312 
ON THE DECREASE OF RAIN 
itself, must act as an obstacle to the wind, continues A 
stream of air, then, meeting an obstacle, leaps over it ; in so 
doing it is forced against the adjoining parallel stream of air, 
which must also diverge from the straight direction, and similarly 
impinge upon the next stream. But the increased pressure 
produced by the impact causes the streams of air to move more 
rapidly, and to diminish in thickness at the same time ; and the 
disturbance of the streams of air will cease at the point where 
the total decrease of size of the streams is equal to the height of 
the obstacle. It is at least obvious that when a uniform wind 
meets an obstacle, some parts of the air must move more rapidly, 
just as a river moves most rapidly in the narrowest parts of its 
channel. It is is quite in accordance, too, with our common 
experience, that an obstacle increases the velocity and force of 
the wind ; thus the wind is always most fierce at the corner of a 
house, the end of a wall, or the summit of a hill.” 
Mr. Jevons then refers to a diagram which is copied in fig. 9. 
In this diagram a stream of air AB is supposed to be sud- 
denly contracted at CD to half its previous thickness, moving in 
consequence with double velocity. At EF the stream dilates to 
its original size, and recovers its first velocity. 
Mr. Jevons adds — “ It is, I venture to hope, rendered quite 
plain that less rain ivill fall upon the summit of the obstacle than 
elsewhere, the surplus being carried forward to the lee side of the 
obstacle. I entertain no doubt that we have in this process 
a sufficient explanation of the observed deficiency of rain in 
elevated places.” 
Experiments made with gauges placed at the four corners of 
a tower are held by Mr. Symons to lend a powerful support to 
Mr. Jevons’s views. Such experiments were first made by 
Prof. Bache at Philadelphia about the year 1834;^ they have 
I British Association Beport, 1838^ Part II., p. 25. Also quoted 
in British Bainfall, 1878, p. 25. 
