Mr Stevenson on the Defects of Fain-Gauges. 15 



drift, a gauge has been found to have acted as a centre of at- 

 traction, or the reverse, it may safely be concluded that there 

 must be something wrong either in its position or construc- 

 tion. 



II. Having now considered the effects of different situations 

 for gauges, the next error which may be noticed is only a pro- 

 bable one, and is supposed to arise, if it does exist at all, from 

 the suspended water being collected into separate drops of rain 

 instead of being uniformly spread over the surface, in conse- 

 quence of which it is possible that drops may fall without the 

 rim, instead of being split by it, and the receiver be thereby 

 robbed of that fraction of any one of these drops which be- 

 longs to the area of the gauge. On the whole, however, this 

 error may be considered a compensative one, and therefore 

 entirely unimportant, at least in gauges of 2 or 3 feet in dia- 

 meter. 



III. The next error is peculiar to gauges placed on a level 

 with the ground, and is occasioned by the current in passing 

 from its regular surface (where it meets with an uniform re- 

 sistance) to the hole occasioned by the mouth of the gauge. 

 This error is considered by some to be the great objection to 

 large areas. It is not indeed easy to estimate what may be 

 the effect of enlarging the area in this instance, but there 

 seems to be no reason why it should be greater proportionally 

 with the large than with the small area. Perhaps the best 

 way would be to submit this point to the sjiow-test^ as a means 

 of ascertaining whether a proportionally greater quantity enters 

 a large than a small area. 



IV. Evaporation of the rain, arising sometimes from a 

 few drops having rested upon the rim or funnel of the gauge, 

 and being there dissipated, is an error common to all gauges. 

 It is by no means a very serious one, but it may certainly be 

 augmented by increasing the size of the gauge, with the con- 

 sequent diminished slope of funnel which would be necessary 

 in order to reduce the cost of workmanship and general un- 

 wieldiness of the instrument. But it might perhaps be les- 

 sened by coating the funnel with coach varnish or with the 

 fine seed of the Lycopodium or club-fern, so well known for its 



