diurnal Variation of the Barometer between the Tropics. 179 
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 twice every 24 hours 
within the tropics, in steady weather, as had been observed 
by Mess. Cassan and Peyrouse, by Dr. Balfour of Calcutta, 
and others. But since my last arrival in India, I have observed 
that the atmosphere appears to produce a different effect on the 
barometer at sea from what it does on shore. 
As I am ignorant whether this phenomenon has been noticed 
by any person before, I will here give you an abstract of my 
journal, shewing how the barometer has been influenced 
during the whole time since I left England, which will enable 
you 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 irre- 
gular until we were in latitude 2 6° N, longitude 20° W, on 
April 29th ; the mercury in barometers then became uniform in 
performing two elevations and two depressions every 24 hours, 
( which for brevity in mentioning hereafter I will call equatro- 
pical motions.) From latitude 2 6° N to latitude io° N, the 
difference of the high and low stations of the mercury in the 
A a 2 
