On the SuljcO of Nav':gat'i0Ji, 315 



verted into velocity, and the degree of velocity afcertained by 

 the power of the macliine. Tliis experiment fliould be made 

 in a wet dock filled with foa water. 



It lliould aUo be obferved that thcfe experiments will aSbrd 

 a fcalc which will anfwer only for balls of the fame dimenfion 

 as that with which the original experiment was made, and 

 therefore the mould of the original ball fliould be preferved ; 

 but this is not the cafe of the fpring, for the Icale of any 

 fpring- may be eafilv found when the fcale of one is graduated 

 in this manner: 



Let the extremities of the new fpring and of the original 

 fpring be joined, fo that they may a6t againft each other; 

 then pull them afunder until the index of the original fpring 

 (hall touch fome graduation, and then the fcale of the new 

 fpring mav be marked where its index then ftands, with the 

 degree of velocity at which the index of the original fpring 

 ftands ; and in this manner all the nccetfarv graduations may 

 be transferred from one fcale to another, though the fprings 

 fliould be of very dit^l'ient powers; becaufe, fince the ball is 

 to be the fame, the force exerted by each degree of velocity 

 remains unaltered. 



However, in a very high fea this method may be attended 

 with fome inconvenience : the firing may be frequently 

 broken, and a certain deiiciee of error will attend a very {Irong 

 pitchine motion ; in which cafe the following method may 

 be ufcd to afcertain dillance by means of theie waves, which 

 interrupt the operation of the inllrument : 



Let anv fmall floating fubllance be thrown out a-head of 

 a fhip at fea, and let the number of times be noticed when 

 it is fecn diftinctly on the top of a wave ; alfo let the interval 

 of time between the firfl and lall of thefe elevations be ob- 

 ferved by a (lop-watch ; which time being divided by the 

 number of elevations will be the time in which a wave pafies 

 througti ils own brvadtli, (I)ecauie the floating body is not 

 pnAruded by the adfion of the waves.) But a \va\ e pafl'cs 

 throuoli its breadth in the fame time in which a penduknn 

 whole ioiiglh is equal to that breadth performs one vibra- 

 tion *: and tables arc or mav be conlirui'-ted to fliow by the 

 time of vibration the length of the pendidum ; which tables 

 will of coiirle fhow, l»v the tmie of undulation, the (pace 

 throu«.'h which anv wave nuwes in that time, and alio the 

 breadth of tlie wavcf- U'lieref()rc, if a fliip failing right be- 

 fore the wind were to a)ecoiupany tlie lame wave through the 



• (Juairc, whit 1 tfctl litis a fumiir in difUnliing t!:is l.iw of the waves ? 

 + Oi tlie lircadth ;iittl Vt!bi;ity of the waves may be fouiiil by an txpe- 

 riiient wjfh tlic lo^-liiu'. 



fea. 



