318 
ON THE DECEEASE OE EAIN 
of its rim and funnel. To account for the remarkable decrease 
in the rate of decrease from the lowest strata upwards, we have 
only to suppose that a corresponding increase in the rate of 
increase applies to the velocity of the wind from the lowest 
strata upwards ; and this, although it has not been experi- 
mentally proved, will not appear unreasonable when we consider 
that it is by contact with the earth and the objects upon it that 
the velocity of the wind is retarded. For the same reason it 
appears likely, if not inevitable, that the differences in the 
velocity of the wind due to elevation will be greater when the 
wind is strong than when the wind is light, and this explains 
why the deficiency of rain in elevated gauges is greatest in 
windy weather. Lastly, it has been observed that in heavy 
rains the differences are somewhat less than in light rains. 
This is just what we should expect if we remember that the 
momentum of a heavy drop will enable it better to resist the 
force which tends to divert it from its path. In this way I 
explain Mr. Stow’s result, already quoted, that the inclination of 
the rain exhibits a closer correspondence with the elevation- 
difference than does the force of the wind. 
Some practical inferences, as regards the measurement of 
rain, follow from what has been stated. 
1. — The height usually adopted for the receiving surface of 
a rain-gauge is one foot above the soil, and I will not say that 
this height may not be on the whole the best, but if the views I 
have advanced are sound, the indications of a gauge at this 
level cannot be free from error. The disturbing ' ifluence of 
the wind is greatest, as we have seen, at the higher levels, but 
it continues all the way down to the ground, a gauge at one 
foot having been found to collect about 5 per cent, less than one 
whose receiving surface was level with the soil. 
9. — In the construction of rain-gauges it should be an object 
to adopt the form least favourable to the blowing out of rain by 
