S46 Professor Kaemtz on the more important 



the cylinder, and it becomes larger the smaller the angle is 

 which it forms with the axis ; the heating of such a surface will 

 in the first case be much more considerable than in the second. 

 Analysis points out the law of this heating under different 

 angles of inclination, and without dwelling on the subject, I 

 would remark, that it follows from what has been said, that the 

 temperature of the soil, and of the strata of air lying next it, 

 must be so much the higher the greater the height of the sun 

 above the horizon. Hence not merely is the heat at every place 

 greater at the time of noon than in the morning and evening, 

 but also the equinoctial regions are warmer than the countries 

 lying near the pole. 



The simple mathematical law, indicated by the relation be- 

 tween the heating power and the height of the sun, is modified 

 in a variety of ways. Although the atmospheric air is an ex- 

 tremely transparent body, yet, even in its purest condition, it 

 does not allow all the rays to penetrate which reach it. It is 

 itself lighted by them, and these rays reflected from the parti- 

 cles of air, are the cause that the visible firmament does not ap- 

 pear perfectly dark. Even under the most favourable circum- 

 stances, of one hundred rays falling in our regions on the at- 

 mosphere, from the sun when it is in a vertical position, only about 

 eighty reach the surface of the earth ; and then, besides, a con- 

 siderable portion of the heat is at the same time lost in the upper 

 strata of the atmosphere. As each particle of air takes a por- 

 tion of heat from the sun's ray by which it is encountered, so 

 the diminution of temperature arising from this cause will be 

 so much the greater the longer the journey is that these rays 

 have to perform through the air ; and as the space to be tra- 

 versed is much greater when the sun's height is inconsiderable 

 than when it is more elevated, sf:>, for this reason, the difference 

 of heat when the sun is low and high will be still more increased. 



The nature of the surface of the earth has a very great in- 

 fluence on the degree of heat communicated. Thus, dry, loose, 

 rolled masses, such as we meet with in the sandy deserts, con- 

 duct the heat slowly through themselves. Although also the 

 uppermost layers may be strongly heated by the sun, yet, at 

 an inconsiderable depth, we meet with a heat which is but little 

 removed from the mean temperature of the year. Hence in 

 warm summer days, where perhaps the temperature of the air 



