THE GUIDING-NEEDLE ON AN IRON SHIP. 297 



care. A quantity of the best steel wire, as thick as a knitting- 

 needle, is selected and cut into lengths to fit the tubes ; bundles of 

 these wires large enough to enter the tubes are made up and 

 tempered to the degree that experience has proved best for hold- 

 ing captive the magnetic charge. This is imparted to each 

 bundle between the poles of a powerful electro-magnet. Of 

 course, every one of the many slender wires that compose a 

 bundle is itself a magnet ; they lie together with their north poles 

 in contact, and likewise their south poles: hence repulsion — a 

 mutually deteriorating influence — is the result ; and indeed oft- 

 times a bundle of wires loses much of its magnetic strength be- 

 cause the steel is not of a quality and temper to resist the destruc- 

 tive force. 



Each bundle is weighed and its magnetic strength tested, and, 

 in placing them on a card, due care is had to the equal distribution 

 of weight and force on each side of the center, for the characteristic 

 of symmetry is ever kept in view. The magnets, except one, are 

 rigidly set in the tubes, and the latter sealed ; the movable mag- 

 net has screws at one end for the purpose of adjusting the mag- 

 netic axis of the whole system to that diameter of the card which 

 passes through the north and south points : it would be the 

 imaginary dotted line {Z . . . Z) shown in Fig. 6. 



The card as above described, with tubes, magnets, bulb, rim, 

 brass cap, and jewel all in place, weighs many ounces — a heavy 

 weight for the delicate force of terrestrial magnetism to turn about 

 on a pivot, how highly soever both this and the jewel may be 

 polished. 



The essential principle observed in the manufacture of the 

 instrument is to reduce the friction on the pivot to a minimum 

 and increase the moving power — the strength of the magnets — to 

 a maximum ; and this object is greatly furthered by the introduc- 

 tion of the liquid : its buoyant effect upon the card reduces the 

 pressure of several ounces to that of a few grains. 



The liquid has another advantage — it steadies the card, pre- 

 vents all those small oscillatory movements that characterize a 

 dry or air compass, while at the same time enabling the magnetic 

 power to cope more efficiently with its burden. The liquid must 

 fill the bowl completely, otherwise an air-bubble would gather 

 and impede the free motion of the card. 



The same compass may guide a ship into all climates — polar 

 seas and tropical oceans ; but as every change of temperature 

 causes a varying expansibility of the copper bowl and the liquid 

 in it, the former, when filled to complete fullness, would soon 

 burst, were no provision made for expansion. To prevent this, 

 an air-tight case of thin flexible metal is placed in the bottom of 

 the bowl, which contracts or expands with every changing press- 



