1870.] 



On Actinometrical Observations in India. 



225 



affects the coast stations ; and attributes this to the want of more complete 

 knowledge of the contour of the surface, both above and below the sea- 

 level, in these parts. But his own method, in the case m=50, remarkably 

 reduces the effects of local attraction at stations on the arc of meridian 

 and out at sea (in Minicoy, an island 250 miles west of Cape Comorin or 

 Punnse); for the sensible negative quantity at Damargida and positive quan- 

 tity at Kalianpur indicate a deficiency of matter below the first and an 

 excess below the second, which exactly tally with the results independently 

 brought out by relative deflections of the plumb-line as obtained by the sur- 

 vey : and the two large and most important effects, negative at Kaliana and 

 positive at Minicoy, may be said to be almost annihilated by this method 

 of correction. This last case of an excess of gravity out at sea (where the 

 surrounding ocean has a deficiency of matter) being explained by his me- 

 thod, he regards as a very strong argument in its favour. And he finishes 

 by saying that if his method is thus far successful in the particular sup- 

 position of the distribution below, whether in excess or defect, being uni- 

 form, which is most likely not strictly the case, there is every reason for 

 concluding that pendulum-observations give support to the hypothesis 

 regarding the Constitution of the Earth's Crust, when viewed on a large 

 scale, admitting of local peculiarities, like the deficiency of matter near 

 Damargida and the excess near Kalianpur, and the similar deficiency near 

 Moscow. 



III. " Actinometrical Observations made at Dehra and Mussoorie in 

 India, October and November 1869, in a Letter to the Pre- 

 sident." By Lieut. J. H. N. Hennessey. Communicated by 

 the President. Received September 7, 1870. 



Mussoorie, July 22, 1870. 



My dear Sir, — In continuation of my last communication, dated April 

 25, 18/0, I have now the pleasure to forward the actinometrical observa- 

 tions taken, during portions of October and November 1869, with the in- 

 struments of the Royal Society, and in compliance with the suggestions 

 which the Committee of the Society made for my benefit. 



(2) The two actinometers are of the kind invented by the Rev. G. C. 

 Hodgkinson, and described by him in the Proceedings of the' Royal 

 Society, No. 89, vol, xv. Further description or allusion is therefore un- 

 necessary, unless I add that the instrument is easily and accurately worked 

 after but moderate practice, and that it is little liable to accident if rolled 

 up in a padded sheet and packed within its own metal tube. It, however, 

 imposes sensible drawbacks, from the delays incurred in throwing off a 

 suitable amount of fluid into the chamber ; and as this adjustment becomes 

 deranged by any considerable alteration in the radiation, it is impossible to 



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