w, 



[ 289 ]' 



LV. distract of Ohservatmis on a diurnal Variation of the 

 Barometer between the Tropics. By J. Horsburgh, 

 Esq. In a Letter to He^'RY Cavendish, Esq. F.R.S.* 



SIR; Bombay, April 30, If 04. 



HEN I was in London at the conclusion of the year 

 ISOl, I had the pleasure of being introduced to you bv my 

 friend I\Ir. Dalryniple, at which time he presented' you 

 with some sheets of meteorological obfervalions, with ba- 

 rometer and thermometer, made by me in India^, and during 

 a passage from India to England. 



Being of opinion that few registers of the barometer arc 

 kept at sea, especially in low latitudes, I have been induced 

 to continue iny observations since I left England, judging 

 that, even if they were fuuud to be of no utility, they might 

 at least be entertaining to you or other gentlemen who have 

 been making observations of a similar nature. 



During my last voyage I have emploved two marine ba- 

 rometers, one made bv Troughton, the other by Ramsden ; 

 and a thermometer by Frazer. These were placed, exposed 

 to a free current of air, in a cabin v/herc the basonS of the 

 barometers were 13 feet above the level of the sea. 



The hours at which the heights of the barometers and 

 thermometers were taken, viz. noon, four hours, ten hours, 

 twelve hours, sixteen hours, and nineteen hours, were 

 chosen, because at these times the mercury in the baro- 

 meter had been perceived to be regularly stationary between 

 the tropics by former observations made in India in 1800 

 and 1801. it was found that in settled weather in the In* 

 dian seas, from eight A.M. to noon, the mercury in the 

 barometer was generally stationary, and at the point of 

 greatest elevation ; after noon it began to fall, and conti- 

 nued falling till four in the afternoon, at which time it ar- 

 rived at the lowest point of depression. From four or five 

 P. M. the mercury rose again, and continued rising till 

 about nine or ten P.M., at which time it had again ac- 

 quired its greatest point of elevation, and continued sta- 

 tionary nearly till midnight; after which it began to fall, 

 till at four A. M. it was again "as low as it bad been at four 

 iu the afternoon preceding; but from this time it rose till 

 seven or eight o'clock, when it reached the highest point 

 of elevation, and continued stationary till noon. 



Thus was the mercury observed to be subject to a regular 

 i-levaiion and depression twice in every twenty-iour hours 



• From the Transactions of tke Royal .S'om 'y for 1805. 



\V)1. 23. Xo. 95. 'Jan. I80G. T iu 



