454 Mr. Downing on the Drainage of Haarlem Lake. 



servations in this country. Upon the scale I have described, there is a point 

 marked, called the point d'arret, or point of stopping, which is the same height 

 for all the polders in each administration, but not the same for every different 

 administration ; and when the interior waters of the basin have reached this 

 point, then all the windmills and steam-engines raising water from the polders 

 must cease working, although the consequence of this, if the stoppage continue 

 long, must be, that they become, to their great injury, covered with water ; 

 this, however, is the lesser of two evils, — for the point of stoppage has been 

 fixed with a reference to the height of the natural lands, and the lower and 

 weaker of the banks surrounding the polders. 



The consequence, then, of permitting the surface of the basin to rise above 

 this point by any further continuing the mechanical lifting of the water of the 

 polders, would be to flood the natural lands, and cause the water to flow back 

 into the polders over the top of their boundary dykes, not only flooding the 

 lands below, but endangering the bank being cut through by the rush of water 

 over the top. 



Upon the area of the basin, therefore, multiplied into the vertical depth from 

 the point of stoppage to the level that the surface had at the time the self-act- 

 ing sluices closed against the exterior waters, depends the volume of rain that 

 may fall upon the total area of the Hydraulic Administration, without entailing 

 the great injury of a cessation to the mechanical drainage of the polders. 



We may illustrate this by the Rynland. Let us suppose the level of the 

 surface of the water in the basin to be 1 foot 2 inches below the point of 

 stoppage, and the self-acting sluices closed, the external waters being from any 

 cause raised above that point. Let 3 inches' depth of rain be supposed to 

 fall in the week over the whole 305,000 acres ; this will raise the basin of 

 56,000 acres, 3 inches, by that which falls directly upon it, no evaporation 

 being supposed to take place, and the rain upon the polders, being also 

 thrown ofi" as it falls, being raised by the windmills and steam-engine, 3 inches 

 upon 176,000 acres, will be equal to 9'4 inches on 56,000, and 3 inches falling 

 on the natural lands ; and, flowing down on the basin, will raise it 4 inches. 

 Thus, the total rise of the basin would be 1 foot 4-4 inches, or 2 inches above 

 the point of stopping ; the engines raising the water from the polders would, 

 therefore, have to cease working before they had discharged that amount 



