WITH ELEVATION. 317 
(as compared with 1 foot) was 94*5, while on 10 ** rough*’ days 
the mean percentage was only 69*8. 
With these facts before us, it is impossible to doubt that the 
main cause of the decrease of rain with elevation is to be found 
in the force of the wind. Indeed, it may be said that upon this 
point meteorologists are now agreed ; the differences of opinion 
yet remaining relate to the mode in which the force of the wind 
operates to bring about the result observed. “ Eddies of wind” 
in and about the rain-gauge have been frequently spoken of as 
furnishing an explanation of the phenomenon, but if by an 
“ eddy ” is meant (as I presume is meant) a circular or rotatory 
motion, such as occurs in a whirlwind, the explanation seems to 
me imperfect ; for I believe that the effect of an eddy (in this 
sense) would be not to diminish but to increase the quantity of 
rain falling in the vortex of the eddy, just as we see little 
heaps of dust collected in the centre of the tiny whirlwinds 
which are not uncommon in our streets and roads, or as, on a 
larger scale, we see in the deluge of a waterspout. But if we 
suppose that the wind, instead of eddying in circles over and 
within the gauge, simply rehounds, then I think we have an 
explanation which meets all the conditions of the problem. 
That the wand does rebound from obstacles w^hich it meets is a 
familiar fact. That the force of this rebound will be pro- 
portional to the force of the wind is obvious without demon- 
stration. And that the effect of the rebound will be to impede 
the entr}/ of rain into the gauge, seems to me equally apparent. 
I am not aware that there is any observed fact in connection 
with this question which the theory now suggested may not be 
made to cover. In its simplest application it explains the 
progressive deficiency of rain in elevated gauges by the pro- 
gressive increase in the force of the wind in elevated situations, 
and the larger proportion of rain which will thus be blown out 
of the instrument by the rebound of the wind from the interior 
