On the Meajure efa Silp*s Departure. 46y 



found thereby fliould agree with that had from obfcrvation, it may be fafely concluded, 

 that it will meafure departure with the fame degree of certainty, 



The principal advantage which the departure inftrument feems to have over the diftanrc 

 jnllrument, is, that the error of lee-way is intirely avoided by it ; this maybe eafily con- 

 ceived, by confidering that tJie magnetic n?edle points out the angle between the meridian 

 and the line of the {hip's keel, (fuppofing no variation) but Cnce the line of her motion is 

 feldom the line of her keel, this angle is feldom the true courfe ; but in this method the 

 line of the {hip's keel does not enter into confideration, the angle made ufe of is that which 

 is contained between the meridian, fuppofed to lie in the plane of the orifice and the direc- 

 tion of the ftream> which muH be the {hip's true line of motion, and confeejuently this Is 

 the true courfe, without addition or deduftion for lee-way. 



If the moveable orifice at the extremity of the immerfed tube were removed, it would 

 fimplify and improve the inftrument much ; the error which it is intended to correft, is 

 that occafioned by the deviations of the plan? of tlie orifice from perpendicularity, wJiich 

 •when moving in an inclined pofition receives an undue degree of preflure, and mifrepre- 

 fents the departure ; in this fituation the prelTure on the orifice is diminlfhed as the fquarc 

 lof the fine of inclination ; therefore, if the fpouting aperture were to be increafed in area 

 as the fine of inclination is dimini{hed, the error would be exadtly compcnfated, becaufc 

 the fe£tion of the fpouting ftream being then inverfely as its velocity, the quantity of water 

 delivered muft remain the fame, which may perhaps be effefled in the following manner: 

 let a Aiding cover be joined to the fpouting apertuxe, which may increafe or dimini{h it by 

 being moved up or down, and. if the aperture be of fuch a form that its area may be en- 

 larged in the ratio of the elevation of this Aide, a pendulum moving {leadily in the plane 9f 

 the two parts of the tube, may be fo conne£led with the Aide as to give it a motion pro- 

 portionate to the fine of inclLnation, which would have the deCred effed; but the difficulty 

 in this cafe would be to give the pendulum aAow deliberate motion: however, if its vibra- 

 tions were to be regulated by the refiftance of ahr contained in a compreflible cafe, with 

 fmall holes for its efcape and return, all fudden vacillations would be checked thereby, and 

 the pendulum would obey .gradually ev^cry motion of the Ihip which might require this 

 correction. 



If you tlunk that this Paper merits Infertion in your Journal, It is at your difpofal. 



I am, SIR, 



Your much obliged, 

 humble Servant, 



JOHN COOKE. 



M m a Reptri 



