1820.] Mr. Boase on Rain-Gauges. 179 



its oblique descent, it appears to me that the phenomenon of 

 different quantities of water being received by similar rain- 

 guages placed at different heights may be easily explained. 



For let A B, fig. 2, represent the given column of rain, and 

 let the assumed horizontal velocity of the wind be double the 

 mean of the downward velocity, then the faUingrain will describe 

 the parabola A C. Suppose the basin, a, of the pluviameter 

 divided into 10 parts, it is evident that the like basin, b, where 

 the rain falls at an angle of 60", will receive only 8'66 of those 

 parts. Let another gauge be placed at c, where the angle of 

 inclination is nearly 45°, or at d, 30°, or at e, 23^° nearly, and 

 the quantities received will be respectively -Y^ths, -V^s, and 

 .jig^ths, or parts of the basin a. Hence then it follows that the 

 proportion of water received by rain-gauges placed at different 

 heights, cteteris paribus, is as the sine of the angle of inclination 

 of the falling rain. Should hills, trees, or buildings, disturb the 

 horizontal force, the angle of inclination will be varied, but the 

 ratio remains the same. Our rain-guages indicate very accu- 

 rately the quantity of rain received by the surrounding surface, 

 provided that surface is level, on the same plane and height as 

 the guage, otherwise they must differ. Thus let df be a mount, 

 having a tabular top, exactly equal to the assumed column of 

 rain ; it is clear that when the rain falls at an angle of 30° as at 

 d, the top receives only five parts or one half, while the other 

 falls on the side^upon half the area of the top; consequently a 

 guage placed on the side of the mount would indicate only half 

 the quantity of water actually received by the surrounding sur- 

 face. It is, perhaps, owing to this circumstance, that the 

 different sides of the same hills are so unequal in fertility and the 

 growth of trees, particularly on the sea coast, where high winds 

 are more frequent, and the rain generally comes from one quar- 

 ter of the compass. I am, Sir, 



Your most obedient servant, 



H. BoASE. 



P. S. Permit me to offer your readers a more simple mode of 

 reducing the height of the barometer to the temperature of 32°. 

 Multiply the difference of the thermometer above 32° by 3, and 

 subtract the product from the second and third decimal figures 

 of the observed height of the barometer. 



Example : Barometer SO'OOO 



Thermometer, 62° ; difference 30 x 3 = 00-090 



29-910 



This indeed does not exactly agree with your method, except 

 when the barometer is at 30 inches, but the difference is so 

 minute, being only ToVotli of an inch at 29 or 31, as not to be 

 worth notice in ordinary observations. 



m2 



