On the Source of Motions upon the Earth. 149 



In this condition let it be conceived, that the earth takes its place 

 as a planet in the solar system. The forces of terrestrial gravity 

 and cohesion, which had thus brought the constituent materials ot 

 the earth into a state of rest, and had retained them in it, would be 

 to some extent overcome by the attractions of external nature and by 

 the solar heat, and a number of changes and motions would arise m 

 conseauence. First of all, the atmosphere would be brought to its 

 existina state as regards temperature and tenuity ; then water would 

 assume" its present condition ; and to these would succeed the tides^ 

 winds, evaporation, rain, dew, the formation of lakes and rivers and 

 the endless lesser changes and motions occasioned by these. It is 

 by means of the rotation of the earth upon its axis, that these mo- 

 tions are permanently maintained. For, in this way, different por- 

 tions of the earth are successively brought under, and are then to 

 some extent removed from, the influence of external matter ; and 

 there is thus brought about an alternate action of the forces inherent 

 in terrestrial matter, and of those belonging to external nature, liy 

 the solar heat, and by the attraction of certain of the heavenly bodies, 

 the cohesion and the gravity of the matter of the earth, are, to a cer- 

 tain extent, overcome, and various motions are thereby produced. It 

 the external influences continued to operate upon the same portions 

 of matter, these motions would go on in one direction, until the co- 

 herence of these matters would almost be destroyed, and their separa- 

 tion from the globe would perhaps take place. But by means of the 

 rotation of the earth, the portions of matter which have been exposed 

 to these external influences are in a short time removed in a great 

 degree from them ; the solar heat is radiated from the earth ; terres- 

 trial gravity and cohesion resume their comparative ascendancy ; and 

 matter tends back towards that condition from which the opposing 



forces had led it. ^ ^^ c \ t 



The motions which have been mentioned are nearly all ot them ot 

 a purely mechanical or physical sort. The position of certain por- 

 tions of matter on the earth's surface is altered, but the power to 

 effect molecular or chemical changes is wanting ; or, if this is not 

 absolutely the case, such changes can occur only to a very limited 

 extent. Thus, it is perhaps possible that the destruction of cohesion 

 by the solar heat, as in the case of the evaporation of water and the 



speak of one body influencing another, to have employed language which ex- 

 pressed no more than what we really know, namely,— that matter in certain 

 conditions of relation toother matter is thrown into motion ; and to have avoiaea 

 the use of the term force in any way which seemed to imply an existence apart 

 from matter. For force is all that we know of matter ; and, that which we 

 call matter, is force ; or, if we separate them, and make matter the substratum 

 of force, we go farther than we have any reason or occasion to do. io express 

 this, however, on every occasion would be inconvenient; and, I shall theretore, 

 having thus explained my sense of the ordinary modes of speech as regards this 

 subject, continue to adhere to them. (See a letter by Dr Faraday on this sub- 

 ject, published three or four years ago in several of the scientific journals). 



