326 PHILOSOPHICAL TRANSACTIONS. [aNNO l/l^. 



Our author goes on to show the usefuhiess and necessity of considering the 

 doctrine of mixed motion, in all questions relating to the course of rivers, the 

 quantities of water which they discharge, the enlarging or narrowing their 

 outlets, the scouring and deepening their channels, and the motion of the 

 tides in harbours. These he illustrates by several deductions from the equa- 

 tions abovementioned. But, to render these of greater evidence, it were to 

 be wished, that those equations were built on a more solid foundation than a 

 tentative calculus; and that allowance had been made for the velocity impressed 

 on the preceding water in rivers, by the impetus of that which follows, which 

 is omitted by the author in his Theory, both of mixed and simple motion. 



In the second book, this learned writer proposes the state of the laguna of 

 Venice, as a proper example, to demonstrate the usefulness of his new Theory. 

 He considers very minutely the several causes of choaking up the laguna, 

 examines the method proposed by various authors for scouring and keeping it 

 clear, some of which he rejects as impracticable on account of the expence, 

 others as useless, or prejudicial; and lastly, delivers his own opinion. 



The principal causes, which he assigns, of filling up the laguna, are the 

 rivers running into it, and the sea tides setting into it; both of which setting 

 in with a great velocity, carry in a great deal of mud, and deposit it there ; 

 the water running out clearer than it goes in. 



The remedy our author proposes for the river waters, is either wholly to 

 divert the course of the rivers and carry them, by another way, directly into 

 the sea : or at least, to secure their outlets with sluices, so as to suffer them 

 to pass into the laguna, when their waters are clear; but after great rains, 

 when they run foul and turbid, to stop their passage that way, and let them 

 out by the other channel into the sea. And against the effect of the tides, he 

 proposes some works of strong piles, and large stones thrown in between 

 them to be carried directly forward into the sea, to break the violence of the 

 waves, and prevent their washing and carrying away the land. 



He seems likewise to favour a proposal made by the late famous Guglielmini, 

 and some others, to let the tide enter the laguna by more passages than it is to 

 go out at, in order to make it run out with a greater velocity, and thereby 

 scour and deepen the channels. But he thinks this contrivance will scarcely 

 perform all that is expected from it ; besides, it will be attended with great dif- 

 ficulties in making works, and flood-gates of a sufficient strength, to resist the 

 violence of the waters. 



He occasionally combats the opinion of Guglielmini, and most other mathe- 

 maticians who have thought upon the subject, that in order to give a greater 

 velocity to the water of a river, thereby to scour and cleanse the channel, it is 



