CHAMBERS'S INFORMATION FOR THE PEOPLE. 



eight o'clock, it begins a westerly sweep, crossing 

 its mean position about 10 A.M. Its maximum dis- 

 tance to the west it reaches at i P.M. ; when it again 

 begins its easterly course, coming up once more 

 to its mean position about seven o'clock in the 

 evening. 



Besides these regular variations of the declina- 

 tion, there are sudden and unforeseen disturbances, 

 caused by what are called magnetic storms. Earth- 

 quakes and volcanic eruptions have a marked 

 effect on the needle. The Aurora Borealis is 

 invariably accompanied by its disturbance ; and 

 the appearance of solar spots, which are most 

 numerous every ten years, has been associated 

 with special magnetic irregularities. 



Inclination or Dip. If an unmagnetised needle 

 be balanced about an axis, so as to remain hori- 

 zontal, it will, after being magnetised, and placed 

 in the magnetic meridian, no longer lie evenly, but 

 will point downwards. This dip or inclination, as 

 it is termed, of the needle to the horizontal is owing 

 to the position of the earth's magnetic poles. They 

 are deep in the interior of the globe, and by no 

 means coincide with the poles of its axis. Now, 

 if we take a small balanced needle, which can 

 swing vertically up and down, and move it over 

 a bar-magnet, we find that when it is above the 

 middle of the bar, it remains horizontal, its ends 

 being equally attracted by the poles of the magnet. 

 When brought over one of the poles, it stands 

 upright, and at any position between, it is more 

 or less in an inclined direction. Exactly similar to 

 the action of the bar-magnet is that of the earth. 

 A balanced needle will be differently inclined, 

 according to its distance from the two poles. 



Over the north magnetic pole, it stands upright, 

 with its north pole down. In 1831, Captain Ross 

 came upon a spot in 70 5' N. lat. and 263 14' E. 

 long, where the needle stood vertical. That spot 

 must therefore be right over the north magnetic 

 pole. Near the equator, again, there is no dip, 

 and the needle lies horizontal. A line drawn 

 through all such places of no dip is called the 

 magnetic equator. It is an irregular line near to, 

 but not identical with the earth's equator. Charts 

 have been drawn up with lines shewing all places 

 where the dip is the same. To these the name of 

 isoclinic lines and charts is given. 



Like the declination, the dip is subject to period- 

 ical as well as irregular variations. At the begin- 

 ning of the present century, the dip was 70 35' 

 from the horizontal in London, and it has since 

 then been gradually diminishing. In London, it 

 is at the present time about 67 50', and there is 

 an annual decrease of about 2 '6'. It is found that 

 the diurnal or daily variations, as well as the 

 disturbances of magnetic storms, affect the inclina- 

 tion at the same time as they affect the declination 

 needle. 



Magnetic Intensity. If a nicely poised needle 

 be made to swing on a pivot or axis near a magnet, 

 it will vibrate more quickly, or more slowly, accord- 

 ing to the strength of the magnet, and according 

 to its distance from it At the middle of the 

 magnet, it will not vibrate so actively as at the 

 ends ; and the number of vibrations it will make 

 in any time, say in one minute, will be an esti- 

 mate of the magnetic force affecting it. Precisely 

 in this way, then, is the strength or intensity of 

 the earth's magnetism found at any place on its 

 surface. At the equator, the needle, oscillating 



260 



most slowly, shews the magnetic intensity to be 

 weakest there. As we go north or south, it swings 

 somewhat more quickly, indicating an increase ot 

 magnetic force. 



These three indications, then, of the declination, 

 as shewn by the compass needle, of the inclina- 

 tion, as shewn by the dipping needle, and of the 

 magnetic intensity, as shewn by the vibrating 

 needle, are termed the magnetic elements of a 

 place. They are different at each place on the 

 earth, and are always changing, so that each spot 

 has its own magnetic history. 



In recent years, much importance has been 

 attached to these changes of the magnetic elements. 

 An immense amount of labour has been spent in 

 observing and registering the variations of the 

 magnetism at different stations over the globe, 

 but much more will doubtless have to be done 

 before any definite results can be expected. 



The variations that have been found to be 

 regular in their occurrence, having periods either 

 of a year, or of a month, or of a day, can evidently 

 be traced to the influence of the sun or of the 

 moon. These bodies are themselves, in all prob- 

 ability, magnetic like our earth, and they will act 

 as huge magnets at a distance. 



Some of the irregular variations can be traced, 

 as we have said, to definite causes. But of the 

 slow changes which take centuries to effect, no 

 acceptable explanation has yet been given ; and 

 of the causes of other disturbances we know as 

 yet absolutely nothing. 



ELECTRICITY. 



Six centuries before the Christian era, it was 

 known to the Greeks that amber, when rubbed, 

 attracted light objects, such as feathers or pieces 

 of straw ; but for twenty centuries, the fact re- 

 mained an isolated and fruitless one. In the year 

 1600, Dr Gilbert, physician to Queen Elizabeth, 

 recalled this curiosity of nature to the attention of 

 the world, and shewed that many other substances, 

 as well as amber, can acquire a similar power by 

 friction. This attraction he ascribed to the pro- 

 duction in the bodies of a subtle substance, for 

 which he coined the name of electricity, from the 

 Greek word electron, meaning amber. 



For almost two hundred years, the term elec- 

 tricity was confined to the frictional excitement, 

 until, by the discovery of galvanism, it received a 

 wider application. This was but the first of a 

 series of subsequent extensions, and what was, at 

 one time, the whole, forms now but a mere part of 

 the science of electricity. 



The various sources of the electrical excitement, 

 such as friction, chemical action, magnetism, and 

 heat, naturally form the bases for the divisions of 

 the subject. We shall accordingly begin with an 

 account of the electricity of friction, which, if not 

 of most practical importance, is no less interesting 

 in itself than it is instructive. 



FRICTIONAL ELECTRICITY. 



Elementary Facts. A piece of amber, sulphur, 

 glass, or sealing-wax, when rubbed with a dry silk 

 handkerchief, or a woollen cloth, becomes ani- 

 mated with a curious power of attracting small 

 bits of paper, hairs, feathers, and such-like. When 



