gives excellent definition in the 

 early morning, before the heating of the soil is very great. 



The design of the house now under construction for the Snow 

 telescope will be described in a subsequent report. It may be said 

 here, however, that it consists of a skeleton frame of light steel con- 

 struction, provided with a ventilated roof. The floor is to be of 

 canvas, tightly stretched at a height of one foot above the ground 

 and permitting a free circulation of air below. The inner walls of 

 the house (which is 10 feet wide at its narrowest point) are to be of 

 light canvas, so arranged that 'they can be raised or lowered at will. 

 The outer walls of the house are to be covered by canvas louvres, so 

 arranged as to shield the entire bouse from the direct rays of the 

 sun, and permitting a free circulation of air. The stone pier, 27 feet 

 high, on which the coelostat will stand, is also to be shielded from 

 the sun by canvas louvres. The ground surrounding the instrument 

 is fairly well covered with bushes, and the few bare spots can be 

 covered with stretched canvas, if necessary. 



