JO Method ofmeafuring a Ship's Way at Sea. 



The agitations of the fea afFe£l alfo that part of the inftrument which lies within the 

 fliip, and it remains to be confidered what confequences arife therefrom. Heeling, or an 

 inclination of the veflel to one fide, and pitching, or the librations of It fore and aft recline 

 the upright tube, and thereby lengthen the column of water contained ; but fince the dif- 

 charge of water is afFe£led by perpendicular height only, this is but an apparent caufe of 

 error; the fame preffure at the immerfed orifice will elevate the water to the fame perpen- 

 dicular height whatever the length of the column may be, and therefore the velocity of the 

 effluent water will be the fame with the fame degree of preffure whether the tube be ere£l: 

 or inclined ; plunging, or the afcending and defcending motion, occafioned by the waves 

 which alternately elevate and deprefs a veflel in the fea, by altering the weight on the ori- 

 fice increafe or dimlnifh the efflux of water from the tube, to remedy which a part of the 

 upright tube below the water line is made narrow, in the manner hereinafter defcribed, 

 whereby the afcentand defcent of the contained water will be rendered gradual, fo that its 

 altitude may become the mean of feveral alternations of preflure; and fince the waves fuc- 

 ceed each other at equal intervals, and of equal dimenfions for a confiderable time together, 

 this mean height will be fufficiently fteady and exaft ; (fee note D) and though this motion 

 and pitching may feem to difturb the operation of the fpouting aperture, by raifing and 

 finking It above or below the plane of the water line, it occafions equal and oppofite errors 

 thereby, which remedy each other, and narrowing the tube is really ufeful only to procure 

 a more equable flow from the aperture ; but heeling, which Is a more durable pofition, 

 has a tendency to ralfe the fpouting orifice above the plane of floatation ; however, this ele- 

 vation is fo fmall, that the error arifing from it is infenfible. 



Some mechanical diflSculties prefent themfelves in applying this Inftrument to a (hip, and 

 in adapting it to the purpofes of navigation ; firft it may be thought unfafe to make a hols 

 in the (hip's bottom to receive the tube, but the experiment has been tried to fupply fire 

 engines with water on fliip-board, and no inconvenience arofe from the praftlce when the 

 receiving pipe was clofely fitted to the perforation. It might alfo be pafTed, If thouglit 

 preferable, through a hole above the water line, and the neceflary curvature given without 

 the (hip. 



Secondly, It appears from calculation, that the water in the ere£t tube would fometlmes 

 afcend fo high, as to require a tube inconveniently long to contain it ; but if it be cut off 

 any where above the fpouting aperture, and if the upper orifice be clofely ftopped, it will 

 anfwer the purpofe as well as before, becaufe the preflure which the ftopper receives, is 

 that which would fupport the column of water cut off, and therefore the preffure which this 

 Hopper gives, is that which would arife from the weight of the column cut off. Hence 

 the preffure of the water within the tube continuing the fame» the rate of difcharge wUl 

 remain unaltered by fhortening it. 



Thirdly, it is difficult to afcertain the true mean level of the fea when the flilp is in 

 motion, which may often be requilite ^ in this cafe, if a flat piece of metal be fixed on the 



