Prof. Tyndall on Force. 63 



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 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 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 expense of 

 the sun's radiation. 



The sun, by the act of vaporization, 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 per- 

 mits 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 connexion with which is not so obvious. 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 oxide of iron, produced by the falling together of the atoms 

 of iron and oxygen ; here also is a transparent gas which you cannot 

 now see (carbonic acid gas) which is formed by the falling together of 

 carbon and oxygen. These atoms thus in close union resemble our 

 lead weight while 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, separated from each other, and thus enabled to repeat the 

 process of combination. In the building of plants carbonic acid is 

 the material from which the carbon of the plant is derived ; and the 

 solar beam is the agent which tears the atoms asunder, setting the 

 oxygen free, and allowing the carbon to aggregate in woody fibre. 

 Let the solar rays fall upon a surface of sand : the sand is heated, 

 and finally radiates away as much heat as it receives : let the same 

 beams fall upon a forest ; the quantity of heat given back is less than 

 the forest receives, for the energy of a portion of the sunbeams is 

 invested in building up the trees in the manner indicated. Without 

 the sun the reduction of the carbonic acid cannot be effected ; and 

 an amount of sunlight is consumed exactly equivalent to the 

 molecular work done. Thus trees are formed ; thus the cotton on 

 which Mr. Bazley discoursed last Friday is formed. I ignite this 

 cotton, and it flames ; the oxygen again unites with its beloved car- 

 bon ; but an amount of heat equal to that which you see produced 

 by its combustion was sacrificed by the sun to form that bit of 

 cotton. 



