ON THE THEORY OF RAIN. 401 



tions, with reference to the temperature of the seasons, led 



Professor Phillips to the following formula for calculating 



the difference between the ratios of the quantities of rain 



received on the ground and, at any height h, the value of 



the coefficient p depending upon the temperature t' of the 



season : 



t' 



IIO 



Calculated by means of this formula^ the mean height 

 of the point at which rain begins to be formed, is 1747 

 feet ; and the height at which the quantity of rain is 

 only one-half of that which falls on the ground, is 356 

 feet. 



Assuming the mean temperature of newly fallen rain at 

 York to be 48°; and taking the latent heat of vapour at 

 1210° at the temperature of 32° Fahi'enheit — the value 

 adopted by Professor Espy in his Meteorological Reports 

 and Essays, — it will be found that a rain-drop cannot 

 acquire the increase of size indicated by the observations, 

 by the condensation of vapour upon its surface, unless 

 its temperature, when at a height above the ground not 

 exceeding that of the top of the tower of York Minster, is 

 below — 434° Fahrenheit! From this result it is evident 

 that only a very small portion of the total augmentation 

 of a rain-drop can be due to the condensation of vapour 

 upon its surface, and that by far the greater portion must 

 be owing to the deposition of moisture which has already 

 lost its latent heat, or heat of elasticity, and which is, 

 therefore, not in the sate of a true vapour, although on 

 the other hand, its invisibility in the atmosphere under 

 ordinary circumstances, in the form of cloud or fog, 

 renders it difficult to suppose that it can be in the ordi- 

 nary liquid state. We have just seen that at a height of 

 356 feet, the quantity of rain is only one-half of that 

 which falls on the ground; and it is evident, therefore, 



SER III. VOL. I. 3 F 



