Frazer.] -"-l-b [-Dec, 5, 1884. 



A convenient Device to be applied to the Hand Compass. 

 By PersifoT Frazer. 



{Read before the American Philosophical Society, December 5, I884. ) 



In the course of my professional work, it is very often necessary to 

 make rapid recognizance surveys where absolute accuracy of detail is not 

 necessary, and where superfluous equipage is to be avoided as much as 

 possible. As a consequence those who have most experience with this kind 

 of rough geological topography, following to the extent of their ability the 

 example of its most distinguished expositor, Prof J. P. Lesley, rely very 

 largely upon determination of direction by the ordinary hand compass. 

 This has many inconveniences, and is only possible of application by 

 those who have had considerable experience. In the first place, 

 there are no sights to direct the eye, nor any means to prevent the de- 

 rangement of the needle, between the time that the sight is taken, and 

 that when it is read off. Add to this the liability of the ordinary hand 

 compass to be broken if its glass face be unprotected ; or the cover lost, if 

 this be of the ordinary unattached kind. 



The difficulty alluded to last, had already been met, by the hunting 

 case pocket compass which has been recently quite widely distributed. 

 As regards the other difficulties, the old prismatic compass is their most 

 satisfactory solution, except that it is more expensive, more liable to get 

 out of order, and more accurate than the necessities of the case frequently 

 require. A hunting case pocket compass with a bright reflecting inner 

 surface of the cover was provided with a slot, one millimeter in width, 

 reaching from the top to the north point of the compass dial. By holding 

 this compass in the ordinary position for taking sight with the open cover 

 at a distance of the diameter of the compass dial from the eye, it will be 

 found that objects can be seen with sufficient distinctness to enable one to 

 centre them easily, and at the same time the position of the needle can be 

 read off by its reflection in the movable cover. This slot also takes the 

 place of the gnomon in the ingenious compass devised by Major T. B. 

 Brooks, some ten years ago, and like that compass may be used to deter- 

 mine the variation of the magnetic needle. In the absence of local attrac- 

 tion, the time of noon being known, the true north can be determined, 

 and hence the magnetic declination of the place. Or the variation being 

 known, the time of day can be determined by suitable marks on the peri- 

 phery of the disc, or the variation being known, the needle can be used to 

 discover the direction and amount of local magnetic attraction. 



