Mr Stevenson on the Defects of Bain-Ganges. 17 



This rim-eddy has been generally, and with justice, con- 

 sidered as a most formidable source of inaccuracy. 



VIII. The last errors which may be noticed are due to the 

 imperfections of form in the mouth of the gauge, errors in the 

 graduation of the scale, or in other parts of the instrument, 

 all of which will clearly be diminished by an increase in the 

 area. 



From the above considerations it appears that, by increasing 

 the area, the tendency to evaporation, by no means a very 

 serious evil, is augmented, and i}erhaps also the error caused 

 by the passage of the wind from the uniform surface of the 

 ground to the aperture of the gauge. On the other hand, by 

 increasing the area, there is a diminution in the ratio of the 

 squares of the diameters, of the errors arising from the re- 

 bounding and dispersing from the ground into the gauge, from 

 the rebounding and dispersing from the rim and funnel out 

 ^j/the gauge, from the eddy caused by the rim and funnel ; 

 and finally, although not in the same high proportion, from 

 imperfection in form of the receiving mouth or other parts of 

 the instrument. 



It is proper to mention, that in some experiments it has 

 been found that the larger gauge gave a less quantity of rain 

 than the smaller ; from which circumstance many suppose that 

 the small gauge is more correct than the large ; but I think it 

 only appears that in such cases the errors connected with the 

 small gauge are in excess. It would not be proper at once 

 to conclude of two gauges, that the one which gives the greater 

 quantity of rain is the more correct, for it is quite possible 

 that while one gives the correct, another may give too high a 

 result ; and as we do not possess the means of ascertaining 

 which of the two instruments is the correct one, we must, in 

 all cases, give the preference to the one whose /oz-w and con- 

 struction seem best fitted for the object to he attained. 



Having endeavoured to shew the superiority of large gauges 

 over small, I shall now describe the form of instrument which 

 appears to me to be best suited for the purpose. What I con- 

 ceive to be its peculiarity is the smallness or rather the want 

 of a rim^ and the advantageous position of the funnel. 



VOL. XXXm. NO. LXY. — JULY 1842. B 



