THE 



METEOEOLOGY OF JAMAICA 



The Barometer 



This instrument Tvas invented by Torricelli in the year 

 1643, and its principle may be illustrated by the following 

 experiment. 



Take a glass tube about three feet in length, closed at one 

 end, and fill it with mercur3^ Now close the open end by 

 pressure of the finger, invert the tube, dip the end closed 

 by the finger into a bowl containing mercury, and then 

 remove the finger. It will be found that the mercury will 

 fall a few inches in the tube, leaving a vacuum at the upper 

 end ; and upon measurement it will be found that the height 

 of the column of mercury in the tube above the surface of the 

 mercury in the bowl will be about 30 inches, provided that 

 the experiment be made near the level of the sea. 



Torricelli thus demonstrated that the pressure of the 

 atmosphere on any area near the sea-level is counter- 

 balanced by the pressure of a column of mercury on that 

 area whose height is about 30 inches. 



In the barometer the bowl is replaced by a small glass 

 cistern ; the cistern and the glass tube are enclosed in a 

 suitable brass frame ; and arrangements are made for 

 measuring with great accuracy the height of the column 

 of mercury above the level of the mercury in the cistern. 

 This is what is to be understood by "the height of the 

 barometer." 



The brass framework has a thermometer attached to it, 



