MAGNETISM. 



tain ing the compass there is frequently 

 inserted a nut or stop, which, when 

 pressed, bears, by means of a lever 

 within the box, against the card, and 

 arrests its movement : thus giving the 

 opportunity of reading off the number 

 of degrees more at leisure, and there- 

 fore more correctly, than could be done 

 at the moment of observation. 



(270.) Sometimes the sights are, for 

 the convenience of carriage, made, by 

 means of joints at their feet, to fold 

 down over the glass which covers the 

 box, when the compass is not in use. 

 In other cases, they are united by a 

 transverse bar, and made so as to be 

 capable of being removed from the box 

 when the instrument is set aside. 



(2 71.) The instrument, in its common 

 form, as above described, is still ex- 

 ceedingly defective, and incapable of af- 

 fording any very accurate observations. 

 Sufficient evidence of its imperfections 

 may be collected from the narrative of 

 Captain Phipps's voyage; from which 

 it appears that, although the observa- 

 tions were made with all possible care, 

 yet differences in the variation were fre- 

 quently occurring, in the same place 

 and at the same time, amounting to 

 two, three, and four degrees. In one 

 instance, the error was even five de- 

 grees ten minutes. 



(272.) The azimuth compass contrived 

 by Captain Kater is a much more per- 

 fect instrument, combining all the ad- 

 Vantages of the ordinary construction, 

 together with those of being extremely 

 portable, and of being adapted to every 

 purpose of observation, whether at sea 

 or on land. A short description of this 

 compass, but without any figure, is given 

 in the Instructions for the Adjustment 

 and Use of the Instruments intended for 

 the late Northern Expeditions, which 

 were printed by order of the Royal So- 

 ciety in 1818. We have been enabled 

 to illustrate the following account of 

 this instrument by the annexed figures, 

 exhibiting its construction ; for the op- 

 portunity of drawing which we are 

 indebted to the kindness of Captain 

 Kater. 



AB, fig. 64, is a brass cylindrical 

 box, containing the compass, of which 

 the card, CD, is five inches in diameter. 

 The needle, which is perforated in its 

 centre to admit of an agate cap, set in 

 brass, for the purpose (if suspension, is 

 fixed to a circular pir e of talc, over the 

 circumference of whi i a narrow circu- 

 lar ring of card is laid ; the outer mar- 



gin of this card is accurately graduated 

 into half degrees. The breadth of the 

 cylindrical box is exactly one inch, and 

 it is covered as usual by a piece of 



Fig. 64. 



m 



glass. A slanting piece of ivory is fixed 

 to the inner side of the box, so as just 

 to come over the outer edge of the gra- 

 duated circle of the card : a line, at right 

 angles to the circumference of this 

 circle, is marked upon the ivory, to 

 serve as an index for reading off the 

 degrees in the manner to be presently 

 mentioned. 



To the opposite side of the box, at O, 

 a sight is affixed, consisting of a brass 

 frame, in the form of a parallelogram, 

 five inches long. To this frame is 

 adapted a shorter frame, Yf, two inches 

 in length, which slides upon it, and carries 

 the segment of a glass cylinder, ground 

 to a radius of five inches. By means 

 of this piece of glass, when presented to 

 the sun, the rays are collected into a 

 linear focus ; the line of light being 

 thrown on the index on the piece of 

 ivory, may be seen at the same time as 

 the degrees on the card. 



This sight has a hinge where it is con- 

 nected with the box, by means of which 

 it can be folded down upon the glass 

 cover of the box, as seen in fig. 66 : 

 and when thus folded, it raises the 

 needle of the compass by means of a 

 lever under its centre, seen at L, so as 

 to press it against the glass cover, and 

 prevent its moving. 



The sight, to which the eye is applied, 

 and which is shown separately in fig, 

 65, is an inch in height from its hinge 

 to its upper point ; but it may be raised 

 somewhat higher by means of an up- 



