FOR THE LOCAL ATTRACTION OF SHIPS. 25 



guage of my introductory remarks: Mr. Barlow, having given 

 a rational form to the laws of magnetic attraction, which thus 

 acquired a basis in his mind, all that remained was to dis- 

 pose the appropriate species of matter, in an order correspond- 

 ing to those laws : Since the action of any mass of iron or 

 system of masses of iron may be referred to two points, indefi- 

 nitely near to each other, in the general centre of attraction of 

 such mass or system, it followed that, in a ship, all the action 

 might be referred to a fixed point in the vessel, and that the 

 line joining that point and the pivot suspending the needle, 

 would be fixed in position, as regarded the ship, in all parts of 

 the world ; and secondly, since the whole magnetic power of 

 iron resides only on the surface, it is easy to procure a plate of 

 that metal, of inconsiderable weight, that shall have, when 

 placed near the compass, an effect upon the needle equal to 

 that of the great mass of iron, or collection of masses, in the 

 vessel, situated at a greater distance ; and if also the centre of 

 attraction of this plate be fixed in the line above mentioned, 

 then in whatever direction the vessel is placed, or in whatever 

 part of the world the observation may be made, the plate will 

 always produce a disturbance on the needle equal to that of 

 the ship ; so that by obtaining the one by observation, the 

 other, that is, the disturbance produced by the ship, becomes 

 known. Hence, whatever may be the law by which this dis- 

 turbing power is governed, or whatever may be the dip of 

 the needle or the direction of the ship's head, the plate and 

 the iron of the vessel attract the needle with the same force, 

 both in quantity and in direction, and the one becomes known 

 by the observation made on the effect of the other. The plate 

 is to be fixed at a certain depth and distance in the line of no 

 attraction of the vessel *. 



Accordingly, such an instrument, called " Barlow's Cor- 

 recting- Plate," has been introduced into the service, for the 

 purpose of thus counteracting the local attraction of vessels ; 

 and its efficacy having been tried by the most experienced 

 navigators, in every part of the world, it has been found most 

 perfectly to answer this purpose. Few inventions, however 

 perfect in theory, have proved so perfect in practice as this; 

 a result the more remarkable and interesting, on account of 

 the original difficulties of the subject, and the purely philoso- 

 phical grounds of the practical application. 



In this, again, we have an example of what abstract science 

 can effect, indeed of what it has effected, for the advancement 



* Edinburgh Philosophical Journal, vol. xi. p. 65 et seq. Ample details 

 of Mr. Barlow's researches will he found in his " Essay on Magnetic At- 

 tractions." 



