346 ■■■'' Mr. Goidhigham's Report [Nov. 



state. The observations commenced early in the day, after the 

 pendulum had been under the influence of the chill and damp of 

 the nis;ht, and I am led to think retained a much greater degree 

 of cold, than was exhibited by the thermometer, particularly 

 during the time the moisture il might have collected was eva- 

 poratmg by the influence of the sun. Hence the greater number 

 of vibrations at such times. Again, great heat in the middle of 

 the day might be retained by the metal of which the pendulum 

 is composed, in a greater degree, and longer than the air. 

 Hence the greater the expansion of the pendulum, and the con- 

 sequent fewer number of vibrations in 24 hours, than would 

 have been shown under a more equable state of the atmosphere. 

 The differences, exclusive of such parts as are attributable to the 

 different heights of the barometer, though very obvious, are not 

 large; and if the changes in the atmosphere which we have 

 noticed give in one case too small a number of vibrations, and 

 in the otner a number too large, the mean will probably show as 

 correct a result as could be obtained under the most favourable 

 circumstances. Before closing this remark, I shall take the 

 opportunity to state, that the barometer sent with the expedi- 

 tion, though the best to be procured here, and a good one, as 

 far as relates to the rise and fall of the mercury ; yet from some 

 general defect (in the scale probably) showed the height less 

 than it ought ; and by comparisons with the Observatory baro- 

 meter, both before trie expedition left Madras, and after its 

 return, the exact difference was 0-103 inch : this correction has 

 accordingly been applied to all the heights of the barometer 

 registered with the experiments and observations.* 



We shall now proceed to apply the remaining corrections. 

 The specific gravity of the pendulum was ascertained for the 

 Madras experiments in the manner stated in my paper published 

 in the Philosophical Transactions ; t I was anxious to have this 

 verified, and, if possible, by a more accurate apparatus than I 

 was able to submit the pendulum to before ; this it occurred to 

 me might probably be done at the Mint; and I accordingly 

 addressed the Honourable the Governor in Council upon the 

 subject; an order was immediately sent to the Assay-Master, 

 Dr. Aitken, to have the operation performed; this was most 

 carefully done, and the mean result of three trials, on different 

 days, was as follows : — Thermometer 91° ; barometer 29*9551 ; 

 specific gravity 7*966. 



Distilled water was used on the first two days, and rain water 

 on the third day ; the results however differed very little. The 



• Some experiments which I have since made at the Observatory confirm the opinion 

 above given ; during the time, however, of making the experiments, although great 

 changes took place in the thermometer, there were none in the hygrometer ; and it 

 spears to have been when there were considerable changes in the latter, that the greatest 

 ^ect of the atmosphere was felt by the pendulum at the island. 



+ A notice of this paper -^ill be found in the Annals, N. S. vol. iv. p. 382. 



