WITH ELEVATION, . 
315 
phenomenon to be explained. It is not enough to prove that the 
obstacle accelerates the wind locally, and that this local acceleration 
of the wind separates the drops of rain. It must be shown that 
this local acceleration of the wind is an operating cause powerful 
in proportion to the proper velocity of the wind which undergoes 
this local acceleration. If this can be shown — if it can be 
shown that the stronger the wind, the greater will be the effect 
produced by the local acceleration of the wind, then we have only 
to assume (what, indeed, we know quite well) that elevated gauges 
are more exposed to wind than low ones, and the theory, is so far 
complete. 13 ut this is just the link that is missing. Mr. Jevons 
has made no attempt to show that the operation of the cause he 
assumes would be more pow'erful in a strong wind than in a light 
wind, or that it would be more powerful with an upper gauge 
than with a lower gauge. He appears, indeed, to have over- 
looked the necessity of showing this. I have endeavoured, by 
means of diagrams and measurements, to ascertain what would 
actually happen with winds of different velocities undergoing 
local acceleration in accordance with Mr. Jevons’s theory, and I 
find (if I am not mistaken) this curious result, that as regards 
the separation of the rain-drops in horizontal distance, the 
effect, instead of becoming greater, becomes less and less marked 
as the velocity of the wind is increased. 
But it is necessary now that I should give a more distinct 
expression to the law to which I have already more than once 
adverted, namely, that the deficiency of rain in elevated gauges 
bears a close relation to the velocity of the wind. This relation 
has been long recognised, and is admitted by all. Passing over 
the frequent references made to it by the earlier observers, I 
would call attention to the following table, compiled from 
statistics given by Mr. Chrimes of Eotherham in British 
Rainfall, 1869, p. 19. The figjires: represent the means of (in 
most cases) four years : — 
