THE MEASUREMENT OF BAIN. 



9 



than half of the diameter of the gauge. This supposes that 

 the rain will not often reach the ground at a more acute 

 angle than 45°. If the rain descend at that angle, and the 

 rod stand at a height equal to half the diameter of the gauge, 

 any rain which would be intercepted by the rod would have 

 fallen, had the rod not been there, within the area of the top 

 of the gauge, as will be seen by the sketch in the margin. 

 vs ^•■*\'^v^XV\v;> V If the rain come straight down, or 



v^'^^N^^^^S^^Vv^X^^ \ nearly so, as it often does in heavy 





<i;v\^ rain in summer, the rod might stand 

 ^N^'^^^x at a much greater height without 

 Imj!^^ producing any effect. If the rain 

 ^^^^'^' be driven by strong wind nearly ho- 

 rizontally, which would be the case 

 in very exposed situations, then of 

 course a slight elevation of the rod 



will intercept rain which would 

 otherwise pass entirely over the 

 gauge. It may, however, be fairly as- 

 sumed, that in the foregoing observations from the Society's 

 gauges, which are about 7 inches in diameter, the results 

 may be taken as perfectly accurate till the rod rises more 

 than 3^ inches above the top. In taking the observations 

 the gauges have been regularly emptied at the end of every 

 month; and there would only be some amount of error, there- 

 fore, in those months in which the rain exceeded 3^ inches, 

 the amount of error being probably proportionate to the 

 greater depth of rain. 



To remedy this objection for the future, instructions 

 have been given either to tie down the rods, or to detach 

 them from the float, merely using them at the time an 

 observation is taken. The latter method is the best, as the 

 float rises on the surface of the water collected, thereby pre- 

 venting evaporation ; while by tying down the rod, the water 

 would rise above the float, and be subject, to some slight ex-; 



c 



