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LYI. 



ON A HELIOSTAT FOR THE SMITHSONIAN INSTITUTION, 

 WASHINGTON. By SIR HOWARD GRUBB, F.R.S., an 

 Hon. Secretary of the Royal Dublin Society. (Plate XIII.) 



[Read January 15, 1890.] 



In a large heliostat which I have recently made for the Smithsonian 

 Institution of Washington, I have introduced some modifications 

 which may possibly be interesting to Members of the Royal Dublin 

 Society. Some of them were designed to meet certain peculiar 

 conditions imposed upon me in the contract, and some were intro- 

 duced in the hope of improving the efficiency of an instrument 

 which from its necessarily unmechanical construction is unlikely 

 ever to take rank as an instrument of precision. 



The heliostat, or siderostat as it is sometimes called, is, as its 

 name implies, an instrument carrying a mirror or mirrors which 

 reflect the rays from any celestial object in a constant direction, 

 irrespective of the apparent motion of that object due to the 

 diurnal rotation of our earth. 



There are many forms given to these instruments according to 

 the various uses to which they are intended to be applied ; but all 

 these modifications may be classified into two essentially different 

 forms, viz. the double mirror and the single mirror heliostat. 



The double mirror heliostat is a comparatively simple instru- 

 ment. It consists essentially of an axis carrying a mirror at one 

 end, the axis being mounted precisely parallel to the axis of our 

 earth, similarly to the polar axis of any equatorial instrument. 



This mirror is so set as to reflect the light from the object 

 under examination in the direction of the axis itself, i.e. towards 

 the north or south pole. If now in the direction of this axis 

 another mirror be placed capable of being set to any position, the 

 light from the celestial object can be directed into the desired posi- 

 tion, in which position it will remain constant so long as the polar 

 axis is kept rotating at the same rate as our earth, and in an 

 opposite direction. 



