796 



PLATE XLI. 



fig. 569. The form of the curves which show the 

 jiirection of the magnetic needle, in cotisequence of 

 the attraction and repulsion of two poles, situated at 

 A and B. They are found by drawing the lines A C D, 

 BED, so that the sura or difterence of the parts AC' 

 BE, shall be always equal, ACEB being a semi- 

 circle : and the direction D F may be found by making 

 AF to BF as the cube of AD to that of DD. P. 

 688. 



Fig. 570. The arrangement of iron filiogs in the 

 neighbourhood of a magnet. P. 688. 



Fig. 571. The particle of iron A B, lying on a card 

 nearly over the magnet C, assumes, when the card in 

 shaken, first the position D, then, falling to E and E, 

 is left a little further from the magnet than at first. 

 P. 689. 



Fig. 572. An azimuth compass. The box is turned 

 round, until the shadow of the thread A B or AC falls 

 on the . line C D : the position of the needle is then 

 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'Cullbch makes the 

 bottom of the box in the form of a hollow cone, rest- 

 ing on a point, and loaded with a weight, which brings 



the centre of gravity below the point of support, as at 

 H. P. 689. 



Fig 573. A dipping needle. The piece A B 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 chajiged, 

 either by turning the stand half round, or by turning the 

 needle within the stand. P. 689. 



Fig. 574 . . 576. The situations of the lines of equal 

 declination in 1700, 1744, and 1794, in the hemi- 

 sphere, which is bisected by the meridian of London. 

 The first two from Mouiitaine's Tables, the last from 

 Churchman's Chart. P. 691. 



Fig. 577. The actual situations of the lines of equal 

 dip. From Churchman's Chart. P. 69?. 



Fig. 578. The lines of equal dip, calculated from 

 the supposition of a small magnet, situated at the 

 centre of the eartli, directed to a point in latitude 

 75° N. and longitude 70° W. P. 699. 



Fig. 579. A, Six's thermometer; B, the wire with a 

 fine spring, which serves as an index. P. 697. 



Fig. 580. Rutherford's double thermometer. P. 697. 



Fig. 581. Deluc's whalebone hygrometer. A, the 

 slip of whalebone; B, a spiral spring, serving to kee{) it 

 stretched; C, the index. P. 710. 



PLATES XLII, XLIIL 



Fig. 582. A chart of the world, on Mercator's pro- year 1794; and with the trade winds and monsoons, P, 

 jection, from Arrowsmith ; with the dip and variation £71,691. 

 •f the coxapass, priacipally from Cburchman, for the 



