126 



The Rev. Samuel Haughton, M. D., Fellow of Trinity College, 

 Dublin, read the following Paper : — 



On the Evaporation of a Water Surface at St. Helena. 



I have already published in the " Proceedings " of the Academy the 

 results of one year's Observation on the Difference of Rain-fall and Evapo- 

 ration made at St. Helena by Lieutenant Haughton, R. A. ; in 1860-61. 

 Through the kindness of this officer, and of Major Phillipps, commanding 

 Royal Artillery at St. Helena, I am enabled to lay before the Academy 

 on the present occasion two years' Observation on Evaporation and Rain- 

 fall, made at the same place, and under better conditions. 



The former observations were made in a glass cylinder, 9 inches high, 

 and 4*85 inches wide, and gave only the excess of Evaporation above 

 Rain-fall. The present Observations were made in two cylinders, one 

 circumstanced like the last, and the other placed in a large tub of water, 

 so as to have the water inside the cylinder always surrounded by water 

 at nearly the same level, thus giving a better approximation to the true 

 evaporation from a water surface surrounded by water. 



At the same time, Major Phillipps recorded carefully the Rain-fall; 

 and from a comparison of both sets of Observations, I have obtained the 

 actual amount of Evaporation, corrected for Rain-fall, during the two 

 years. 



Part I. — Lieutenant Haughton' s Observations on the Difference 

 of Evaporation and Rain-fall. 



Observations on Evaporation resumed at 10 h 45 m a. m., on Sunday, 

 3rd August, 1862, with the old glass as before ; and another, about the 

 same size, immersed in water up to the zero point. 



A is the depth of the water evaporated in one week in the new glass 

 immersed in water. 



B, ditto, in the former glass. 



Inches. Inches. 







A. 



B. 



August 



10, 1862, . . 



. 1-00 



1-30 



n 



17, ... . 



. 150 



2-10 



'j 



24, . . . . 



. 1-00 



1-40 



ii 



31,. . 



, 1-20 



1-70 



Sept. 



7,. . . . 



. 1-50 



2-00 



>> 



14 



. 1-80 



2-40 





21, . . . . 



. 1-50 



2-00 



? ) 



28* . . . 



. 1-10 



1-60 







Inches. 



Inches. 







A. 



B. 



October 



5, . . 



. . 1-60 



2-10 



n 



12, . . 





210 



ii 



19, . . 



. .' 1-40 



1-80 



ii 



26, . . 



. . 1-60 



2-30 



NOVEMB. 



2, . . 



. . 1-60 



2-30 



ii 



9, . . 



. . 135 



2-00 



11 



16. 







ii 



23,f. . 



. . 2-10 



3-55 



11 



30, . . 



. . 1-15 



1-90 



* The level of the water is always below zero at the end of the week, 

 f For two weeks. 



