WITH ELEVATION. 
31S 
been repeated more recently by Mr. Dines at Hersbam, Surrey;^ 
and the conclusion arrived at by both these observers is that 
the decrease of rain at the top of the tower as compared with the 
ground is always most marked in the windward gauge and least 
marked in the leeward gauge, the latter sometimes showing no 
decrease at all. This is no doubt just such a result as Mr. 
Jevons’s diagram would lead us to expect. Nevertheless I doubt 
if the explanation given by him is, to any material extent, a true 
explanation. 
In discussing Mr. Jevons’s theory, a question suggests itself 
at the outset, which, although it may admit of an answer, it 
may clear the ground to state. Is it actually in accordance with 
our experience that obstacles to the passage of the wind increase 
the velocity of the wind ? Is not our experience the very 
reverse of this ? The windiness of elevated situations, as the- 
tops of hills, is surely not due so much to the obstruction 
occasioned by the hill, as to the comparative freedom from 
obstruction which is proper to stations raised above the general 
level of terrestrial objects. The increased force of the wind at 
sea or on extensive plains seems to demand a similar explanation, 
I believe there can be no doubt that the effect of obstacles to the 
passage of the wind is, on the whole, to retard the velocity of the 
wdnd. And yet I do not urge this as an objection to Mr. 
Jevons’s theory. 
Let us imagine two tubes, having their mouths exposed to the 
same current of air. One tube shall be free throughout, the 
other shall have its calibre constricted by a diaphragm at one 
part to half the area elsewhere. I suppose there can be no 
doubt that the wind will pass more slowly through the con- 
stricted tube than through the other ; yet it will be equally true 
^ British Bainfall^ 1877 ^ p. 15. Meteorological Maga^ine^ 
August f 1878. 
