20 DERANGEMENT OF THE COMPASS BY THE 



which, independent of chance, have resulted from scientific 

 investigations and experiments directed to a specific object. 

 Of these, the Safety-lamp of Sir H. Davy, and his present 

 chemico-electric guard to the copper of vessels, and the Cor- 

 recting- Plate of Professor Barlow, form memorable examples ; 

 and in all these cases, the value of the discovery is only equal- 

 led by the extreme simplicity of the application." 



Nearly seven hundred years have now elapsed, since the 

 compass began to be employed, in Europe, as a nautical in- 

 strument; and yet it is only within a short period that an im- 

 perfection has been discovered in it, which detracts much from 

 its real value for such a purpose; namely, that, with the ship's 

 head at different points, the needle does not continue to point in 

 the same direction; the difference, in some cases, being so great 

 as to lead to the most fearful errors and uncertainties. The ge- 

 neral nature of this effect will be understood, by considering, 

 that the upper parts of all masses of iron attract that end of the 

 compass needle, which, when freely suspended, dips below the 

 horizon, that is, the north end in the northern hemisphere of 

 the earth, and the south end in the southern hemisphere. 

 Now, in vessels of all kinds, the iron which enters into their 

 construction or equipment, is nearly symmetrically disposed 

 with respect to the longitudinal section of the ship. Sup- 

 posing the ship to be divided from stem to stern, equidistant 

 from the sides, by a mathematical plane, such as would be 

 formed if the vessel were so cut in halves, but the halves not 

 separated from each other, there is an equal quantity of iron 

 on each side of this section. When therefore this section is in 

 the magnetic meridian of any place, or in other words, when 

 the direction of the freely-suspended needle is the same as 

 that of the section, which can only happen when both point 

 to the magnetic north and south, then the action of the 

 iron in the vessel is either coincident with, or directly op- 

 posed to, the magnetic action of the earth : in either case 

 the lateral direction of the needle is not disturbed, it is not 

 drawn either towards the east or towards the west. But 

 when the ship's head is on any other point of the compass, 

 and particularly when it is towards the east or the west, the 

 two forces acting on the needle, that of the earth and that of 

 the iron of the vessel, being exerted nearly at right angles to 

 each other, one drawing the needle towards the north, and 

 the other drawing it towards the east or west, it becomes 

 drawn more or less out of its natural position, according to 

 the direction of what is called the resultant of these two forces ; 

 that is the mean direction which results from one of them 

 drawing the needle one way, and the other drawing it the other 



