1844.] 87 



VII. — Observations on Evaporation made at the Red-hills 

 near Madras in 1844, by Lieut. Ludloiv, Engineers. 



This series of observations was undertaken with the in- 

 tention of ascertaining with as much exactness as possible, 

 what the actual evaporation from the surface of a large sheet of 

 water is, and what proportion it bears to the evaporation shewn 

 by an evaporator exposed on the land in its neighbourhood. 



The evaporator employed was a flat brass vessel exposing 

 a surface of exactly 1 square foot, and containing when filled 

 to within an inch of the edge 1 gallon of water. It was 

 painted white inside and was enclosed up to the level of 

 the edge with a double wooden case, in order to prevent the 

 brass from becoming heated. 



A -gallon of water was poured into the evaporator early in 

 the morning, and the following morning what remained un- 

 evaporated was drawn off by means of a syphon into the 

 gallon measure. The measure was then carefully filled up 

 from a graduated glass tube, and the quantity evaporated thus 

 measured with great accuracy. 



Two evaporators of this description were used, the registers 

 of which when exposed together under exactly similar circum- 

 stances, had been tested and found to agree well. One was 

 then placed in the middle of the Red-hill tank, (which has a. 

 surface of several square miles,) being supported on a post 

 firmly fixed, and the other was exposed on the land at a dis- 

 tance of about Jths of a mile from the tank. 



From a comparison of the registers of these two evaporators 

 the following results have been obtained. 



During April the evaporation on the tank to that on the 



land was as 1 : 1*3897 



May ---------- 1 : 12744 



June 1 : 1*2396 



July - - - 1 : M881 



August - - 1 : 11736 



Mean of the 5 months. 1 : 1*2531 



