54 Mtthod of meafuring a Ship's Way at Sea. 



In climates where the (ky is feldom obfcured, fuch an inftrument would afford a fteady 

 guide to the mariner, not being liable to the errors of variation, aurora borealis, or to the 

 deflections of libration, which difturb the fufpended card very condderably. 



F. It is a faft, well known at fea, that the apparent wind is compofed of the true wind, 

 and of the wind occafioned by the fhip's motion ; therefore it is that a fhip failing on dif- 

 ferent tacks, or with a different degree of velocity in the fame line, will have the wind in a 

 different point, becaufe one of the component forces is altered in pofition in the former, 

 and in intenfity in the latter cafe. What we experience in riding through a fhower of rain 

 is a familiar example of this fadl ; while the horfe's motion is uniform, the dire£tion of the 

 drops appears to be conftant, but if it be encreafed, the direftion of the drops will be more 

 in the rider's face ; if it be diminilhed, it will be more behind him ; and if his road be 

 changed to the right er left, the fhower will apparently alter to a point more oppofiie to his 

 motion than the point it fcemed to be in before. 



G. Should it be confidered advifeable to difencumber this inftrument by feparating the 

 lee-way machinery from it, it may be annexed to the compafs box in the following 

 manner : 



A metal rod is to be extended down the (tern pod, and fo hung as to be turned eafily on 

 its axis by a vane at the bottom, which is to be a£l;ed on by the ftream. To the upper part 

 of this rod, juft below the level of the deck, a crofs bar of a few inches long is to be annexed. 

 The compafs box muft reft on an upright foot, extending a little below the deck, and fup- 

 ported by a focket, in which it is to turn freely. Juft below the deck a crofs bar is to 

 be annexed to this foot equal to that on the ftern poft rod, and the extremities of the two 

 crofs bars are to be connefted by two flender equal bars moveable on pivots, fo that the 

 four bars may form rhomboides, in confequence of which the oppofite fides muft be always 

 parallel, and the motion of the foot will be the fame as that of the ftern poft rod : therefore 

 the vertical fedtion of the compafs box, which pafles through the centre and fleering mark 

 of the box, will be parallel to the vane } and fince the ftream will point the vane exadtly to 

 the fhip's true line of motion, it follows, that the fteering mark on the compafs box will 

 be pointed to the true line of motion alfo, and the angle contained between this mark and 

 the north point of the card will confequently be the true magnetic courfe clear of the error 

 of lec-way. 



lV.—Ot> 



