Olfervaims on the Meafure of a Ship's Departure. ad^ 



cryftalllne, in its living ftatc, is to that of water nearly as 18 to 17 ; that the water im- 

 bibed after death, reduces it to the ratio of 2 1 to 20 ; but that/on account of the unequable 

 denfity, of the lens, its efFefl: in the eye is equivalent to a refraiStton of 14 to 13 for its 

 whole fizc. Dr. Wollafton has afcertained the refraflion out of air, intothe centre of the 

 recent cryftalline of oxen and flieep, to be nearly as 143 to 100 j into the centre of the 

 cryftaliine of fifti, and into the dried cryftalline of (heep, as 152 to too. Hence, the re- 

 fraQion of the cryftalline of oxen in water, Ihould be as 15 to 14 : but the huma» 

 cryftalline, when recent, is decidedly lefs reftadlive. 



(To be continued.) 



V. 



Ow the Meafure of a Ship's Departure hy the Re-allion of the Fluid through which it paffeu 



By JoHK Cooke, Efq. 



S I R, 



To Mr. NICHOLSON. 



July 22, 1801, No. 14, Kildare Street, Dublin. 



OiNCE the year 1793, when I firft communicated my fcheme to you for afccrtaining 

 diftance at fea, by the afcent of water in a tube oppofed to the ftream, it occurred to me 

 that departure might be meafured in the fame manner, and of feveral plans which I con- 

 ceived for this purpofe, the following feems to be the moft prafticable. It is an inftru- 

 ment Cmilar to that defcribed in the 48th page of the 4th Vol. of your Journal, with thefc 

 alterations: 



Firft, the horizontal and ereft parts of the tube muft be two diftindl pieces, the former 

 being moveable in an horizontal circle on the extremity of the latter, fo that the ere£i part 

 of the tube maybe fixed to the veflel, and made water tight, without impeding the motion 

 of the horizontal part of it. 



Secondly, the rod which in the diftance inftrument is attached to the lee-way vane, is in 

 this inftrument attached to the bottom of the horizontal tube, fo that the orifice of it may 

 be pointed thereby in any horizontal dire£lion, and fcrewed faft in that pofition ; and the 

 lee-way vane, which is unneceflary according to this method, is to be removed. 



Thirdly, the extremity of the horizontal tube is to be fo formed, as to be capable of 

 moving up and down on horizontal pivots with two plates, like the fins of a fifli extending 

 behind it to keep the plane of the orifice in a vertical pofition, notwithftanding the heeling 

 of the {hip, which differs from the apparatus annexed to the diftance inftrument in thisi 

 that the objed of the former is to prefent the orifice perpendicularly to the ftreani iti all 



Vol. V* — October j8oi. Mm cafes. 



