430 Description of Crow's Seaman's Octant 



sailors, are able to go to any extent into the basis of the science 

 of navigation, and wlio continually are working their various 

 problems without ever thinking further than that they are to 

 add together certain numbers and subtract others ; and that 

 if they do this right, they at last arrive at the desired result ! 

 This instrument will insensibly impress upon the minds of 

 those who use it, that in every triangle all the parts bear a 

 rigid and definite relation to each other, and that having 

 certain things given, certain others can thence invariably be 

 found; and that any three things that can be brought to the 

 shape of a triangle, become, by that, objects of simple com- 

 putation, whether they are distance, difference of latitude and 

 departure, or the main mast, a main shroud, and the hori- 

 zontal line joining that mast and shroud along the deck. 



I may be permitted here to recommend to the nautical 

 world not to correct any of the courses when the vessel has 

 been on one tack for the whole day, and has made the same 

 lee-way throughout. This will often be the case, and there 

 may be twenty courses on the log-board. Work them all, 

 without any correction; and having reduced them to one course 

 and distance, made good, then allow the lee-way and variation 

 upon that one course. 



This instrument may be had at various prices ; according to 

 which it is fitted up as follows, viz. 



With two tangent screws, for slow motion : one to the arch 

 and the other to the slide. 



With glasses complete, so as to be at the same time a 

 Hadley's quadrant. 



With the slide to revolve, so as to be able to work oblique 

 as well as right-angled plane trigonometry. 



With a scale of chords, by which it will work all cases of ob- 

 lique and right-angled spherical and plane trigonometry. 



Testimonials have been received from J. A. Dotchen, Esq., 

 Teacher of Mathematics; Olinthus Gregory, LL.D., Prof, 

 of Mathematics, &c. ; M. C. Friend, Lieut. R.N. F.R.S.; 

 T. L. Tiarks, F.R.S.; P. P. King, Capt. R.N. F.R.S. &c. ; 

 P. Lecount, Lieut. R.N. F.R.A.S.; W. G. W.Owen, Capt. 

 R.N.; Thomas Lynn, Capt. Hon. East India Company's 

 Service; E. Riddle, Esq. F.R.A.S., Master of the Mathe- 

 matical School, Greenwich Hospital : From these the follow- 

 ing are selected. 



" I have carefully examined the instrument invented by 

 Mr. Crow, of Gravesend, in order to facilitate the keeping a 

 ship's reckoning, and to supersede the necessity of recurring 

 to tables for that purpose. 1 have no hesitation in testifying 

 that the instrument is founded upon correct principles, and 



that 



