202 A TREATISE 



question, I applied to Mr. Farquhar, who, I under- 

 stood, had paid some attention to this subject ; and 

 was favoured with the following very obliging and 

 instructive letter : 



TO DOCTOR BALFOUR. 



DEAR SIR, 



<c You likewise desire me to give you some ac- 

 count of the regular diurnal variations of the Ba- 

 rometer which take place in this country ; and which, 

 I said, I conceived to be peculiar to tropical climates, 

 from the otherwise unaccountable silence of every 

 author whose work I had been able to consult on 

 the subject. The first intimation of this was from 

 Mr. Henry Trail, who informed me that he had 

 observed the Mercury to rise every night till about 

 eleven o'clock, when it became stationary. I imme- 

 diately repeated his observations, and found that the 

 fact was certain ; but that there was likewise another 

 diurnal variation, which had escaped his notice. Af- 

 ter numerous observations, at all hours during the 

 day and-night, I found that the Mercury is subject 

 to the following variations, with the utmost degree of 

 regularity, throughout the whole year. Fiom six 

 in the morning till between seven and eight, it is sta- 

 tionary ; it then rises till nine, sometimes, though 

 rarely, till ten, when it remains stationary till noon ; 

 it then descends, and is lowest at three, and con- 

 tinues stationary till eight ; when it begins to rise, 

 and continues till eleven, and is then at the same 

 height that it was at nine in the morning. 



On 



