326 Mr. Gassiot Description of a Rigid Spectroscope. [June 15, 

 TABLE IV. 



Readings taken by Mr. Beckley at Kew Observatory. 



Temp. Temp. Micrometer 



Date. Air. Prism. reading. 



April 3 57'5 56'8 573 



,,28 57-2 56-8 5-53 



3 55-1 54-3 5-66 



6 56-0 54-5 5-41 



9 55-0 58-4 5-44 



May 3 53*6 58'4 5'47 



Decrease of zero in the month, -0026. 



On the 5th of May the spectroscope was removed to the rooms of the 

 Royal Society, Burlington House, where it still remains. 



The result of observing under a varied temperature of 40 Fahr., 

 the carrying of the instrument from the Minories to Kew Observatory, and 

 subsequently to the Royal Society, without affecting the readings, may be 

 taken as evidence that with ordinary care the spectroscope can now be 

 used with reliance as to the rigidity of its construction, thus fulfilling the 

 conditions which are indispensable for obtaining correct observations. 



It will be observed that it was my intention to have made arrange- 

 ments with Mr. Coxwell for the observations being made with his bal- 

 loon, but the weight of the entire apparatus (approaching two hundred 

 weight), and still more the difficulty of obtaining a uniform temperature 

 throughout the prisms, renders observing in this manner very difficult, if 

 not impracticable ; I therefore suggested to Mr. Stewart that, if the ob- 

 servations were made in different latitudes, the object sought would be 

 obtained in a more satisfactory manner. 



The best, and probably the most satisfactory mode of observing, would 

 be to obtain the sanction of the Admiralty to allow the spectroscope to be 

 placed on board one of Her Majesty's vessels about visiting various lati- 

 tudes ; continued observations could then be made, and the result thereof 

 from time to time forwarded to Kew Observatory. 



Mr. Stewart writes me, that to this time it has been assumed, without 

 proof, that the change of the coefficient of terrestrial gravity does not in 

 itself alter any other coefficient of a body ; and if a reason is asked, 

 none can be given, since gravity is a force of the nature of which men of 

 science are confessedly ignorant, and that it would therefore be very desir- 

 able that experiments should be undertaken with the view of setting this 

 matter at rest. 



It is to determine this, as far as the index of refraction is concerned, that 

 the spectroscope I have described has been constructed, and the assistance 

 of the President and Council of the Royal Society will be asked, in order 

 that the observations may be made with this apparatus by some trustworthy 

 observer, on board any of Her Majesty's ships, from one point to another 

 of the earth's surface. 



