150 PNEUMATICS. 



A thermometer should always be attached to 

 the barometer, as a necessary appendage ; and, by 

 the side of it, a scale of correction, to show how 

 much to add or subtract from the height of the 

 mercury in the barometer for the degree of tem- 

 perature ; for, it is evident that the mercury in the 

 tube will be affected by heat and cold, in the same 

 manner as the thermometer, and, on that account, 

 it will not show the true weight of the atmosphere. 

 This correction is, therefore, very necessary. 



Ever since it was observed that a change of 

 weather generally accompanied or followed a 

 variation in the height of the barometer, it has 

 been used as a prognostic of the weather ; and 

 hence it is frequently called a weather-glass. A 

 great variety of observations have been made by 

 different persons, relative to the effect which cer- 

 tain changes of weather have upon this instrument; 

 and thence they have derived a set of rules, that 

 assist in enabling one to foretel the changes in the 

 weather. 



But these are by no means so certain, and so 

 much to be depended upon, as many suppose. 

 So numerous are the causes that affect the state of 

 the atmosphere, with which we are but little ac- 

 quainted, that no instrument can be implicitly de- 

 pended upon as foretelling the alterations that are 

 to happen. 



The following are the principal circumstances 

 to be observed in using the barometer as a weather- 

 glass. 



1. The rising of the mercury presages, in gene- 

 ral, fair weather ; and its falling, foul weather ; as 

 rain, snow, high winds, and storms. 2, In very 

 hot weather, the falling of the mercury foretels 



