3l6 On the SuhjcEl of Nav'tgatmi, 



ica, her rate of failing would be known by the time of the 

 wave's undulation : but, fiace the fliip receives the impulfe of 

 the wind more powerfully than the wave, it moves failer and 

 inakes more way than a wave by the breadth of all the waves 

 which it pafles through ; and hnce a fliip pitches on paffing 

 over each wave, and fuice the number ot pitches may be re- 

 giltered, without trouble, bv an ofcillating inlirument, (like. 

 the watch which fliows the number of fteps which the wearer 

 makes,) the number of waves whofe breadth is to be added 

 to the progrefs deduced from the tabular rale is known ; 

 whereby the diilance made bv a {h\]p Jallmg before the luuid 

 may be found, 'provided the time of "undulation be aicertained 

 whenever there is rcafon to fufpeft a change in the motion 

 of the fufface of the fea. But when a Hiip fails npfjn a windy 

 the diftance deduced from thefe data requires correftion. 

 Suppofe two ihips placed on the fame wave, and that one 

 runs dire<Stly before the wind and keeps tiuie with this wave, 

 and that the other fails in a courfc which forms an angle 

 with the direction of the wind, retaining, however, her place 

 on the fame wave ; it is evident that then the fpace defcribed 

 by the former Will be to the fpace defcribed by the latter, as 

 the cofme' of this angle to radius: alfo, if thefe fliips, pur- 

 fuing thele courfes, pafs over the" fame waves, or the iame 

 number of (imilar waves, the direct path of the former in 

 paffing thefe waves is to the oblique path of the latter as the 

 coline .of the fame angle to radius alfo: therefore, in failing 

 -wpon a iv'ind, the reluit which this method of calculation 

 •gives, is to the true diftance as tlie cofine of the angle which 

 the rhomb forms with the direftion of the wind is to radius, 

 and may be corrected accordingly : or, if this angle be af- 

 fumed as a courfe, and the refult obtained by this method 

 be taken as difference of latitude, the corrcdcd difhince will 

 be found in the diftance column of the common nautical 

 tables. 



But both thefe methods muft prove erroneous where the 

 {liip is borne away by a current; and the common method 

 of eftimaling the eifect of a current by the apparent motions 

 of a funken body is fallacious; becaufe the boat may be in 

 quiefcent water, and the current may exift only below ; or 

 the upper and lower ftrata of water may l)e parts of the fame 

 current flowing in the lame dire^Hion with different degrees 

 of velocity : therefore anv theory which points lo a true mea- 

 ■fure of currents Hiould be announced, though it may require 

 lon<T experience to pcrtict it. 



Let two fliip? lie-io ai a confiderable diflance afunder (fup- 

 pofe twelve miU<): "if a gun be iircd from each irom a filu^ 



tion 



