314 On the Suljfci of Navigation. 



prevent which, the ftring is to he of fuch a length as to ren- 

 der ihe angle through which it vibrates verv niiniue, wherchv 

 its tentlency to deflcft the ball from a rectilinear path will be 

 diminidicd ; and if (as Mr. Boyle and others alTcrt) the 

 greateft height of a natural wave be no more than fix feet, il 

 will not require a verv long radius to render the angle fub- 

 tcnded by a fine of fix feet fo fniall, that the divarication 

 Occafioned by it may become inconfiderable. 

 ■ This may be lubjccted to experiment in an eafy manner : 

 kt a boat, wjth the inflrument annexed, be drawn on a canal 

 through ftrch a ferpentine line in an horizontal plane as it 

 xvould dclcribe in a vertical plane in paffing over waves, and 

 the et^cft on the ball w ill be the fame in both cafes ; by which 

 rpeans it will be poffible to find the error occafioned by pitch- 

 ing, and the length of fi:ring neceflfarv to reduce it. 

 - It may occur as an objection, that the immerfed ball can- 

 iiot be kept at the farne depth afall limes, and that it may 

 receive different degrees of refiftance from the fame velocity 

 at different depths, which muft occafion error : but the expe- 

 riments of C. Coulomb, publillied in the Phvfical JVJemoir? 

 of the French National Inftitute during the third quarter of 

 the year 8, tome iii. p. 288, prove the rcverfe ; his words 

 nre — " L'on peut rondure de cette experience que lorfqu'un 

 corps fubmcrgc fe mcut dans un fluidc, la preflion ou la hau- 

 teur du fluide au defl'us du corps n'augmente pas fenfiblemcnt 

 fa refiftance." 



But the crcntefi: difficuliv attending; this fcheme is fo form 

 the fcale ot ihe fpring that is to afccrlain the icveral degrees 

 of prelTtire which a glol)e of certain dimenfions meets from 

 the leveral rcquifite degrees of velocitv. For this purpofe, 

 let a man be furnidied with a flender chain one yard in 

 iength, fo hiftcned between his flioes as to limit the length 

 tof his itcps : let him keep his eye fixed on the vibrations of 

 a pendulum, and, by fie].pingto the full length of his chain 

 in time with the ofcillations, his motion will be equable, and 

 the rate of it will be exactly known. If a man fo prepared 

 fhould draw a boat along a ftagnant canal, the rale of the 

 boat's motion will be equable alfo, and al'eertained ; and this 

 rate may be varied to anv required rate within certain limits 

 by altering the length of the pendulum: then if the inftru- 

 Tiient* be attached to the boat, the expanfion of the fpring at 

 •each degroe of celerity with which the man proceeds may be 

 marked on the fcale : for the higher degrees of velocity the 

 man's motion mufi be increafed bv fome mechanic power, 

 as the axle in the wheel, whereby additional Ibrce may be con- 

 vened 



