248 Professor TyndaVs Lecture on Force. 



tion. Suppose a higli mountain on the earth's surface ; on ap- 

 proaching 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 rota- 

 tion as much as it previously augmented it ; and thus the action 

 of all fixed bodies on the earth's surface is neutralized. But sup- 

 pose the mountain to lie always to the east of the moon's meridian, 

 the pull then would be always exerted against the earth's rota- 

 tion, the velocity of which would be diminished in a degree cor- 

 responding 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 dimin- 

 ish the velocity of the earth's rotation. The diminution, though 

 inevitable, is, however, too small to make itself felt within the 

 period over which observations 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 millstones ; that heat has an origin 

 totally diflferent 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 expense of the 

 sun's radiation. 



The sun, by the act of vaporisation, lifts mechanically all the 

 moisture 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 glaciers 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 towards 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 connection with which is not so ob- 

 vious. Trees and vegetables grow upon the earth, and when 

 burned they give rise to heat, and hence to mechanical energy. 

 Whence is this power derived ? You see this oxyd of iron, pro- 

 duced by the falling together of the atoms of iron and oxygen;, 

 here also is a transparent gas which you cannot now see — car- 



