Observations on Rainfall. 99 



with the horizontal line as we ascend, a greater surface in the 

 globular gauge is presented to the rain which is driven up the 

 inclined slopes into the gauge, and so an apparent increase 

 with elevation was the result. 



In Mr. Symons' " British Rainfall," for 1867, the Rev. T. E. 

 Crallan pointed out that in the path described by the falling 

 raindrops the drops approached each other as they neared the 

 earth. An article upon the same subject in " British Rain- 

 fall," for 1870, by the Rev. F. W. Stow, also shows that the 

 diminution above the ground bears a close relation to the 

 velocity of the wind. 



The mouth of a rain gauge being placed horizontally, it is 

 clear that in the case of high elevations the rainfall is driven 

 at a small angle to the mouth of the gauge, that it does not 

 present an equivalent area of surface to the reception of the 

 rain as a rain gauge placed at a lower level, in which the rain 

 enters at a large angle to the surface. The observations which 

 have been made clearly show that in times of high wind the 

 diminution is greater with elevation, but with heavy rains in 

 periods of calm weather there is liitle or no difference in the 

 rain collected at various elevations. The observations that 

 have been published by the British Association Committee in 

 the case of York, show that the diminution with altitude was 

 in proportion to three times the square root of the height ; this 

 property of the square root of the height representing the 

 velocities at which the rain-drops would fall according to the 

 laws of gravitation. It is also well to bear in mind that the 

 movements of air over the surface of the earth follow the same 

 natural law as movements of water or other fluids ; the velocity 

 is smallest owing to friction on the surface of the earth and 

 increases as we ascend. A drop of rain of sufficient weight so 

 as not to be materially interfered with by the air would fall 

 like other bodies, and would fall a distance in the first second 

 represented by the figure 1, in the second second by the figure 

 3, in the third second by the figure 5, and so on, the odd 

 numbers representing the proportionate amount of space 

 travelled in every second of time. The horizontal force of the 

 wind diminishing as we near the earth, it is quite clear that a 

 falling drop of rain, when there is any wind, becomes more perpen- 

 dicular as it strikes the earth than at any point above its surface. 

 The explanation, therefore, of the apparent diminution of the 

 amount of rain collected is due to the diminished area the 

 gauge presents to rain when driven at a varying angle. 



During the past three years at the author's residence a 

 rain-gauge 4 feet above the surface of the ground has collected 

 6o"30 inches of rain, while one placed i foot above the surface 

 of the ground has collected 61-32 inches. 



