Light Science for Leisure Hours

discovery was impressed upon me in a very unpleasant manner. I had purchased a pocket-compass, and was very anxious-not, indeed, to test the instrument, since I placed implicit reliance upon its

ecognised, matters assumed a less cheerful aspect. We were unwilling to compromise our dignity as 'explorers' by asking the way-a proceeding which no precedent in the history of our favourite travellers allowed us to think of. But evening came on, and with15 it a summer thunder-storm. We were getting thoroughly tired out, and the h?c olim meminisse juvabit with which we had been comforting ourselves began to lose its force. When at length we yielded, we learned that we h

robably discovered a long time ago, for 800 years before our era the Chinese applied the magnet's directive force to guide them in journeying over the great Asiatic plains, and they must soon have detected so marked a peculiarity. Instead of a ship's compass, they made use of a magnetic car, on the front of which a f

d it on the 13th of September, 1492, during his first voyage, and when he was six hundred miles from Ferro, the most westerly of the Canary Islan

the declination is towards the west in Europe. In Columbus's time it was towards the east. Thus

iarity exists; they seek to determine its extent, how far it is variable-whether from time to time or from place to plac

n, and the western parts of Australia; nearly the whole of the Atlantic Ocean; Greenland, the eastern parts of Canada, and a small slice from the north-eastern part of Brazil. All these form one region of westerly declination; but, singularly enough, there lies in the very heart of the remaining and larger region of easterly magnets an oval space of a contrary character. This space includes the Japanese Islands, Manchouria, and the

magnetic needle from a full obedience to the true polar summons. Or the comparative effects of oceans and of continents might be called into play. But unfortunately for all

icist, was the first (I believe) to point out that 'the progressive movement of the magnetic needle towards the west appeared to have become continually slower of late years' (he wrote in 1814), 'which seemed to indicate that after some little time longer it might become retrograde.' Three years later, namely, on the 10th of February, 1817, Arago asserted definitively that the retrograde movement of the magnetic needle had commenced to be perceptible. Colonel Beaufoy at first oppugned Arago's conclusion, for he found from observations made in London, during the years 1817-1819, that the westerly motion still continued. But he had omitted to take notice of the circumstance, that London and Paris are two different places. A few years l

to 23° west of north. We see that it is now necessary to explain, moreover, how it has happened that this gradual change has ceased, and has given place to a return towards the preceding state of the globe.' 'How is it,' he pertinently asks, 'that the directive acti

easterly magnets now runs, as we have said, across Canada and eastern Brazil in one hemisphere, and across Russia, Asiatic Turkey, the Indian Ocean, and West Australia in the other, besides having an outlying oval to the east of the Asiatic continent. These lines have swept round a part of th

nishing, and as worthy of careful study as any embraced in the w

from a silk thread without torsion,-it will be found to exhibit each day two small but clearly perc

eight in the morning, returns towards the west to attain its greatest westerly excursion at a quarter-past one. It then moves again to the east, and having reach

l, never exceeding the fifth part of a degree, while the mean position of the needle lies some 20° to the west of north,

easy to trace in M. Arago's results a sort of effort on the part of the needle to turn towards

observation of our suspended needle. Such a variation has been long since recognised. It is found that

is clear, therefore, that if we travel from one hemisphere to the other we must find the northern dip of the needle gradually diminishing, till at some point near the equator the needle is horizontal; and as we pass thence to southern regions, a gradually increasing southern inclination is presented. This has been found to be the case, and the position of the line along which there is no inclination (called the magnetic equator) has b

on to the Antarctic Seas was the discovery, if possible, of the southern magnetic pole. In this he was not successful. Twice he was in hopes of attaining his object, but each time he was stopped by a barrier of land. He approached so near, however, to the pole, that the needle was inclined at an angle of nearly23 ninety degrees to the horizon, and he was able to assign to the

less than 75°; in other words, the needle was inclined only 15° to the vertical. In 1791 the inclination was less than 71°. In 1831 it was less than

hing at London and Paris, the magnetic equator must be approaching these places, and this (in the present position of the curve) can only happen by a gradual shifting of the magnetic equator from east to west alon

e, for instance, be further from Paris now that the needle is inclined more than 23° from the vertical, than in 1671, when the inclination was only 15°

e intensity of the magnetic action-the energy with which the needle seeks its position of rest. This is not only vari

coincident with the northern magnetic pole, which we have seen lies to the north of the American continent, there are two northern poles, one in Siberia nearly at the point where the river Lena crosses the Arctic circle, the other not so far to the north-only a few degrees north, in fact, of Lake Superior. In25 the south, in like manner, there are also two poles, one on the Antarctic circle, about 130° E. long., in Adélie Land, t

l element which seems to correspond with the law discovered by Sabine? There is one very important element. The position of the perihelion of the earth's orbit is such that the earth is nearest to the sun on about the 31st of December or the 1st of January. There seems nothing rashly speculative, then, in concluding that the sun exercises a magnetic influence on the earth, varying according to the distance of the earth

the discovery by Sabine that the moon exercises a distinctly traceable effect upon the magnetic needle seems to me a very remarkable one. We receive very little light from the moon, much less (in comparison with the sun's light) than most persons would suppose, and we get absolutely no perceptible heat from h

f a stately ship at27 anchor on the scarce-heaving breast of ocean. Suddenly a change is noted; irregular jerking movements are perceptible, totally distinct from the regular periodic oscillations. A magnetic storm is in progress. But where is the centre of disturbance, and what are the limits of the storm? The answer is remarkable. If the jerking mo

the aurora in high latitudes, northern and southern. Probably they never happen without such a display, but numbers of auroras escape our notice. The con

times last for sever

n the magnetic needle, the question will naturally arise, Has the sun

rate of about 7,000 miles in a minute, across a part of the solar disc. Now it was found afterwards that the self-registering magnetic instruments at Kew had made at that very instant a strongly marked jerk. We learned, also, that at that moment a magnetic storm prevailed at the West Indies, in South America, and in Australia. The signalmen in the telegraph statio

ematic increase and diminution in the intensity of magnetic action, and that the period of the variation was about eleven years. But at the same time, a diligent observer29 had been recording the appearance of the sun's face from day to day and from year to year. He had found that the solar spots are in some years more freely displayed than in others. And

for a cause producing at once both sets of phenomena. There is as yet no certainty in this matter, but it seems as if philosophers would soon be able to trace in the disturbing action of the plane

Magazine, June 18