620 



MAGNETISM. 



the iron becomes, as it were, a connecting link in 

 the succession of the particles of the magnetic fluid, 

 holding them in a developed state, while the magne- 

 tizing bars are communicating a further degree of 

 intensity. The annexed wood-cut represents Du 



Hamel's method, where A B, A' B' are the two 

 steel bars ; A B' and A' B the two cross pieces of 

 soft iron ; and a 6, a' b' are the two bundles of 

 magnetized bars. .flSquinus improved this method 

 by connecting the ends of the steel bars by strong 

 magnets, instead of soft iron rods. He likewise 

 made the rods to move at a small distance from each 

 other, and inclined them in a contrary direction to 

 that of Du Hamel. 



Horse shoe magnets are formed by bending 

 straight bars somewhat in the form of horse shoes, so 

 that the two extremities may point in one direction, 

 and be near each other. Connect the two extremi- 

 ties with a piece of soft iron, after the manner of 

 Du Hamel. The horse shoe may then be magne- 

 tized after the manner of Du Hamel or /Equinus. 



There are many other facts that might have been 

 stated with regard to the magnetizing of iron, steel, 

 nickel, cobalt, &c., but the nature of this work ren- 

 ders it necessary that we should confine ourselves to 

 the BMre important points in this as well as the rest 

 of the sciences. 



The most simple magnetical instrument is the 

 horizontal needle. This needle consists of a bar of 

 hardened steel, magnetized. The bar is made of 

 various forms, being commonly rectangular, but 

 having a broad portion in the centre. There is a 

 hole made in the middle of the broad portion, 

 which is tapped with a screw, to receive an ex- 

 terior screw turned upon a brass cap, into which 

 is fitted a piece of agate, on which the needle is 

 balanced upon a steel point. This form of the 

 needle is exhibited in the accompanying wood en- 

 graving. 



Another form of the horizontal needle is repre- 

 sented top of next column. 



The needle, thus prepared, is suspended in a box. 

 made of brass or wood. Below the needle a cir 

 cular card is placed, whose circumference is div ided 

 into degrees or minutes, or more commonly int.) 

 thirty-two equal parts, called points. The axis ot 

 the needle passes through the centre of the card. 

 The four cardinal points are marked on the card : 

 and the intermediate divisions are for the purpo se of 

 very nice observations, laid off in degrees and min- 

 utes ; and, should great delicacy be required, the 

 needle is made to carry a vernier. When the appa- 

 ratus is used for land surveying or astronomical 

 purposes, the box is furnished with sights. W hen 

 the compass is used for observations at sea, the card 

 is also suspended upon a point, and the box in w hich 

 it is contained, is hung upon gimbols; the whole 

 forming a sort of universal joint, in order that the 

 needle and card may retain the horizontal position, 

 independently of the heaving of the ship. The 

 inside of the compass bowl or box is white, with the 

 exception of a black line, called the lubber line, 

 drawn parallel to the keel of the vessel, for the pur- 

 pose of showing the helmsman the inclination of the 

 ship's course to the magnetic meridian. The re are 

 various modifications of the sea compass, or mar iner's 

 compass, as it is commonly called ; but the chief 

 causes of the superiority of the one over the oth er, lie 

 in the excellence of the workmanship, rather than 

 in any slight modifications in the form. 



The Azimuth compass, or that intended for mea- 

 suring the azimuth, or direction of any object, corn- 

 pared with the magnetic meridian, differs little in 

 form from the sea compass, with this exception, t hat 

 it is furnished with sights, and the box in which the 

 needle is suspended is capable of an horizontal motion 

 round a pin in the centre of the bottom. 



Variation compass. A compass only intended for 

 ascertaining small changes in the declination of the 

 magnetic needle, either annual or diurnal, is com- 

 monly more limited in its arc of vibration, and is 

 generally longer than the needles used for more 

 common purposes ; it is also furnished with conveni- 

 ences for reading closer than the common needle; 

 otherwise any needle so mounted and furnished as to 

 be very sensible and capable of being read off to 

 minute divisions might be used as a variation com- 

 pass. 



The following cut represents the variation needle 

 employed by Colonel Beaufoy, in making a valuable 



