260 



SCIENTIFIC RECREATIONS. 



J.S3/W 



Fig. 265. Compass. 



will 



with " counter-irritants " another. But the usual way is to "swing the ship," and 

 so adjust the compass. Swinging the ship means turning her round point by 

 point, and marking the deflection of the needle with reference to a certain 

 object. The amount of deflection at each point is read and noted, and 

 subsequently taken into consideration when sailing. 



The Azimuth Compass is a mariner's compass fitted with brass uprights 

 slit through the centre, through which the heavenly bodies may be seen. 



These are the sights. The card is divided into 

 degrees and quarters. A fine wire is fixed upon 

 one of the sights, and in the other slit is a prism 

 to reflect the divisions of the card to the eye. 

 The object the azimuth distance of which it is 

 desirable to know is looked at through the slit, 

 and bisected by the wire. The divisions of the 

 scale are at the same time reflected, and the 

 number read gives the azimuth distance required. 



The compass has led us away slightly from 

 our consideration of the electro-magnet, but we 

 now examine it and its effects as briefly as possible. 

 An electro-magnet is formed by wrapping a copper wire round a piece 

 of soft iron shaped like a horse-shoe ; the wire should be insulated with silk. 

 If the wire be wound round the iron in the same direction, and a current be 

 merely sent through the coil, it will be found that the horse-shoe iron is highly 

 magnetic, but if the current be stopped the power is lost. Such magnets 

 will carry weights much heavier than themselves, and by careful consideration 

 of certain laws, and with reference to the number of 

 coils and the strength of the current, these magnets will 

 sustain a weight some thousands of times greater than 

 their own weight. 



If we cover a non-magnetic piece of iron with a 

 wire coil, and taking a magnet turn it rapidly beneath 

 the wire-bound iron, so that the magnetic poles approach 

 each other alternately, an electrical current will be 

 generated in the wire. The electro-magnetic machine 

 is thus made ; but although strong currents may be 

 generated as a source of motive power it is a failure. 



To Faraday our knowledge of magneto-electricity is due. " He knew" 

 (says Professor Tyndall in his interesting work, " Faraday as a Discoverer ") 

 " that under ordinary circumstances the presence of an electrified body 

 was sufficient to excite by induction an unelectrified body. He knew 

 that the wire which carried an electric current was an electrified body, and 

 still all attempts had failed to make it excite in other wires a state similar 

 to its own." 



But while he was making his experiments on the induction of electric 

 currents he noticed that at the time the current was passing from the battery 



Fig. 266. Electro-Magnet. 



