Royal Society. 59 



The change of intensity of the dipping needle, in as far as it is 

 owing to a variation of the dip, would only be in the proportion of 

 3726 to 3732 • whereas, its actual amount is found to be one eighty- 

 third part of the whole. This, therefore, seems to imply changes in 

 the general magnetic intensity of the earth ; but the author, limiting 

 his present inquiry to the variations in the dip, concludes that the 

 times of the day when these changes are the greatest and the least, 

 are such as indicate a constant inflection of the magnetic pole to- 

 wards the sun during the diurnal rotation, and to point to the sun as 

 the primary agent in the production of these changes. 



May 15. — A paper was also read, entitled, " Experiments relative 

 to the 'Effect of Temperature on the Refractive Index and Dispersive 

 Power of Expansible Fluids, and on the Influence of these Changes 

 in a Telescope with a Fluid Lens." By Peter Barlow, F.R.S. 



In a paper lately read to the Society, the author stated that he had 

 not detected any change in the focal length of the telescope by 

 changes of temperature j but he has since ascertained that, in order 

 to obtain the brightest and most perfect image, the distance of the 

 object-glass requires a minute adjustment, amounting to 0*134 of an 

 inch, corresponding to an elevation of temperature from 57° to 84°, 

 or a depression from 57° to 31°. , 



In order to introduce greater clearness and precision, the author 

 proceeds to define certain terms which he finds it necessary to em- 

 ploy. By the length of the telescope, he would be understood to 

 mean the distance between the object-glass and the focus • by the 

 Jiuid focus, that between the fluid lens and the focus -, and by the 

 focal power of the telescope, he means the focal length of a telescope 

 of the usual construction, which gives the same convergency to the 

 rays,. or produces an image of the same size. 



As it is difficult to determine the refractive index of the fluid under 

 different circumstances, from which their effect on the focal power of 

 the telescope might be computed, Mr. Barlow endeavoured to ascer- 

 tain by direct observations the effect of changes of temperature on 

 the power of the telescope, and thence computes the corresponding 

 change in the refractive index of the fluid. The result is the amount 

 of adjustment already stated. The correction for angular measure- 

 ments was the 60th part of a second in every minute for every degree 

 of thermometric change ; a quantity which, he observes, is too small 

 to deserve notice, except in cases of extreme delicacy. The disper- 

 sions at 3 1° and at 84° are in the ratio of 3067 to 3084. The change 

 in the refractive index between 32° and 212°, supposing it to in- 

 crease uniformly, would be about one tenth of the whole, a proportion 

 which is very nearly the same as the actual expansion of the fluid. 

 Hence the author considers it as probable that in this, and all other 

 expansible fluids, the index of refraction varies directly as. the den- 

 sity : on the other hand, it would appear that the dispersive ratio re- 

 mains at all temperatures constantly the same. 

 . May 22d. — A letter was read from Thomas Andrew Knight, Esq., 

 addressed to the President, containing " An Account of some Cir- 

 cumstances relating to the Economy of Bees." 



12 • In 



