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XXVI. — Observations on the Temperature of the Ch'ound at Trevandrum, in India, from May 

 1842 to December 1845. By John Caldecott, Esq., Astronomer to the Rajah of Travancore. 

 Communicated in a Letter to Professor J. D. Forbes. 



(Read 15th March 1847.) 



122 Pall Mall, Feb. 3, 1847. 



My Dear Sir, — I have taken the opportunity of a short visit to England to bring with me 

 a complete copy, to the end of ] 845, of the Observations of Terrestrial Temperature, made at 

 Trevandrum (of which I have already sent you a partial account), and which I have now the 

 pleasure to forward to you herewith, to be dealt with in any way you may think proper. 

 Should you think them worthy of presentation to the Royal Society of Edinburgh for publica- 

 tion in their Ti-ansactions, it wiU be gratifying to me. The observations made with the 12 feet 

 thermometer are now at an end, in consequence of the fracture of the instrument during a high 

 wind last year ; but, after rejecting those made immediately on the introduction of the instru- 

 ments, say all those of 1842, there will still remain three entu-e years of trustworthy read- 

 ings, which I hope may be found to be sufficient for any purpose for which such observations 



in a tropical region may have been considered desirable. 



Jno. Caldecott. 



Professor Foebes, Edinburgh. 



EXPLANATION. 



These three thermometers, made by Messrs Adie and Son of Edinburgh, were put down on May 1, 

 1842. They are buried at the respective depths of 12 French feet, 6 French feet, and 3 French feet, on the 

 top of the Observatory hill, which is composed of the stone called " laterite." The situation is about 200 

 feet above the sea. 



In December 1841 and January 1842, they were suspended in the Observatory, and compared with a 

 standard thermometer by Traughton and Simms, as follows, viz. : — 



No. 1, or the 12 feet thermometer, by a comparison made 12 times in 24 hours for 29 days, i. e., by 348 

 compai'isons, requires, in order to reduce it to the standard thermometer, the addition to the readings here 

 given of 2°133. The greatest deviation from this mean of any single comparison is 0°'54. 



No. 2, or the 6 feet thermometer, by the same number of comparisons, requires the addition of 2°-172 ; 

 the greatest deviation from which is 0°'69. 



No. 3, or the 3 feet thermometer, by the same number of comparisons, requires the addition of 2°-922 ; 

 the greatest deviation from which is 0°"57. 



Latitude of Trevandrum Observatory, ... 8° 30' 32" N. 

 Longitude from Greenwich, 5'' 7"* 59^ E. 



[It may here be stated, that the original registers furnished by Mr Caldecott include four observa- 

 tions, daily (except on Sundays), at 6 a.m., Noon, 6 p.m., and Midnight. The extent of the Tables is, con- 

 sequently, very considerable ; and, in printing them, it was necessary to ascertain whether these six-hourly 

 observations possessed a value proportional to the great additional space which they would have occupied. 

 A careful examination shewed that the variation from one hour to another, in the different thermometers, 

 was due mainly to the influence of the heat of the air in expanding the exposed part of the column of spirit, 

 and did not at all indicate the coui'se of the diurnal curves at different depths ; nor were there data given 

 for eliminating this discrepancy. It was, therefore, decided to publish the 6 A.M. observations alone,* as 

 representing fairly the general result, and as being less liable to error from the irregular expansion of the 

 exposed column of spirit by the intense heat of the tropical sun. 



It will be observed that all the temperatures in the Eegister, and the means, are TTNOOEKEOTED for the 

 large index error of the instruments. A corrected summary will be given, together with a few deductions, 

 at the conclusion of the paper. — J. D. F.] 



* The monthly means, four times a day, are, however, given in a separate Table. 



VOL. XVI. PART III. 5 D 



