BAROMETRICAL VARIATION. 



r 



and a Ihermometer by Frazer. Thefe were placed expofed Tropical varia- 

 to a free current of air in a cabin, where the batons ot the rometer- 

 barometers were 13 feet above the level of the fea. 



The hours at which the heights of the barometers, and ther- 

 mometers were taken, xiz. noon, 4 hours, 10 hours, 12 hours, 

 14. hours, and 19 hours, were chofen, becaufe at thefe times 

 the mercury in the barometer had been perceived to be regu- 

 larly ftationary between the tropics, by former obfervations 

 made in India in 1800 and 1801. It was found that in fettled 

 weather in the Indian feas, from 8 A M to noon, the mercury 

 in the barometer was generally ftationary, and at the point df 

 greateft elevation ; after noon it began to fall, and continued 

 falling till 4 afternoon, at which time it arrived at the lowed 

 point of depreffion. From 4 or 5 P M the mercury rofe again, 

 and continued rifing till about 9 or 10 P M, at which time it 

 had again acquired its greateft point of elevation, and con- 

 tinued ftationary nearly till midnight; after which it began to 

 fall, till at 4 A M it was again as low as it had been at 4 after- 

 noon preceding ; but from this time it rofe till 7 or 8 o'clock, 

 when it reached the higheft point of elevation, and continued 

 ftationary till noon. 



Thus was the mercury obferved to be fubjeft to a - regular 

 elevation and depreffion twice in every 24 hours in fettled 

 weather ; and the loweft ftation was obferved to be at about 4 

 o'clock in the morning and evening. I remarked that the 

 mercury never remained long fixed at this low ftation, but 

 had a regular tendency to rife from thence till towards 8 in the 

 morning and about 9 in the evening, and from thofe times 

 continued ftationary till noon and midnight. 



In unfettled blowing weather, efpeciallyat Bombay during 

 the rains, thefe regular ebbings and flowings of the mercury 

 could not be perceived; but a tendency to them was at forae 

 times obfervable when the weather was more fettled. 



In the ftieets, which I formerly prefented to you, were 

 evinced thefe elevations and depreffions twice every 24 hours 

 within the tropics, in fteady weather, as had been obferved 

 by MefiT. Caftan and Peyroufe, by Dr. Balfour of Calcutta, 

 and others. But fince my laft arrival in India, I have obferved 

 that the atmofphere appears to produce a different effed on the 

 barometer atym from what it does on Jhore^ 



Vol. XIII.— January, 1S06, C As 



