[ 146 ] 



XY. 



A NEW FORM OF POSITION-FINDER FOR ADAPTATION TO 

 SHIPS' COMPASSES. 



BY SIR HOWARD GRUBB, F.R.S., Vice-President, Royal Dublin 



Society. 



(Plate XII.) 



[Read, November 17 ; Received for Publication, November 20, 1903 ; 

 Published, February 10, 1904.] 



This instrument possesses some advantages over the ordinary 

 form. The figure (p. 148) is a section showing the construction. 

 Plate XII. is reproduced from a photograph of the instrument 

 itself. 



The purpose of the instrument is to obtain from a ship's deck, 

 with a moderate amount of accuracy, the magnetic bearing of 

 ?ome landmark or vessel which is in sight. It is not an operation 

 that can be effected with great exactitude, as it is generally taken 

 on a vessel in motion, and can only be correct for the precise 

 moment at which it is taken. It is usually a somewhat difficult 

 "observation, more particularly if the vessel be rolling in a 

 heavy sea. 



The ordinary construction consists of a framework which is 

 laid upon the covering-glass of the binnacle, and centred upon it 

 by means of a little pin which fits in a hollow ground in the centre 

 of the glass cover. This frame supports a piece of tube mounted 

 at a convenient angle, on looking through which a portion of the 

 divided compass-card, which is opposite the tube, is seen through 

 a lens, as well as a pointer, mounted across the tube, to form an 

 index. At the upper end of the tube, on a swivel, a prism is 

 mounted ; and by turning a button this prism can be brought into 

 such a position that a portion of the landscape or horizon can be 

 seen by reflection in it. If now the eye be so placed that one-half 

 the pupil be used to view the landscape through this prism, and 

 the other half to view the card and pointer, a coincidence can be 

 made between the three objectsa 



