110 Mr. Chilton on an improved Rain Gauge. [Aug. 



the quantity of rain that falls in a single shower, at any other 

 season. But to answer all purposes, it must be provided with a 

 cover, in the centre of which is inserted a funnel, whose top has 

 the same area as that of the top or bottom, of the prismatic 

 vessel above. To prevent evaporation, the orifice of the funnel 

 is furnished with a valve against which a weak spring, attached 

 to the inside of the cover, presses with a force just sufficient to 

 close it, but which is overcome by the weight of a few drops of 

 rain. It is evident that in a shower the water will open the 

 valve, and after it has passed into the body of the gauge, the 

 valve will close the orifice again, suffering, however, the drain- 

 ings of the funnel to pass along the pendant wire by cohesive 

 attraction. 



This top, with its funnel and appendages, may be fitted on the 

 body of the gauge, like the lid of a common tea-canister. 



The water being thus introduced into the gauge, the method 

 of determining its altitude in inches and decimal parts depends 

 upon the following fundamental statements, in connexion with 

 the simple operation of weighing the water in the gauge. 



Fundamental Principle. 



A cubic inch of distilled or rain water, under a medium pres- 

 sure and temperature, weighs 252*525 grains, according to the 

 latest corrections. Now this number, multiplied by 100, the 

 area of the funnel, in square inches, or that of the top or bottom 

 of the body part of the gauge, gives 25252*5 grains for the 

 weight of 100 cubic inches of water. Supposing this quantity of 

 water in the gauge, it would evidently form a stratum on the 

 bottom of one inch in height ; and if we conceive this stratum to 

 be divided by horizontal sections into 100 equal parts, these 

 parts would form strata, each of which would be the -pio-th of an 

 inch in height ■ and, being equal to a cubic inch, would weigh 

 252'525 grains. Let us further suppose that one of these strata 

 is subdivided into 10 equal parts by sections in the same direc- 

 tion, each of these parts would evidently form a stratum of water, 

 whose height would be only the T .jJ__th part of an inch ; and 

 being equal to the 10th part of a cubic inch, would weigh 

 25*2525 grains. 



Having then the weight of 100 cubic inches corresponding to 

 one inch in altitude ; the weight of one cubic inch to the T ^„th 

 of an inch ; and the -^th of a cubic inch to the T _'- Tr th part of an 

 inch ; it is easy to see that the height of the water in the gauge 

 may be obtained by making one or other of the above numbers a 

 divisor to the corrected weight of the water, in troy grains. But 

 this trouble is rendered unnecessary by the use of the following 

 tables : — 



