Method of meafunng a Sh'tp^s Wa)^ at Sea. 51 



top of a pole, and held out of the fliip fo as to defeifd the external orifice from the percuffion 

 of the water, fhe contained water will link to the true mean level, and the pofition of the 

 fpouting aperture, if fixed in a Aiding plate, may be adjufted thereby* 



Fourthly, if dirt or weeds get into the inftrumeot, it may be cleared by forcing out the 

 contained water by means of the plunger hereafter defcribed ; or if this plunger and the 

 ftopper be drawn out, the water will fpout up forcibly in failing at a moderate rate, and 

 thus remove the obftruftion. 



The advantages expeded from this inftrument are, that it propofes a perpetual meafure, 

 which adapts itfelf to every alteration of the fliip's motion, and therefore corrects that error 

 arifing from the log, the ufe of which fuppofes the (hip's motion to be uniform during the 

 invcrvals of experiment, which is highly improbable. 



And fecondly, that the inftrument being funk to a confiderable depth, will not be af- 

 feGed by fuperficial currents occafioned by wind, to which the log is expofed. 



The caufes of error which alFedt this inftrument, are the fame which attend the methods 

 of navigation in general ufe. 



The error of yawing is that which arlfes from unfteady fteering, and will defletl the 

 orifice from the true pofition. 



If it be admitted, that the generality of fcamen fteer fo as to confine the yaws within 

 certain limits, the true line of motion may be confidered as a Terpentine line, which has a 

 fixed ratio to the right line which joins its extremities in all cafes ; but if the quantity of 

 difcharged water which reprefents a mile, be found from a mean of feveral experiments 

 made at fea (which experiments are of courfe afFe£ted by yawing) it follows that the ftand- 

 ard meafure thus eftabliftied needs no corre£tion, the effedt of yawing being proportionate, 

 and being allowed for in afcertaining the ftandard quantity. 



The error of lee- way may be correded, if a metal rod be extended along the axis of the 

 upright tube, and pafling through its bottom into the water below it, it (hould be capable 

 of being turned round in any dire£l:ion by the ftream a£ling on a vane like that of a wea- 

 thercock, which is to be annexed to it below the tube ; on the top of the tube is to be 

 placed a circular plate engraved with the points of the compafs, through the center of 

 which this rod is to pafs, and to bear an index round it, which will point out the angle of 

 lee way on the compafs plate. Since the ftream below will keep the vane in the fhip's 

 true line of motion, if this rod be furniftied with a circular protuberance, nearly filling the 

 tube below the fpouting aperture, the fudden current of the water within it will be thereby 

 checked and rendered gradual, as was propofed before ; and if this rod be divided into two 

 parts juft below the protuberance, and if the termination of the lower part be fquare, and 

 if the bottom of the protuberance contain a fquare focket, thefe parts may be occafionally 



- * Or othenwiie, if the external aperture of the tube itfelf, or of any other tube, could be placed at right 

 nngles to the fhip's motion, the water would during that time ftand at and (hiw the external level.— N. 



Ha feparated 



