186 Dr Fleming on a Simple Form of Bain-Gauge. 



The position of the gauge in a well-trimmed grass plot, at 

 a distance from houses, walls, or trees, seems to me to ob- 

 viate all risk of any extra water getting in, as the points of 

 the grass effectually prevent any such occurrence. Those ob- 

 servers, however, who are particularly fastidious on the sub- 

 ject, may adopt a brush to be placed around the margin of 

 the gauge, as recommended by Mr Thomas Stevenson, civil 

 engineer, in this Journal for July 1842. 



The size of the gauge, or the area requisite for receiving a 

 fair amount of the falling rain, can scarcely be said to have 

 attracted sufficient notice. If we assume that the accele- 

 ration of the current would take place, by passing from a 

 grassy surface over the open space of the mouth of the 

 gauge, and a corresponding derangement of the motion of 

 the rain-drop, then it must follow, that the larger the area, 

 the greater will be the amount of error from this source. In 

 order to put this to the test, I placed in the same grass-plot 

 with a Thom's gauge of seven inches in diameter, three other 

 gauges of one, two, and three inches in diameter, and ob- 

 tained the results for three months, viz. : — 



1842. 

 Sept. 

 No. 1. Diam. = 1 inch, 3-689 



2. ... =2 ... 3-551 



3. ... =3 ... 3-921 



4. ... =7 ... 3-7o0 



The grass-plot was not free from eddies, arising from 

 buildings, trees, and walls ; but as all the gauges were 

 nearly similarly exposed, and as they gave nearly similar 

 results, I am inclined to think, that a receiver of one inch 

 in diameter is as trustworthy as one of seven inches. When 

 the instrument is to be made of copper (the most suitable 

 ingredient), the small size reduces the expense. I have 

 noticed the index-iod as divided into inches and tenths. 

 The eye, after a short practice, has little difficulty in halving 

 or quartering the tenths ; and this is a degree of accuracy 

 as great as the circumstances of the case warrant us in 

 aiming at. The inequality in the fall of rain, at two places 

 within less than a mile of each other, forbid us to expect any 

 very accurate correspondence among gauges, even at mo- 



