Method ofmeafuring a Ship's Way at Sea. J3 



wind, and, confcquently, no current, and the veflel being then ftationary with regard to 

 the earth, the rate and diredion to the ftream will be the fet and drift of the current. 

 However, fince no adequate method is known to determine the true dire£tion of the wind 

 in a current in all cafes, this obfervatioa would be ufelefs j but that there are fome grounds 

 to fufpeft that within the tropics the apparent deviations from the trade winds arife from 

 ^e deflcdling force of current winds. If this be fo, and if the general motion of the 

 trade wind be affumed as the true dire£lion of the wind, this rule will (hew the fet and 

 drift of the currents in a great part of the ocean. 



NOTES. 



A. This law has been denied in cafes where a diminution of preflure on the back part of 

 the moving body arifes from its motion, but the altitude of the water in this tube is afFedled 

 only by the abfolute, and not the relative, impulfe of the impinging current (which is ap- 

 parent from the form of it), and Mr. Vince's late experiments tend to prove the truth of 

 the theory in fuch cafes. 



B. If the fpouting aperture be very fmall, the theory is true, but when the aperture is 

 large, it is not fo certain for this reafon ; and alfo becaufe it will be more convenient to re- 

 ceive a fmall than a large quantity of water into the fliip, the hole through which the water 

 ifiues fliould be very fmall, with even thin edges. See Vince's Hydroft. p. 54, 



C. It may not be ufelefs to obferve, that if the fpace reprefented by the inftrument in 

 paffing over any certain number of fuch waves be obferved and divided by the number 

 of thefe waves,, it will give what is called the breadth of the wavej and fince the celerity of 

 a wave ia demonftrated to be as the fquarc root of its breadth, the rate at which the wave 

 moves may be found by a table at any time by the help of this inftrument. 



D. Nairne's marine barometer is formed on this plan, and alfo the tide guage. 



E. It may be curious^ if not ufeful,, to obferve, that it is pofliblc to fteer by the fun 

 alone without the aid of magnetifm. 



If therabe a large watch, whofe hand performs one revolution in twenty-four hours, 

 with fuitable graduation on the dial plate ; this dial plate, with a perpendicular ftyle in the 

 centre, when reclined according to the latitude, b&tomes a fun dial j and when the fliadow 

 of the ftyle coincides with the hand of the watch, the ftyle muft hang over the true meridian : 

 therefore if this meridian be fet to the (hip's courfe by a motionlcfs compafs card, and if the 

 helm be fo managed as to keep its fiiadow conftantly on the hand of the watch, the true 

 pofition of the ftiip will be preferved until the time of the watch or the elevation of the ftyle 

 . require correftion. It is poffible to effed.the fame thing by a ftar, by having a dial plate. 

 formed of fome xeflefting fubftance. . 



Id 



