^56 Professor Kaemtz on the more important 



dryer. During the winter months this increase of the absolute 

 and diminution of the relative humidity continues till about 

 2 o'clock P.M., and as at that time the heat diminishes and pre- 

 cipitations ensue on the cold surface, so during the afternoon and 

 night the amount of vapour is smaller, and the air is relatively 

 moister, until on the following day a similar course of changes 

 is repeated. But in summer the phenomena are quite different, 

 when, during a long-continued east wind, fine weather occurs 

 with a high temperature. As the surface is then much heated, 

 the warm masses of air rise with rapidity, and mechanically 

 carry upwards with them the masses of vapour which they 

 contain. Although in this manner the evaporation always con- 

 tinues, yet the absolute amount of vapour resulting from this 

 ascending stream of air exhibits a maximum about 10 o'clock, 

 and now it diminishes till the warmest hours of the day in the 

 afternoon, without ever being so inconsiderable as was the case 

 at sunrise. We can understand how the dryness of the air in- 

 creases very rapidly under these circumstances. If during the 

 decrease of temperature this ascending stream of air becomes 

 very inconsiderable, the absolute quantity of vapour again in- 

 creases, owing to the continued evaporation ; and this is the 

 case more especially at the time of sunset, when the vapours 

 which have ascended to the upper regions during the day again 

 sink down ; and now a second maximum occurs, and as during 

 the night the vapour is precipitated as dew, the amount of va- 

 pour diminishes till the following morning ; so that we have 

 during the course of the day two maxima and two minima, 

 while the relative humidity is pretty regularly changed from 

 morning till noon, and from that time till the end of the night. 

 This course, which I have now given as that for summer, is 

 deduced from observations continued for several years at Halle. 

 It appears that many differences of climate occur ; but such 

 observations as would enable us to discuss them minutely are 

 altogether awanting. Thus the measurements made by Dr 

 Neuber at Apenrade, on the sea coast, shew a much smaller 

 diminution of the amount of vapour about the time of noon 

 than I fpund at Halle, and this is also confirmed by hourly 

 observations which I made, with the same instruments I em- 

 ployed at Halle, during the months of July and August 1837, 

 at Deep near Treptow on the Rega, close to the shore of the 



