16 Mr Stevenson on the Defects of Bain-Gauges. 



singular property of repelling water, in that respect resembling 

 the bloom of fresh cabbage leaves, on which the drops of rain 

 or dew being prevented from spreading, are always found to 

 preserve their spherical form. 



V. The next error, like one formerly noticed, is peculiar to 

 gauges placed on a level with the ground, and is caused by the 

 resilience or dispersion from the surrounding ground of the 

 rain and hail, which are thus made to fall into the receiver. 

 Should there be grass growing close to the edge of the gauge, 

 as is generally the case, there is the farther risk of drops lodg- 

 ing among the blades of grass, and being afterwai'ds blown 

 into it. 



Now a very few drops being blown into a small gauge would 

 more materially affect the result than if the same number were 

 blown into a larger gauge. By enlarging the orifice this error 

 would increase simply as the diameter, while the quantity of 

 rain that should be collected would be increased as the square 

 of the diameter, and thus the great advantage of large over 

 small areas is in this instance rendered evident. 



VI. Resilience and dispersion, or the rebounding of the 

 drops of rain, hail, &c. off or out of the gauge, arising from 

 their impact against the rim or oblique sides of the funnel, 

 is a serious evil in all forms of the instrument, and the loss 

 occasioned in this way is, I suspect, greater than is generally 

 believed. By enlarging the orifice the error would be dimi- 

 nished in the same high proportion as the last. 



VII. The eddy occasioned by the rim catching the wind 

 produces an error somewhat like the last in its extent, and it 

 may be diminished in the same proportion by simply increas- 

 ing the area. Sir John Leslie, in his article on Meteor- 

 ology, in the Encyclopaedia Britannica, has stated that there 

 would be an advantage in having large areas, and describes 

 the rim-eddy in the following words : — " We may suspect 

 that the measure of the rain, hail, or snowy flakes receiv- 

 ed by the ombrometer is not exactly proportioned to the 

 extent of surface which it presents ; for while torrents pour 

 down from the heavens, an eddy plays about the rim of the 

 ba.sin, deranging the regularity of the discharge,'' 



