S48 Professor Kaemtz on the more important 



of temperature which take place during the day. If the sun 

 is under the horizon, the source of heat is removed, and the^ 

 earth is cooled, inasmuch as its surface radiates to the sky the 

 heat acquired by day, and hence the heat is diminished during 

 the whole night, till at length, some time before the rising of 

 the sun, warmth again arrives accompanying the rays of light of 

 the dawn. It is the superficial thin layer of the earth''s surface 

 which first loses its heat in this manner ; it is cooled at last to 

 a lower point than the portion beneath, which during the day 

 had acquired heat by conduction, and, as an exchange now 

 ensues between the two, the cooling is less considerable than it 

 would have been, did that exchange not take place. 



We have to take into consideration the same circumstances 

 in the night cooling, as in the action of the sun. If the ground 

 is a bad conductor of heat, the communication between the 

 upper and the more deeply situated layers takes place but 

 slowly, and the same places in which the heating during the 

 day was so considerable are also remarkable for their cooling 

 during the night. But it is quite diff'erent with regard to very 

 large masses of water. The uppermost portions are hardly 

 cooled when they sink down, owing to their greater specific 

 gravity ; warmer portions take their place, which in a short 

 time meet with the same fate, and thus the cooling during the 

 night is just as inconsiderable as the heating during the day. 

 Owing to the occurrence of clouds which is so very frequent, this 

 decrease of heat is still further diminished. As the clouds act 

 like a screen and diminish considerably the radiation by night, 

 the decrease of heat is smaller ; and just as cloudy days in sum- 

 mer are much colder than clear ones, so obscure nights are 

 Avarmer than clear ones. 



What has been said of the changes that occur in the course 

 of the day, also holds good as to the variations which take 

 place in the course of the year. In the interior of a continent 

 the difference between the heat of summer and winter is much 

 greater than it is on the sea-coast; nay, so remarkable is this fact, 

 that, in passing from the west coast of Europe to the interior, 

 the increase of this difference can be traced almost step by step. 



Rise and Fall of the Barometer explained. — Without fol- 

 lowing up more deeply the above propositions, or forming 

 conclusions regarding the resulting form of the isothermal 



