212 M. Arago on Artesian Wells 



Mariotte have been recently repeated from more accurate data, 

 especially in what relates to the gauging of the Seine. And we 

 shall now present the conclusions, as they are arranged in an 

 excellent work of M. Dausse, the engineer, which has not yet 

 issued from the press. 



The basin of the Seine (and we shall here limit and terminate 

 it northwards at Paris, since we can so easily measure the quan- 



the nature of the bodies themselves, their exposure, and the clearness of the 

 atmosphere, exercise the greatest influence over the phenomenon, it may be 

 clearly understood, that, even an approximation to the definite quantity of 

 dew which is deposited in each country, would be one of the most complicated 

 problems in physics. 



Dr Dalton has found that the mould of a garden saturated with moisture 

 contains in bulk not less than seven-twelfths of water. The quarter, and even 

 the half, of this water may disappear, without materially affecting vegetation. 



It would appear that, in all countries, the layer of water which evaporation 

 removes from any sheet of water, in the course of a year, is just about as 

 thick as that which the rain deposits in its place. The experiment, however, 

 would need to be repeated with vessels of a much larger size than those which 

 have been hitherto employed by meteorologists. The observations made now 

 nearly a century ago by Bazin, give a larger quantity from the evaporation 

 of earth saturated with moisture, than that supplied by pure water itself. 

 This result seems not at all likely ; but, in all such subjects, it is only expe- 

 riment that can autjioritatively determine. 



I shall conclude this note by a remark of the late Professor Leslie, who, 

 without adding any thing to our knowledge, as yet so imperfect, on the cause 

 of evaporation, has notwithstanding pointed out, in this phenomenon, a de- 

 velopment of mechanical power most striking to the mind, especially when we 

 reflect on the silent manner in which it is accomplished by Nature. 



Let it be conceded that the water which is annually elevated from the globe 

 by means of evaporation, is equal, in all climates, to the quantity of water 

 which falls in each of them. This evaporated water is disseminated through- 

 out the atmosphere at all heights. We may, however, procure a kind ot mean 

 between these extremes of the ascending movements, by conceiving that the 

 evaporated water is all raised to a certain mean height, and no farther. The 

 annual evaporation, so far as regards its mechanical effects, would be thus re- 

 presented by a known mass of water being elevated vertically to the extent 

 of a number of yards, which is also known. But the work of this sort which a 

 man could eflTect during a year has been determined ; and the comparison of 

 the two results demonstrates, that evaporation represents the labour of 

 (80,000,000,000,000) eignty billions of men. If we suppose, then, that 

 (800,000,000) eight hundred millions is the population of the globe, and that 

 one-half of them could work, then the power employed by nature in the for- 

 mation of clouds, would equal what could be rendered by two hundred thousand 

 times the constant labour of the whole human race. 



