C. GENERAL PHYSICS. 201 



stability of the circular needle, as shown by its oscillations 

 to the right and left, is greater than that of the ordinary 

 needle; and its moment of inertia is, in fact, equal in all po- 

 sitions. No difficulty is experienced in locating the position 

 of the magnetic axis of the circular needle. These needles 

 are magnetized instantaneously by means of a powerful soft- 

 iron electro -magnet. He concludes that the circular com- 

 pass-needle is an instrument worthy of navigators ; and by 

 perfecting its construction we shall come into possession of 

 a simple, sensitive, stable instrument, constituting a veritable 

 improvement on the present arrangement. It is even stated 

 that the extreme sensibility that can be given to this instru- 

 ment may render it advantageous in magnetic observations. 

 A portion of the errors of the instrument, due to local at- 

 tractions, may be corrected by means of a circular magnet ; 

 but this is not to be recommended, as new complications are 

 thereby introduced. Bull. Ilebcl. Assoc. Scientifique. 



COKEECTIOXS OF THE COMPASS OX IEOX SHIPS. 



From an elaborate memoir, by Garbich, on the theory and 

 practice of the deviation in compasses on iron ships, we take 

 the following directions for effecting the compensation of 

 the compasses. In order to avoid the employment of large 

 masses of iron, it is best to use two iron rods placed diamet- 

 rically opposite to each other. To determine exactly the 

 distance of these rods from the centre of the compass, it is 

 best to turn the ship's head toward that point in the hori- 

 zon at which the quadrantal deviation is a maximum, after 

 first allowing for the semicircular deviation ; then, by mov- 

 ing the rods to or from the centre of the compass, to annul 

 the maximum quadrantal deviation. This error being thus 

 compensated, it will be found that a portion of the rolling 

 or heeling deviation is also removed. The semicircular de- 

 viation is then best compensated, by means of two magnets, 

 as follows : Under the centre of the compass is fastened a 

 non-magnetic metallic parallelopipedon of square section, one 

 side of which is parallel to the keel ; the lower side of this 

 should be fastened to a metallic disk of the same material, 

 and this so fastened by screws to the base of the binnacle 

 that its position can not be altered with reference to the 

 keel of the vessel. On the upper end of this parallelopipc- 



12 



