458 



Mr. Downing on the Drainage of Haarlem Lake. 



visit England to examine into the working of the far-famed Cornish piimp- 

 ing-engines. As to the amount oi power to be applied to laying dry the Lake, 



an interesting practical problem presents itself. We must, in the first in- 

 stance, set up a power to lay dry the polder exactly equal to that required 

 ever after to maintain it in that state. If we apply to the exhausting of the 

 water a greater power than that required to keep it dry afterwards, we shall 

 be burthened with the interest upon a too great first cost, and subsequent 

 maintenance, having thus purchased too dearly the earlier laying dry of the 

 polder by an expenditure not commensurate with the gain in time. At first 

 sight it would appear impossible to fulfil the conditions of this problem, and so 

 indeed it would be, if the power to be employed in permanently Iteeping dry 

 the land on the bed of the Lake were required to act continuously, as it must 

 do, in the first instance, in laying it dry. The power required in the engines 

 must then be obviously equal to the maximum power required to meet the rain- 



