MEMOIR ON BOSTON HARBOR. 103 



it is not suffered to rest in those places where the maximum velocity of the current is 

 sufficientlj' great to move it again on the returning tide. That part of the deposit of 

 the ebb which is left on the borders of the flats will be carried farther up by the trans- 

 porting power of the flood, increasing in strength as the tide rises; but if this matter be 

 left during the period of still water between flood and ebb on the upper and inner ed-^e 

 of the flats, it will not be carried off again, because at these points the ebb stream, hav- 

 ing no back-water to assist its momentum, never acquires velocity enough for that pur- 

 pose. It soon, in fact, leaves these higher spots bare. A single illustration, which has 

 been mentioned by Professor Treadwell, suffices to convey an idea of this gradual accu- 

 mulation. When the Milldam was built, a barrel of tar in a good condition was found 

 buried several feet in the soft mud. 



In this manner the capacity of the reservoir is diminished by natural causes, of which 

 alone I am now speaking ; and as the reservoirs become smaller, the united sections, or 

 capacity of the channels, by which the water is returned to the sea on the ebb, will ne- 

 cessarily undergo a similar reduction. A diminution of the reservoir occasions an injury 

 to the harbor proportional to the space lost ; it reduces the size of the channels simply 

 because less water requires a smaller passage, and is not capable of keeping open the 

 original passage.* 



This description of the character of Boston Harbor opens the way to a brief enunci- 

 ation of the leading principles which should govern any plan of improvement, either spe- 

 cial or general. 



The first fundamental principle to be observed is the preservation of the water re- 

 ceptacle, to its fullest extent ; though not necessarily in any actual or particular form.f 

 Tidal mud-lands may be occupied to advantage, provided compensation is made by deep- 

 ening the reservoir, and adding to the quantity of the water in the reservoir. The ve- 

 locity being, in general, proportional to the square root of the depth,! and the quantity of 

 water proportional to the depth nmltiplied by the velocity, it follows that our reservoirs 

 could be, at any time, improved for scouring purposes by excavating systematically the 

 flats which are bare at low water. Owing to this law of velocity, fully established by 



* The diminution in the reservoir caused by dikes, embankments, and made land, gradually produces of 

 course the same effect. It is stated by William P. Parrot, Esq., Civil Engineer, that the area of the peninsula 

 has been increased to three times its original size by this filling-up process. In this manner the action of the 

 natural causes of deterioration is greatly promoted ; but as the purpose here is only to lay down general views 

 and principles, it is not necessary to dwell upon the significancy of this statement. 



t The English engineers generally. 



J Zendrini, and others. 



