18 



Tropical varia- 

 tion of the ba- 

 rometer. 



KAROMETRrCAL VARIATION, 



As I am ignorant whether this phenomenon lias been noticed 

 by any perlon before, I will here give you an abftraft of my 

 journal, (hewing how the barometer has been influenced 

 during the whole time fince I left England, which will enable 

 you to form an idea whether I am right in concluding that the 

 barometer is really differently afFe6ted at fea from what it is 

 on fliore, at thole places in India where the obfervations have 

 been made. 



The lirft flieet begins with the obfervations made on board 

 ihip» 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 ibe mercury in barometers was flu6tuating and irre- 

 gular until we were in latitude 26^ N, longitude 20o W, on 

 April 29th ; the mercury in barometers then became uniform in 

 performing two elevations and two depreffions every 24 hours, 

 (which for brevity in mentioning hereafter I will call equatro- 

 pical motions.) From latitude 26° N to latitude 10* N, the 

 ditference of the high and low ftations of the mercury in the 

 barometers was not (6 great, as it was from latitude 10* N 

 acrofs the equator, and from thence to latitude 25** S. Within 

 thefe laft-mentioned limits, the difference of high and low 

 Nations of the mercury in the barometers was very confi- 

 derable, generally from five to nine hundred parts of an inch, 

 both in the daily and nightly motions. 



When we reached the latitude of 28" S, longitude 27° W, 

 June 7th, the mercury in barometers no longer adhered to 

 the equatropical motions ; but then, as in high north latitudes, 

 its rifmg and falling became irregular and flu£luating during our 

 run from latitude 28° S, longitude 27 «. W, (moftly between 

 the parallels of 35^ and 36° S,) until we were in latitude 27° S, 

 and longitude 51® E, on the ] 1th of July. The mercury then 

 began to perform the equatropical motions, and continued 

 them uniformly, during our run from the laft-mentioned po- 

 lition, up the Madagafcar Archipelago, acrofs the Equator, 

 until our arrival at Bombay July 31(1. 1802. 



Augaft 6th, 1 802. When the barometers were placed on 

 fliore in Bombay, the mercury, for (he firft fix days,, appeared 

 to have a fmall tendency towards performing the equatropical 

 motions, but not equally perceptible as when at fea, the dif- 

 ference between the high and low flations of the mercury in 



the 



