Astronomical and Nautical Collections. 1 25 



may represent a plate of which the thickness varies very slowly 

 near the centre, and decreases from that point to the circumference. 

 Its action on points near its centre must be sensibly the same as 

 that of any other plate of a constant thickness, and of a very great 

 extent. With regard to such a plate, I must here remark, that in 

 the extract of my first memoir, the action of a plate of great extent 

 has been erroneously compared with that of a hollow sphere, of 

 which the ray is supposed to increase without limit ; for the por- 

 tion of the spherical surface most remote from the attracted point 

 is as much greater in extent as the actions of all its points are 

 weaker, so that its total action is a finite quantity, which is not to 

 be neglected, as I had supposed. [Whether or no this correction was 

 suggested by the doubt expressed in the translation inserted in 

 these collections,'the ingenious author has not thought it necessary 

 to inform us ; but the having been able to afford a useful hint, to 

 a mathematician like Mr. Poisson, is an occurrence too flattering to 

 the translator's vanity to allow him to pass it by in silence.] 



In a similar manner we may assume that a spheroid greatly 

 elongated approaches very near to the form of a needle or bar, of 

 which the diameter decreases from the middle to the extremities, 

 varying at first very slowly ; and its action on points near its mid- 

 dle can differ but little from that of a bar, of which the diameter is 

 constant, and very small in proportion to its length. When, there- 

 fore, we have experimentally observed the actions of a bar, or a 

 plate, magnetized by the influence of the earth, on points very near 

 the middle points of these bodies, we may compare our theory with 

 observation in this new point of view. In order to facilitate this 

 comparison, 1 have taken care to enunciate distinctly, in my memoir, 

 the principal consequences of the calculation, which appear to be 

 most deserving of experimental confirmation. 



The second paragraph of the memoir relates to a question which 

 is curious with regard to the theory, but still more important in a 

 practical point of view, and which has lately excited much atten- 

 tion in England. I allude to the means of destroying the deviations 

 to which the compass is subject on board of a ship, and which are 

 occasioned by the magnetic action of the guns, the anchors, and 



