NATURAL PHILOSOPHY. 159 



use in steering. One way of effecting this is by attaching at 

 opposite sides, to the rim of the cover, a semi-circuhir arc or band 

 of a convenient width, having along its middle a narrow slit, by 

 means of which it may be directed to the sun or other heavenly 

 body; or through which the sun's light, shining over the centre, 

 and on the edge of the card, shows by a bright streak on a dark 

 ground the compass-bearing by observation. This being com- 

 pared with the bearing, determined astronomically, gives a differ- 

 ence, which is the error or deviation of the compass from the true 

 meridian. On a surface projecting from the rim of the bowl, or 

 on the rim of the cover, are graduations, which are read as usual 

 by a zero mark on the other rim. When the sun's light is too 

 faint to give a distinct streak, or in observations of moon, star, or 

 planet, the object may be viewed through the slit, either directly, 

 or as reflected from the glass of the cover beneath the slit. In 

 such cases the observation is made by taking the usual reading of 

 the card at the lubber line, and also the reading on the rim, giving 

 the angle between the lubber line and the object. According to 

 their position, the sum or the difference of these readings gives 

 the compass bearing of the object ; and this compared with the 

 true azimuth, gives the error or deviation from the meridian. 



As a high wind acting on the continuous arc may cause the 

 compass to have a tremulous motion, in order to avoid or lessen 

 this inconvenience in such cases, the arc is removed and replaced 

 on the side toward the object by a short piece about an inch high, 

 and on the other side by a shorter piece, each piece having in it, 

 as in the arc, a narrow slit. The piece toward the object being 

 fitted with a reflector, which may be either of the usual sort, with 

 a hinge so as to be turned according to the altitude of the object, 

 or it may be a portion (about an octant) of a glass cylinder fixed 

 horizontally; the object reflected in either of those ways may thus 

 be viewed through the slit or hole on the opposite side. There is 

 yet another way of attaining the end in view. Graduate a rim of 

 the bowl or cover of the common binnacle compass, putting a 

 proper zero-mark on the other rim, by turning the cover so as to 

 bring a bar of the roof into the shadow of the opposite bar, — the 

 zero-mark indicates the angle between the object and the ship's 

 head. This, with the azimuth of the object and the usual reading 

 of the card suffices, as above shown, to give the true meridian, 

 and the deviation of the compass from it. — Scientific American. 



CONSTANT GALVANIC CURRENT. 



The following observations may have occurred to others, but 

 not having met with them published, they may be of value, as 

 tending to the perfection of our scientific instruments, by provid- 

 ing the source of a constant galvanic current, of large quantity 

 and very great intensity. Tiie bichromate of potash battery fur- 

 nishes a current of great force, and its simplicity, economy, and 

 convenience of management would make it preferable to the 

 double fluid batteries, but for its want of constancy when a current 



