m 



' ' APPENDIX, 



I alfo tried two perpetual Jogs ; one invented by Mr. 

 RufTell, the other by Foxon, both conftruded upon this 

 principle, that a Spiral, in proceeding its own length in the 

 diredion of its axis through a refilling medium, makes 

 one revolution round the axis ; if therefore the revolutions 

 of the fpiral are regiilered, the number of times it has gone 

 its own length through the water will be known. In both 

 thefe the motion of the fpiral in the water is communicated 

 to the clock-work within board, by m>eans of a fmall line, 

 faftened at one end to the fpiral, which tows it after the 

 (hip, and at the other to a fpindle which fets the clocks 

 work in motion. That invented by Mr. RufTell has a 

 half /piral of two threads, made of copper, and a fmall 

 dial with clock-work, to regifter the number of turns of 

 the fpiral. Foxon's has a whole fpiral of wood with one 

 thread, and a larger piece of clock-work, with three dials, 

 two of them to mark the diftance, and the other divided 

 into knots and fathoms, to ihew the rate by the half 

 minute glafs, for the convenience of comparing it with 

 the log. 



This log, like all others, is liable to the firft error, as 

 well as to the fecond. The third it partakes of in a very 

 fmall degree, only alleging the reckoning by that quan- 

 tity which the fpiral is thrown towards the ihip ; whereas- 

 in the log the fame circumftance afFefts the whole rate 

 for the hour* The fourth it is entirely free from, as well 

 as the fifth. It will have the advantage of every other in 



R fmooth 



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