352 Professor Helmholtz on the Law of the [April 12, 



We have another power which produces motion on the surface of 

 the earth. I mean the attraction of the sun and of the moon producing 

 the tides. 



All the other phenomena on the surface of the earth are produced 

 by the radiation of the sun, by the sunbeams ; and the greater part of 

 those changes which occur on the surface of our earth, are caused by 

 the heat of the sun. As the heat of the sun is distributed unequally 

 over the surface, some parts of the atmosphere become heated more 

 than other parts ; the heated parts of the atmosphere rise up, and so 

 winds and vapours are produced. They come down at first as clouds 

 in the higher parts of the atmosphere, and then as rain upon the sur- 

 face of the earth ; they are collected as rivers, and go again down into 

 the sea. So you see that all the meteorological phenomena of our 

 earth are produced by the effect of the solar beams by the heat of the 

 sun. 



The light of the sun is the cause of another series of phenomena, 

 and the principal products of the light of the sun are plants, because 

 plants can only grow with the help of the sun-light. It is only by 

 the help of the sun-light, that they can produce the inflammable 

 matter which is deposited in the bodies of plants, and which is extracted 

 from the carbonic acid and the water contained in the atmosphere, and 

 in the earth itself. 



This may give you an idea of the sense and bearing of the general 

 principle on which I purpose to speak. As many English philosophers 

 have been occupied with working out the consequences of this most 

 general and important principle for the theory of heat, for the energy 

 of the solar system, for the construction of machines, you will hear 

 these results better explained by your own countrymen ; I shall abstain 

 from entering farther into this part of the subject. At the same time 

 that Mr. Grove showed that every force of nature is capable of bring- 

 ing into action every other force of nature, Mr. Joule, of Manches- 

 ter, began to search for the value of the mechanical equivalent of heat, 

 and to prove its constancy, principally guided by the more practical 

 interests of engineering. The first exposition of the general principle 

 was published in Germany by Mr. Mayer, of Heilbron, in the year 1842. 

 Mr. Mayer was a medical man, and much interested in the solution of 

 physiological questions, and he found out the principle of the conser- 

 vation of force guided by these physiological questions. At the same 

 time also, I myself began to work on this subject. I published my 

 researches a little later than Mr. Mayer, in 1845. Now, at first sight, 

 it seems very remarkable and curious, that even physiologists should 

 come to such a law. It appears more natural, that it should be 

 detected by natural philosophers or engineers, as it was in England ; 

 but there is, indeed, a close connection between both the fundamental 

 questions of engineering and the fundamental questions of physiology 

 with the conservation of force. For getting machines into motion, it 

 is always necessary to have motive-power, either in water, fuel, or 

 living animal matter. The constructors of machines, instruments, 



