Mr. Downing on the Drainage of Haarlem Lake. 459 



fall in some definite period, as, suppose, one month, together with any infiltration 

 from the external basin. 



Observations on the rain-fall of Holland, extending over a period of 

 ninety-eight years, had shown that the greatest depth of rain in any one 

 month was 6 -524 inches more than the evaporation for the same period. If 

 we allow 1-47 inches for infiltration, we have a total depth of 8 inches to be 

 thrown off the land in one month, and this has to be raised from the level of 

 the future drains in the polder to that of the water in the Ringvart, a height of 

 about 16 feet. A simple calculation gives 1087 horse-power as necessary to 

 effect this in twenty-five working days per month. It was, therefore, resolved 

 to put up three engines, of 350 horse-power each, and these were located in the 

 positions shown on the map. From the power thus required to keep dry the 

 future polder, we may, proceeding in a retrograde order, readily deduce the 

 time in which the first operation of laying dry the lake may be effected by that 

 force which we have shown to be necessary to its permanent maintenance, 

 namely, 1087 liorse-power. The Lake at 13 feet of depth contained about 

 800,000,000 tons of water, which had to be raised an average of about 6^ feet 

 of vertical height. "With no delay from accidents, or frost, or from the point 

 of stopping being reached, this might be effected in about fourteen months. 

 From avariety of imforeseen causes it required thirty-nine months; and the weight 

 of water actually raised was, as indicated by the counters attached to the engines, 

 upwards of 1,100,000,000 tons. The annexed woodcut, p. 460, shows the rate of 

 progress in each successive month, and by joining the lowest points in each suc- 

 cessive year we may perceive how, in each winter season, the rain-fall, with 

 the absence of evaporation, gained upon the power of the engines ; the general 

 average lowering of the surface was at the rate of about 4 inches per month ; 

 every inch in depth upon the surface weighing upwards of 4,000,000 tons, we 

 may thus form an opinion how prejudicial and expensive such risings must have 

 been, more especially in the latter months, when it had to be raised nearly the 

 full depth of the Lake. 



The estimated expenditure at the commencement of operations was 

 £687,500 ; the result, up to the last published account, which includes the full 

 completion of all the works, was an actual expenditure of £827,200, being an 

 excess of £139,700. 



