8 Description of a new Refieciing Compass. 



iiiarv, as it possesses the advantage peculiar to itself, of deter- 

 mining the point of station by the bearings of two objects only, 

 f.he positions of which arc known; and may even in certain con- 

 fined circumstances be employed in determining the station of 

 the observer by the bearing and estimated distance of one known 

 object. 



These considerations led to the construction of the instrument 

 which I am about to describe. It war, invented in 1811 by 

 Captain Kater, F.R.S.: and on a survey made with it the same 

 year, was found capable of a degree of accuracy beyond any ex- 

 pectation that had been previously formed of it. 



The box containing the compass is about 2\ inches diame- 

 ter, but may be made of any size that may be thought con- 

 venient. Its depth is such as to allow no more space than is 

 sufficient for the card to float freely. The card is very accu- 

 rately divided iiito 360 degrees from the 7ioith towards the east, 

 and every tenth degree is distinctly numbered. The needle, 

 which is rendered strongly magnetical, is fixed beneath the card 

 vnth its north point at the 180th degree. It has an agate cap, 

 and is made as light as is consistent with that form best calcu- 

 lated for retainir.g the magnetic virtue. 



Fig. 1. Plate i. represents the instrument; A is the sight 

 next the eye. It slides rather stiffly in a dovetail on the outside 

 of the box, and is prevented by a shoulder from going beyond 

 the bottom. Very near the top of this sight on the inside, and 

 projecting over the edge of the compass-card, is fixed a small 

 plane silvered glass or mirror B, at an angle of about 40°. The 

 mirror is of the same breadth as the sight; and when the sight is 

 pressed home to the shoulder, the lov/er edge of the mirror nearly 

 touches the glass of the compass. Between the glass of the 

 compass and the upper part of the mirror there is sufficient 

 room for an opening which contains the segment of a convex 

 lens of about three-fourths of an inch focus, in a line perjjen- 

 dicular to the centre of the lens, and immediately above the uj)- 

 per edge of the mirror a small hole is drilled, through which is 

 seen the object the bearing of v.hich is to be determined. On 

 the opposite part of the box is an open sight with a vertical hair 

 or wire : this sight is of such a length as to admit of its being 

 folded down on the glass of the compass, and a contrivance is 

 added, by which the needle is throwti off the point at the same 

 time, thus securing it from injury when the instrument is not 

 in use. 



To use the Inslrnneni. — Raise the sight, when the compass- 

 card will vibrate ft-eely ; look through the small hole above tb^ 

 lens, and cover the object with the thread of the opposite sight ; 

 check the vibration of the compass-card by inclining the box, 



keeping 



