1824.] Mr. Chillon on an improved Rain Gauge. 109 



families Limacidcc and Helicida ; but as a proof that such ana- 

 logy does exist, I need only observe that Ferrussac has named 

 two of the genera of the latter family Helkarion and Helicolimax, 

 on account of their similarity to the genera Avion and Umax of 

 the former family. 



Article VII. 



Dcscviption of an improved Rain Gauge. By Mr. George Chil- 

 ton, Lecturer on Chemistry.* 



The quantity ofjain that falls in any particular district being 

 an important item in Meteorology, any improvement in the 

 instruments of observation by which that quantity can be deter- 

 mined correctly must be acceptable to the cultivators of that 

 department of science. In the common construction of the rain 

 gauge several causes of error are manifest, which when taken 

 separately, might be deemed trivial, but whose combined effect 

 is such as every accurate observer must be desirous of avoiding. 

 It is well known that fluids undergo changes in bulk by changes 

 of temperature, as well as by those of barometrical pressure ; 

 and that any mode of measuring the dimensions of a fluid, ex- 

 posed to the influence of these fluctuating causes, provided it 

 does not make due allowance for them, must be erroneous. 



In addition to these causes of irregularity, the cohesion of the 

 fluid, which is necessarily connected with the measurement by 

 graduated rods, renders it impossible to determine the true 

 height of it. 



But besides these obvious causes of inaccuracy, the fluid in 

 the common construction of the rain-gauge is too much exposed 

 to spontaneous evaporation. This might, in part, be remedied 

 by narrowing the neck of the funnel, but here another difficulty 

 arises : if the aperture, by which the water enters the gauge be 

 too small, the funnel, in a smart shower, might be filled to over- 

 flowing ; by which a part of the water would be lost. 



The following is a description of a rain gauge constructed on 

 principles, by the help of which, the quantity of rain that falls 

 into it can be accurately determined in inches of altitude with- 

 out being affected by the causes of error alluded to above. 



An essential part of the rain gauge is a prismatic vessel, figs. 1 

 and 2 (see p. 1 13), whose top and bottom are, each 10 inches 

 square, inside measure, with any convenient height. 



This is all that is necessary for occasional experiments, as for 

 instance, to determine the quantity of rain, snow or sleet, that 

 may fall in winter when the evaporation is inconsiderable ; or 



* American Journal of Science. 



