10 Mr Parker, On Garnot's principle [Oct. 31, 



tion of X, and thus Carnot's principle is proved to hold for the 

 cycle just described. 



It will be noticed that we have only considered the total 

 quantity and not the quality (or wave length) of the radiation 

 absorbed by the plant. We have merely shown that Carnot's 

 principle is satisfied when the plant grows by absorbing heat, 

 without enquiring whether any or what conditions must be 

 satisfied in order that the plant may be able to absorb heat. 

 Now it is observed that a plant cannot grow when all the radia- 

 tion which falls upon it is dark. Thus there is a further question 

 for consideration; but at present we do not appear to possess 

 sufficient data to discuss it. We shall therefore merely point 

 out that the question cannot affect the validity of the preceding 

 reasoning. 



It follows from what we have said that ' transpiration ' is 

 essential to vegetable life. In fact, a plant would be choked if 

 kept in a place constantly saturated with aqueous vapour. On 

 the earth the atmosphere is prevented from being constantly 

 at the point of saturation by the succession of day and night : 

 in other woi'ds, ' cold and heat, day and night *,' are necessary 

 to vegetation. 



If a planet be placed so that no heat can reach it from ex- 

 ternal sources, its temperature will generally become practically 

 constant and its atmosphere saturated with vapour. On such a 

 body vegetable life would be impossible ; and we may infer from 

 Carnot's principle that our beds of coal cannot have been pro- 

 duced by the internal heat of the earth alone, but must be due to 

 solar radiation. 



If we make a few assumptions, Carnot's principle easily leads 

 us to some further results. Thus if we suppose the condition 



of the air to be the same in two places, 6 and 6' will be the 



a a' a 



same in both. If therefore we put x = ~ -^ — ^ q, we may write 



Q 



X — -^ — ^ \q, where X is the same in both cases. Consequently, 



V — Vo 

 a 



since -~ is nearly equal to unity, we see that q practically varies 



as x(d — OJ. Next, since x depends on the difference between 

 the temperature of the air and the dew-point, and therefore on 

 6 — 6^, we will assume that, for a given time, it varies as the 

 intensity of solar radiation and 6 — 6^ conjointly. Hence, for a 

 given time, q varies as the intensity of solar radiation and {6 —dgY 

 conjointly. For example, Jupiter is about 5"2 times as far away 

 from the sun as the earth, and at this distance the intensity of 



* Gen. viii. 22. 



