170 Mr. T. Hopkins on the Causes of the 



Table II. 



Table of the heights of the dry- and wet-bulb thermometers, 

 and the difference between the two, together with the dew- 

 point and height of the barometer at Plymouth for three 

 years. 



As however the aqueous vapour of the atmosphere presses 

 on the mercury of the barometer separately and independently, 

 it has been attempted to be shown that the variable pressure 

 of the vapour arising from difference in the quantity in the 

 atmosphere at different periods of the day, combined with 

 change of the gaseous pressure resulting from alteration of 

 surface temperature, and that the two causes acting together 

 produced the double undulation of the barometer; to this 

 view therefore we will direct our attention. 



The temperature near the surface of the earth at Plymouth, 

 as well as at the other places, rises from about 5 in the 

 morning till about 2 in the afternoon ; and when the wet 

 bulb, as well as the dry thermometer, is used, as it was at 

 Plymouth, it is seen that the temperature of the latter rises 

 more than that of the former, or of the dew-point, and eva- 

 poration must consequently become progressively more active; 



