362 Capt. Graydon's Celestial Compass, 



vernier I is brought to coincide with such point or degree upon 

 the compass plate, as will correspond with the course desired 

 to be steered. For example : in the drawing, the compass 

 plate is represented as set proper tor steering a course due 

 north. 



The person who steers the vessel has only to keep it in such 

 a position that the shadow of the round rod s, or of the inter- 

 section of the cross wires O, may fall upon the surface q, so as 

 to coincide with the perpendicular mark thereon. The line 

 A B or F G will then remain in the plane of the true meridian, 

 and the vessel will therefore proceed in a course due north. 

 In like manner any other course may be steered, by setting 

 the compass plate F G accordingly, and keeping the shadow 

 of the rod s upon the perpendicular mark upon the surface of 

 the ivory*. 



This method of steering is particularly applicable to high 

 northern and southern latitudes, as the heavenly bodies remain 

 constantly above the horizon, as long as navigation is practi- 

 cable in those parts of the world, where the sky is generally 

 unclouded and clear overhead ; which circumstance is stated 

 in Capt. Lyon's late account of his unfortunate attempt to reach 

 Repulse Bay, from which the following is an extract, viz. : 



" Although the fogs in the Polar regions are so frequently 

 mentioned in the course of recent narrations which have been 

 published, I believe they are generally understood as resem- 

 bling our English fogs ; which is not, in fact, the case. — In 

 the northern seas, these vapours rarely rise to above a hundred 

 feet from the sea, and a sky of most provoking brilliancy is 

 frequently seen overhead." — Capt. Lyoris Voyage of Discovery, 

 page 43. 



An instrument on the construction above mentioned, for 

 steering without magnetic aid, was ordered by the Lords of 

 the Admiralty, and sent out with H. M. S. Hecla, on the Polar 

 expedition, in the year 1824. 



For ascertaining the Latitude 'whe^i the Hoi-izo7i is obscured. 

 {See Plate I.) 

 The platform H I being fixed over or parallel to the vessel's 

 keel, as above described, the instrument is to be turned upon 

 the axis or pivot of the compass plate F G, until o on the outer 

 division is brought to coincide with o on the vernier I. The 

 round rod 5 is to be set to the sun's declination as above de- 



• When the surface of the ivory cannot be distinctly seen by the person 

 who steers, owing to its position with regard to the sun, a piece of semi, 

 transparent glass is used (in place of the ivory); the shadow is then seen 

 behind the glass. 



scribed. 



