80 ANNUAL OF SCIENTIFIC DISCOVERT. 



" Standing upon one of the London bridges, we observe the current 

 of the Thames reversed, and the water poured upwards twice a day. 

 The water thus moved rubs against the river's bed and sides, and heat 

 is the consequence of this friction. The heat thus generated is in part 

 radiated into space, and then lost, as far as the earth is concerned. 

 What is it that supplies this incessant loss? The earth's rotation. Let 

 us look a little more closely at the matter. Imagine the moon fixed, 

 and the earth turning like a wheel from west to east in its diurnal ro- 

 tation. Suppose a high mountain on the earth's surface ; on approach- 

 ing the moon's meridian, that mountain is, as it were, laid hold of by 

 the moon, and forms a kind of handle by which the earth is pulled 

 more quickly round. But when the meridian is passed, the pull of the 

 moon on the mountain would be in the opposite direction; it now 

 tends to diminish the velocity of rotation as much as it previously auo 1 - 

 mented it ; and thus the action of all fixed bodies on the earth's sur- 

 face is neutralized. But suppose the mountain to lie always to the 

 east of the moon's meridian, the pull then would be always exerted, 

 against the earth's rotation, 'the velocity of which would be diminished 

 in a degree corresponding to the strength of the pull. The tidal-wave 

 occupies this position ; it lies always to the east of the moon's meridian, 

 and thus the waters of the ocean are in part dragged as a brake along 

 the surface of the earth ; and as a brake they must diminish the veloc- 

 ity of the earth's rotation. The diminution, though inevitable, is, how- 

 ever, too small to make itself felt within the period over which obser- 

 vations on the subject extend. Supposing then that we turn a mill by 

 the action of the tide, and produce heat by the friction of the mill- 

 stones; that heat has an origin totally different from the heat produced 

 by another mill which is turned by a mountain stream. The former is 

 produced at the expense of the earth's rotation, the latter at the ex- 

 pense of the sun's radiation. 



" The sun, by the act of vaporization, lifts mechanically all the mois- 

 ture of our air. It condenses and falls in the form of rain ; it freezes 

 and falls as snow. In this solid form, it is piled upon the Alpine 

 heights, and furnishes materials for the glacieys of the Alps. But 

 the sun again interposes, liberates the solidified liquid and permits 

 it to roll by gravity to the sea. The mechanical force of every river 

 in the world, as it rolls toward the ocean, is drawn from the heat of 

 the sun. No streamlet glides to a lower level, without having been 

 first lifted to the elevation from which it springs by the mighty 

 power of the sun. The energy of winds is also due entirely to the, 

 sun ; but there is still another work which he performs, and his con- 

 nection with which is not so obvious. Trees and vegetables grow upon 

 the earth, and when burned they give rise to heat, and hence to me- 

 chanical energy. Whence is this power derived ? You see this oxide 

 of iron, produced by the falling together of the atoms of iron and ox- 

 ygen ; here also is a transparent gas which you cannot now see, car- 

 bonic acid gas, which is formed by the falling together of carbon 

 and oxygon. These atoms thus in close union resemble our lead 

 weight Avhile resting on the earth ; but I can wind up the weight and 

 prepare it for another fall, and so these atoms can be wound up, sepa- 

 rated from each other, and thus enabled to repeat the process of com- 

 bination. In the building of plants, carbonic acid is the material from 



