C. GENERAL PHYSICS. 147 



of these compasses is sufficiently exact for tlie purposes of 

 navigation, but is not entirely satisfactory in a scientific 

 point of view. It is questionable whether the theory really 

 represents the semicircular as "well as it does the quadrantal 

 deviation, and to settle this point there is great need of more 

 observations. The so-called permanent and sub-permanent 

 magnetism of the ship were undergoing a constant and rapid 

 chano-e during the progress of its voyage, such a change as 

 would correspond to a transfer of magnetism from aft for- 

 ward, and to a rotation from right to left in the direction of 

 the force. The ratio of the hard to the soft iron force was 

 slpwly varying at each compass, and for the different com- 

 passes it ranged between four and sixty-seven. There were 

 compasses on board at which the attraction of the hard and 

 soft iron forces coincided, from which it follows that in no 

 case was the ratio of the hard to the soft iron force the same 

 in the coefficient "B" (of Poisson's notation) as it was in the 

 coefficient "C." So far as can be judged from the observa- 

 tions discussed in this report, in the case of a vessel " swung" 

 for the first time, it is impossible to make any reliable estimate 

 of the ratio of the hard to the soft iron force in the coeffi- 

 cients "B" and " C," and, therefore, it is also impossible to 

 make any reliable estimates of the changes that the devia- 

 tions of the compasses will undergo on a change of magnetic 

 latitude. HarJcness' Observations on Terrestrial 3fag7ietism, 

 Washington^ 18lS. [Smithso7iian Contributions.) 



THE BEST CONDITIONS FOR ELECTRO-MAGNETS. 



Du Moncel has communicated to the French Academy the 

 results of his study upon the problem. What are the dimen- 

 sions to be given to the electro-magnet, and the dimensions 

 of the wire to be employed in the coil, in order to attain the 

 best possible conditions, the resistance of the circuit and the 

 force of the battery being given? He has, 



1. Established the principle that a given helix produces its 

 maximum effect when its own resistance {x) is greater than that 

 of the remainder of the galvanic circuit in the ratio of 1 to 1 -f-a?. 



2. That, for the same diameter of the coil, the helix which 

 gives the best results is that whose thread has a size and a 

 length such that its resistance is equal to that of the exterior 

 circuit. 



