172 Mr. T. Hopkins on the Causes of the 



shown by the ordinary thermometer ; the second, the tempe- 

 rature of the wet-bulb thermometer, as kept down by the cool- 

 ing influence of evaporation ; and the third gives the differ- 

 ence between the two first. Now as this difference arises from 

 the extent of the evaporation, the numbers of the difference 

 may be taken to express the force and amount of evaporation, 

 and to indicate the additional vapour that is discharged into 

 the atmosphere. This force or amount at 5 o'clock in the 

 morning is l c, 21, from which time it increases to 3 0, 39 at 10 

 o'clock. So that during this time, five hours, the increase in 

 the force of evaporation is 2 0, 18; and this in the temperature 

 theory must be held to be sufficient to overcome the lightening 

 effect of a rise of 5°*86 of temperature, and also to raise the 

 mercury of the barometer to the full extent of the morning 

 rise ! After this time, from 10 to 1 o'clock, the temperature 

 rises further from 52°-84 to 55°'83 or 2°*99 ; and during the 

 same period the force of evaporation increases 1°'89, that is, 

 from 3°"39 to 4 0, 28. Thus we are required to believe, that 

 from 5 to 10 in the morning, 2 0, 18 of evaporation overcame 

 the lightening influence of 5°'S6 of temperature, and in addi- 

 tion raised the mercury of the barometer; and from 10 to 1 

 in the day, 1 0, 89 of evaporation not only failed to overcome 

 the lightening effect of 2 0, 99 of temperature, but allowed this 

 relatively small amount of temperature to produce the further 

 result of a fall of the mercury of the barometer. Or put in a 

 tabular form, say that from 



5 to 10 o'clock, 5°*86 of temperature and 2° - ]8 of evaporation caused a rise. 

 10 to 1 o'clock, 2°-99 of temperature and l°-89 of evaporation caused a fall ! 



That is, where temperature, the influence which lightens the 

 atmosphere, is relatively great and should cause a fall, the 

 mercury of the barometer rises; and where the influence of 

 temperature is relatively small and should cause the vapour 

 to produce a rise, the mercury falls ! This must be erro- 

 neous. 



In the same place, at Plymouth, from 1 o'clock until 4 p.m., 

 as may be seen in the table, the temperature falls ; and as far 

 as that temperature acted the atmosphere would of course be- 

 come heavier. At the same time evaporation shows vapour 

 is passing into the atmosphere; it ought therefore to follow 

 that the barometer should rise, and considerably too, through 

 the operation at the same time of both the causes which are 

 supposed to contribute to the production of a rise. But the 

 barometer does not rise ; on the contrary, it falls, and conti- 

 nues falling until 4 o'clock. These facts and reasonings prove 

 that neither the daily variations of surface temperature, nor 

 the different amounts of vapour pressure, nor both taken to- 



