XXXVII. A Treatise on Rivers and Canals. By Theod.. Aug. Mann, Member of the Imperial and Royal Aca-demy of Sciences at Bruffels; communicated by Jofeph Banks, Efq, P. R. S..

Read June 24, 1779.

TO JOSEPH BANKS, ESQ. P. R. S..

SIR,

OUR election to the Presidency of the first literary

You

and scientific Society in the world, to a chair fo long and fo gloriously occupied by the great NEWTON;; joined to the friendship you have been pleased to honour me with fince my being first known to you: has encou-raged me to fend you fomething of my compofition, as the best way of expreffing my fincere respect and attach-ment to you, and my profound veneration for the illuftrious Body which has chofen you for its head. Though various circumstances, by carrying me very early into fo reign countries, have made me from my youth almost

an

an alien to my native foil, and put me in a fituation which apparently must make me ever remain fo; yet, neither time nor distance could ever weaken, much less obliterate, my tender attachment to it, or my ardent wishes for its welfare.

These confiderations will, I hope, merit a favourable acceptance from the Royal Society of the following piece, which I have the honour of addreffing to you; and an indulgent condefcenfion for its imperfection in every respect, and particularly in point of style. Five and twenty years absence from my native country, and the neceffity of converfing during that time in different foreign languages, must unavoidably have filled mine, without my being fenfible of it, with idioms and expreffions in no wife English.

As to the subject I have undertaken to treat on this occafion, I was guided in the choice thereof by the motive of saying something that might be useful to my native country. The great number of extensive and magnificent canals, which have been cut through almost every part of England of late years, for the use of internal navigation, and which do honour to the public spirit of the nation, merit to be confidered in a scientifical as well as in a commercial light. Their waters have their laws of motion different in many cafes from those of rivers:

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rivers: they are liable to many accidents which the others are not, and of a different nature. These accidents do not become fenfible till many years after their construction, and are better prevented in time than remedied when they happen. I have long lived in a country famous for its navigable canals, and have been much employed, under the eyes of the government of it, upon that fubject. I only mention this to fhew, that I have not undertaken to treat a fubject to which I am an utter ftranger.

There are, moreover, many confiderations concerning the laws of motion in rivers and canals in general, the velocity of their currents in proportion to the quantity of their declivity, and the means of ascertaining therefrom the respective heights of the interior parts of continents, which merit the attention of a natural philofopher. I shall venture to offer my thoughts and obfervations (fome of which, I believe, are new) on all these subjects in the enfuing Differtation, which I fubmit entirely to the judgment of the Royal Society, and shall efteem myself happy if I fucceed in it, fo as to be of any use to my country, and to be able to testify, at the fame time, my profound respect and veneration to you, DEAR SIR, and to the illustrious body over which you preside.

VOL. LXIX.

4 D

SEC

SECTION I.

Different ufes for which canals are made, with an account of the principal authors who have wrote concerning them.

1. Artificial canals are to be confidered in a double light; as facilitating commerce by means of internal navigation, and as preventing inundations by carrying off the too great abundance of water from low and flat countries, fuch as are Holland, Flanders, &c. In these last named countries they ferve at once for both purposes; and it is in this double light that I shall confider them in the ensuing discourse. If canals for draining have fluices upon them, particularly at the end whereby they difcharge their waters, as is univerfally the cafe in the Low Countries, they differ in no wise from navigable canals: if they have nothing to sustain their waters in them, they are to be confidered in every respect as rivers or rivulets, and follow the fame laws. It muft, therefore, be carefully kept in mind, that whenever I mention canals, I mean those only whofe waters are kept up by fluices, and never those without them, which I include, without diftinction, under the common appellation of rivers; for

they

If I mistake not,

they are no more than artificial ones. all the navigable canals in England are of the first fort; that is, have their waters kept up, and let off by fluices. This neceffary diftinction will take away all ambiguity from what I have to say on canals throughout the following discourse.

2. But that I may fulfil the task I have undertaken, it is neceffary first of all to lay down fuch principles on the nature of rivers and canals in general as have been demonstrated true both by calculation and experience; to the end, that we may deduce from thence the true laws of motion of their waters, and the quantity of declivity of their beds: for this purpose, and because a large volume would hardly fuffice to comprise all the demonstrations of these principles, which, confequently, I am obliged to omit in this treatise, it will not be amifs to mention the principal authors who have treated this fubject in different ages and countries, in whofe works the demonftrations of all the principles I fhall lay down may be found, if any one doubts the truth of them. These are the following.

SEXTUS JULIUS FRONTINUS, de Aquæ-ductibus Urbis Romæ, cum Notis POLENI, impreff. 1722.

JOHN BAPTIST ALEOTTI, Hydrometrician to the Duke of Ferrara, and to Pope CLEMENT the VIIIth.