PAPERS BY MEMBERS 67 



raphy necessary in elementary education are also easily dem- 

 onstrated with the sphere. 



The sphere now in the Academy building was invented 

 by Wallace W. Atwood, Secretary of the Society and Di- 

 rector of the Museum. It was constructed, installed and pre- 

 sented to the Academy by Mr. La Verne W. Noyes, Presi- 

 dent of the Board of Trustees, in order to broaden and to 

 promote the educational and scientific work of the Academy. 

 The Construction 



The material used in constructing the sphere is very light 

 galvanized sheet-iron, 1-64 of an inch thick, which has been 

 pressed to the proper curvature and soldered to the equatorial 

 ring and to a much smaller ring about the entrance to the 

 sphere. The separate sheets lap sufficiently to be soldered 

 upon one another. The platform and horizon table are of 

 wood and rest upon a very strong steel frame. 



The diameter of the sphere is fifteen feet. The weight, 

 exclusive of the platform, is a little more than 500 pounds. 

 This weight is carried by a tube 2^/2 inch tube attached to the 

 outside of the sphere along the line of the equator and resting 

 upon three wheels as shown in the cross section view. The two 

 lower wheels carry the greater portion of the weight but the 

 third and upper wheel, above the door, resists a certain thrust 

 due to the inclined position of the sphere. The stationary 

 platform within the sphere is supported in part by steel 

 trusses resting upon the frame work of the museum balcony, 

 and in part by two upright pillars which rest upon the great 

 I beam of the mainfloor of the Museum. This platform car- 

 ries a circular horizon table, below which the sphere is ob- 

 scured from view, and above which there is a complete hemis- 

 phere on which the stars are represented. 



The observer in this sphere is located on the surface of 

 the Earth at North Latitude 41 degrees 50 minutes. Celes- 

 tial spheres constructed for localities having other latitudes 

 north or south would be placed at other angles and certain 

 other constellations would be represented. Thus a celestial 

 sphere constructed for Buenos Aires, to represent the south- 

 ern heavens, would be so placed that the observer would enter 

 from the north polar region and see the southern constella- 

 tions, not visible at Chicago, observe the courses of Sun and 

 Moon north of him but fail to see any of the constellations 

 about the north pole of the heavens as seen from the latitude 

 of Chicago. 



Attached to the steel structure supporting the sphere is 

 a small electric motor, which propels the two lower wheels 

 supporting the sphere and their rotation causes the sphere 

 to rotate. 



