1862.] on Force. 533 



rotation, the velocity of which would be diminished in a degree corre- 

 sponding to the strength of the pull. The tidal wave occupies this 

 position — it lies always to the east of tlie 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 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 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 carbon ; but an amount of heat equal to that which you see 



