APPENDIX. Xiii 



No. IV. 



DESCRIPTION OF A PLUVIAMETER AND AN EVAPOROMETER CON- 

 STRUCTED AT BENARES. 



By James Prinsep, Esq. 1st Jan. 1823. 



The Pluviameter with which I register the fall of rain at Benares, is made on the 

 Syhon principle, the quantity is read oft* by simple inspection to any required ac- 

 curacy ; the loss by evaporation is almost prevented ; and the instrument does not 

 require to be handled in emptying it. Figure 3 represents the one now in use. It 

 is composed of a bason at top of sufiicient area, screwing on to the brass cap of a 

 graduated glass tube, into which it opens by a small neck with a loose valve of talc 

 to keep out dust, &c. 



The tube contains a small glass syphon whose longer leg passes through the bot- 

 tom of the tube, water-tight, and enters the lid of a copper reservoir below. 



Each degree of the scale on the tube may correspond to .01 inch depth of rain on 

 the area of the bason : (in mine each = .005 inch.) When 100 degrees are accumulat- 

 ed, or the tube is filled, the whole is drawn off by the syphon into the lower cylinder, 

 where it marks unit upon a similar syphon connected exteriorly with the latter. 

 This likewise empties itself when it has once become full. 



Tin cases are applied for the protection of the glass tubes. 



Fig. 2 was the original form of the instrument, and shews what may be done with 

 broken bottles and retort necks and tubes, where brass workers are not to be met 

 with. 



Fig. 1 is an Evaporometer invented with the same view of reading off the depth of 

 Evaporation by inspection, and magnified. The ratio of the area of the bason, and 

 length of the tube may be such that each degree on the latter may mark .01 or .001 

 inch depth of Evaporation from the former. 



The instrument is charged by drawing the piston down to zero, and filling the 

 tube with distilled water to a mark in the narrow neck at top. The water is then 

 passed upwards into the cup, and the diminution ascertained by drawing down the 

 piston until the neck-level is adjusted ; and readmg off the degrees marked by tho 

 piston's upper edge. 



The lower cork serves to insure the prevention of leakage. 



D 



