296 [Dec, 1845. 



ino- to the several periods of observation, in order to establish the 

 ratio above referred to, between the land surface and the water 

 surface at each of those periods. 



With these data, and a simple formula of easy application, it 

 will be found practicable, not only to establish the ratio of supply 

 from the surrounding slopes, to the whole quantity of rain falling 

 during a season, but also the ratio which evaporation on land, 

 bears to that on the water surface. 



The quantity, or bulk of water, removed from any source, such 

 as a lake or pond, during any given period of observation, will 

 consist of first, the draught, or flow from the outlet, and second, of 

 that removed by evaporation. 



This quantity, determined by actual guaging, is readily referred 

 to the known area of water-surface, simultaneously observed — and 

 the depth to which it would cover the same surface if again res- 

 tored, is thereby ascertained. 



The supply of this quantity may be derived from 1st, the store 

 previously accumulated in the pond or lake ; 2d, from rain which 

 falls directly on the ivater- surface during the period of observa- 

 tion ; or 3d, from the drainage, or flow from the surrounding 

 slopes. 



When the level of the lake falls during the period of observa- 

 tion, it is evident that something has been taken from the accu- 

 mulated store, and if the amount of depression of level, and the 

 depth of rain guaged, be not together equal to the sum of 

 draught and evaporation observed, it is evident that the third 

 source of supply — viz : the drainage from the water-shed, must 

 have furnished the balance. If, on the other hand, the level of 

 the source have risen, something has been added to the store of 

 water previously accumulated ; and in this case the depth of rain 

 and the supply from the water-shed, have exceeded the sum of 

 draught and evaporation, and the measures of level and of area 

 show the amount of this excess. In either case the supply de- 

 rived from the surrounding slopes is easily eliminated, since all 

 the other terms of the equation are known. 



From the 7th of August, to the 1st of November, of the pre- 

 sent year, — the time in which the observations, surveys and gaug- 

 ings, required for the purposes above designated, were made in the 

 district of Spot Pond, 9 miles north west from Boston, — the depth 



