390 OhservatioHs on a d'mrnal Variation 



in settled weather ; and the lowest station was observed 16 

 be at about four o'clock in the morning and evening. I 

 remarked that the mercury never remained long fixed at 

 this low station, but had a regular tendency to rise from 

 thence till to\\ ard« eight in the morning and about nine in 

 the evening, and from those times continued stationary till 

 tioon and midnight. 



In unsettled "blowing weather, especially at Bombay 

 during the rains, these regular ebbings and flowings of the 

 mercury could not be perceived ; but a tendency to them 

 was at some times observable when the weather was more 

 settled. 



In the sheets which I formerly presented to you were 

 evinced these elevations and depressions t\sicc every twenty- 

 four hours within the tropics, in steady weather, as had 

 been observed by Messrs. Cassan and Peyrouse, by Dr. Bal- 

 four of Calcutta, and others. But since my last arrival in 

 India I have observed that the atmosphere appears to pro- 

 duce a different effect on the barometer at sea from what 

 it does on shore. 



As I am ignorant whether this phaenomenon has been 

 noticed by any person before, I will here give you an ab- 

 stract of my journal, showing how the barometer has been 

 intlueiiced during the whole time since I left England, 

 which will enable vou to form an idea whether I am right 

 in concluding that the barometer is really differently affected 

 at sea from what it is on shore, at those places in India 

 where the observations have been made. 



The first sheet begins with the observations made on 

 board ship, in my voyage from London towards Bombay, 

 in the months of April and May 1802. 



From the time of leaving the Land's End, April 19th, the 

 motion of the mercury in barometers was fluctuating and 

 irregular until wc were in latitude 26° north, longitude 20'' 

 west, on April 2f)th ; the mercurv in barometers then be- 

 came uniform in performing two elevations and two de- 

 pressions every twenty-four hours, (which for brevity in 

 mentioning hereafter I will call equatropical motions.) 

 From latitude 26"* north to latitude 10" north, the differ- 

 ence of the high and low stations of the mercury in the 

 barometers was not so great as it was from latitude lO" 

 north across the equator, and from thence to latitude So** 

 south. Within these last-mentioned liniils the difference 

 of high and low stations of the mercury in the baronaeters 

 was very considerable, generally from five to nine hundred 

 parts of an inch, both in the daily ami ii!i.lu!v motions. 



Wlicn 



