134 BENJ. PIKES, JR., DESCRIPTIVE CATALOGUE. 



according to circumstances in the following manner : In 

 summer time, when the diurnal variations of temperature are 

 great, regard is to be had to the time of day at which the 

 experiment is made. In the morning, supposing the differ- 

 ence between the temperature of the air and the constituent 

 temperature of the vapor to be small, it is to be recollected, 

 that the accession of heat during the day is great, and that 

 the difference will therefore probably increase. If the point 

 of condensation should at the same time be lowered, it is in 

 indication of very fine weather. If, on the contrary, the heat 

 of both should increase with the day in nearly equal pro- 

 gression, rain will almost infallibly follow, as the heat of the 

 air falls with the setting sun. In showery weather, the in- 

 dications of this instrument vary rapidly three or four de- 

 grees ; and a person making observations at short intervals 

 of time, may easily predict the approach of a storm. 



Price, $14.00. 



