S24' On Rain-Gauges. 



at the lower one ; because the eddy winds alter the oblique 

 course of the direct rain and wind to a more perpendicular 

 descent, the more so as they approach the receiving surface of 

 the latter. These eddy winds are formed by the springing 

 back of the direct current from the sides of the houses, &c. 

 upon which they forcibly strike ; and the more violent they are, 

 the greater of course must be the effect of the eddies. Hence 

 a gauge near the ground surrounded by walls or houses, though 

 its situation be tolerably open, will receive the rain more fi-eely, 

 and in a greater ratio, than one at the top of a building 20, 30, 

 or 40 feet high ; because the springing back or recoiling of the 

 current alters the deflected course of the rain to a more ver- 

 tical fall into the I'ecipient part of the gauge ; a circumstance 

 that may in some measure be traced in the drops of rain on 

 coming within a few yards of the ground. 



In the winter months, when the sun's rays have but little 

 power to raise the vapour plane, the clouds are generally very 

 low : so that we are sometimes enveloped by thick fogs and 

 mists ; and a change in their temperature and electrical state 

 will cause a sudden condensation, and con veil them into rain 

 in a more rapid manner than appears to be effected in an op- 

 posite season : hence it happens that rain and wind are more 

 frequent in the winter than in the summer months, and that the 

 redundant quantity in the lower gauge is projiortionably less in 

 the latter season. It may be necessary to observe, in confirma- 

 tion of the greater density of atmosphere near the earth's sur- 

 face, that when it was inclined to condense by meeting with 

 supervening cold currents, I have, after a dewy night, often 

 measured from the rain-gauge ^^^ of an inch in depth ; the 

 residuum probably amounted to yi^ more. So great a quantity 

 of dew does not appear to be collected in a rain-gauge placed 

 at a considerable height from the ground : for in a fine clear 

 evening soon after sunset, we sometimes see a limit to the height 

 of the dew, by its purple or lilac tinge. 



We also know by barometrical experiments that the atmo- 

 sphere near the ground is thicker and much heavier than that 

 above it ; therefore, the nearer the gauge is to the ground, the 

 greater will be the quantity of rain received in it ; arising 

 chiefly from the enlargement of the di'ops and the eddy winds 

 below. It is probable too, that a rain-gauge placed 200 or 300 

 feet from the ground may be higher than the base of the rain- 

 fraught cloud ; in that case the comjiarative quantity of rain 

 collected in it must be further diminished. 



I am not aware that the manner in which the influence of 

 the wind upon the rain takes place, has been attempted to be 

 explained in any plausible way. 



It 



