806 
ON THE DECKEASE OP EAIN 
fundamental one. Is the apparent increase of rain at a lower 
level as compared with a higher level a real increase, or is it 
only apparent? In other words, does the rain, as it approaches 
the earth, actually increase in quantity, or is the apparent 
increase due to some imperfection in our methods of measure- 
ment? Without attempting to answer this question at once, 
I will examine seriatim the most important theories which 
have been put forward, taking first those which proceed on the 
assumption that the increase is real. 
Of this class, the theory which has attracted most attention 
is that which was advocated by Prof. Phillips, whose observations 
at York have been already referred to. According to this view, 
the drops of rain in their descent being colder than the lower 
strata of air through which they fall, condense moisture upon 
their surface and so increase in size. This theory appears to 
have been first suggested by Benjamin Franklin, who, in a letter 
to Dr. Thomas Percivall dated 1771,^ compared a drop of rain 
to a bottle of cold water condensing dew upon itself when 
brought into a warm room. That rain, even in our hottest 
days, he adds, comes from a very cold- region, is obvious from its 
falling sometimes in the form of ice. The same view received 
the support of Arago in a paper published in the Annuaire 
dll Bureau des Longitudes for the year 18^4. Prof. Phillips, to 
whom this theory appears to have occurred independently, thus 
states it in his first report to the British Association on the rain- 
gauge experiments which had been entrusted to himself and Mr. 
Gray: — “It is therefore rather as a matter of very probable 
inference than a plausible speculation that I offer the hypothesis 
that the whole difference in the quantity of rain at different 
heights above the surface of the neighbouring ground is caused 
by the continual augmentation of each drop of rain from the 
I Memoirs of Thomas Percivall, M.D., Appendix B. 
