130 Astronomical and Nautical Collections, 



Mr. Barlow has also proposed to employ his instrument in a 

 manner which may be called the reverse of that which has been 

 here described. Before the ship sails, he finds, by trial, a position 

 ofthe plate fixed near the compass, such that, in all directions of 

 the ship, it produces the same deviation with that of the ship itself, 

 and in the same direction. Hence he infers, that if the deviations 

 are not very great, the united actions of the iron and of the plate 

 will occasion deviations about twice as great as those which either 

 cause would produce alone. So that when the true variation is 

 required in the course of the voyage, the observation being made 

 twice in succession, once with and once without the plate, the dif- 

 ference of the two directions gives a measure of the effect ; and 

 we obtain the true natural variation, by adding this difference tp 

 the variation first observed, when the employment of the plate has 

 diminished it, and by subtracting the diflference, when the plate has 

 increased the variation. 



In order to form an estimate of the degree of generality of this 

 mode of correction, I have inquired whether we could produce 

 with a single sphere, for every direction of the earth's magnetism, 

 the same deviations in a horizontal needle as are due to a system 

 of spheres given in magnitude and in position, and magnetrzed, as 

 well as the required sphere, by the influence of the earth. The 

 investigation shows that this is only possible when the bodies fulfil, 

 as in the former case ofthe destruction of their effect, two separate 

 conditions ; a certain quantity depending on the magnitudes and 

 the dispositions of the given spheres must become evanescent, and 

 another quantity which, in the former case, was to be positive or 

 evanescent, must now be evanescent or negative. This method, 

 therefore, is not more extensively applicable than the former ; its 

 application is less simple ; and it becomes impracticable when the 

 deviations are extremely great, though this is precisely the case in 

 which the remedy is most wanted. 



But when all the given spheres have their centres in the same 

 horizontal plane which contains the compass, it is always possible 

 to destroy their effect, or to imitate it by means of a single sphere 

 placed in a proper situation. Its centre must be in the same plane 



with the rest, and we may find the directions of two right lines 



