DESCRIPTION OF PLATES. xliii 



ascertained by that of the card E, which is fixed on it. The compass is kept always 

 in a horizontal position, by means of a double suspension on the gimbals E G. 

 Instead of this suspension, Mr. M'Culloch makes the bottom of the box in the form 

 of a hollow cone, resting on a point, and loaded with a weight, which brings the 

 centre of gravity below the point of support, as at H. P. 535. 



Fig. 573. A dipping needle. The piece AB is brought into such a situation, 

 that the line drawn on it coincides with the middle of the vibrations of the needle. 

 The position of the needle may be changed, either by turning the stand half round, 

 or by turning the needle within the stand. P. 535. 



Fig. 574 . . 576. The situations of the lines of equal declination in 1700, 1744, 

 and 1794, in the hemisphere, which is bisected by the meridian of London. The 

 first two from Mountaine's Tables,* the last from Churchman's Chart.f P. 536. 



Fig. 577. The actual situations of the lines of equal dip. From Churchman's 

 Chart. P. 537. 



Fig. 578. The lines of equal dip, calculated from the supposition of a small 

 magnet, situated at the centre of the earth, directed to a point in latitude 75 N. and 

 longitude 70 W. P. 537. 



Fig. 579. A, Six's thermometer ; B, the wire with a fine spring, which serves as 

 an index. P. 545. 



Fig. 580. Rutherford's double thermometer. P. 545. 



Fig. 581. Deluc's whalebone hygrometer. A, the slip of whalebone ; B, a spiral 

 spring, serving to keep it stretched ; C, the index. P. 554. 



PLATES XLII. XLIII. 



Fig. 582. A chart of the world, on Mercator's projection, from Arrowsmith ; 

 with the dip and variation of the compass, principally from Churchman, for the year 

 794 ; and with the trade winds and monsoons. P. *437, 536. 



* Ph. Tr. 1757, p. 329. Account of Methods, 4to, 1758. 

 t Churchman's Magnetic Atlas, 1794. 



