430 Mr. T. Hopkins on the Diurnal Changes 



thus shown that the range of vapour pressure was greatest, 

 not where the temperature was the most raised, and where 

 evaporation must have been the greatest, but in the latest and 

 coolest month, and on the highest mountain ! And in Sep- 

 tember the pressure increased to the latest period of the day, 

 not near the surface, the source of evaporation, but on the 

 high mountain. These irregularities show that some cause 

 was in operation, which determined the vapour that had been 

 produced by evaporation from the surface of the earth in the 

 warm and comparatively dry month of June to continue in- 

 creasing at the low level up to eight in the evening, but to ac- 

 cumulate only to a moderate extent, whilst on the mountain 

 it accumulated to a much greater extent, but not later than 

 until noon. In the cooler month of September, however, the 

 vapour accumulated to about an equal extent, and about the 

 same times, on the low level and on the high mountain, pre- 

 senting a great difference between the action of the vapour in 

 June and in September. The absolute pressure of the va- 

 pour, it will be recollected, is greater in the lower than in the 

 higher strata ; but the increase of that pressure is greater in 

 the higher part in the dry and warm month of June, while it 

 is only equal in the moist and cool month of September, 

 showing that it was not merely expansion and diffusion of the 

 vapour produced by evaporation that were in operation, but 

 that some other cause was at work, which made the vapour 

 accumulate on the mountain more than on the plain in June» 

 but not in September;'-'J'^/^>J"''''"'J.''''^J \^> jugi^v/ yiij .to jw| 



In high latitudes thfe'^^i^ *(!)? 'thS' Vi;j:5tttif i^"tfiei6fig't'1ft 

 winter, and the most in summer. In Halle, in Prussian 

 Saxony, for instance, it is 4*509 in January, and in July 

 11*626, almost three times the amount; and the same kind of 

 difference between winter and summer is found in other 

 northern parts. Generally it may be said to be the least in 

 winter and in cold climates, and the most in summer and in 

 warm climates. 



When the dew-point, contiguous to the surface of the 

 earth, is the nearest to the temperature, which is, say, at four 

 or five in the morning, both the temperature and the dew- 

 point are the lowest. From this time the temperature rises 

 more than the dew-point, until the former reaches the highest 

 point for the day. There is consequently in the lower part 

 of the atmosphere an increasing difference occurring between 

 the temperature and the dew-point up to the time of the high- 

 est temperature. But this does not take place in the same 

 degree in the higher strata, as in them the dew-point pro- 

 gressively approximates to the temperature, until at some 



