Light Science for Leisure Hours

ould be looked upon as a maniac who should speak of the influence of Jupiter's moons upon the cotton trade. Yet, as he proceeds to show, there is an

of a sovereign's intrinsic value, we find ourselves led to the diurnal motion of the stars, and that by no very intricate path. For, What is a sovereign? A coin containing so many grains of gold mixed with so many grains of alloy. A grain, we know, is the weight of such and such a volume of a certain standard substance-that is, so many cubic inches, or parts of a cubic inch, of that substance. But what is an inch?31 It is determined, we find, as a certain fr

parent diurnal motion of the sidereal system, that is, on the real diurnal rotation of the earth. We may look on the meridian-plane in which the great transit-telescope of the Greenwich Observatory is made to swing, as the gigantic hand of a mighty dial, a h

ece,' and it is of the correctness of

of his watch. It would, indeed, be possible to detect any gross change of rate; but for the purpose of illustration I assume, what is indeed the case, that the clock is very accurate, and therefore that minute errors only are to be looked for ev

vations extending over a long interval of time, to determine the length of their rotation-period within a second or two. Supposing their rotation uniform, we at once obtain an accurate measure of time. Supposing their rotation n

(in telescopes of sufficient power) with continents and oceans, which are rarely concealed by vapours, this planet is in other respects fortunately situated. For it is certain that whatever variations may be taking place in planetary rotations must be due to external agencies. Now, Saturn and Jupiter have their satellites to influence (perhaps appreciably in long intervals of time) their rotation-movements. Venus

lues he obtained was34 forty-six seconds too great. He then took a period of two years, and being misled by the erroneous values he had already obtained, he missed one rotation, getting a value two minutes too great. Thirty years ago, two German astronomers, Beer and Madler, tried the same problem, and taking a period of seven years, obtained a value which exceeds the true value by onl

rate of our test-planet with the earth's rate during the past 200 years; and therefore, if the earth's rate vary by more than one-hundredth of a se

ed now, astronomers have looked around them for information available at once on this interesting point. The search has not35 been in v

the evidence offered by the earth was corrected, Newton was prepared at once to accept and propound the noble theory which rendered his name illustrious. Again, men wished to learn the true shape of the earth, and went hither and thither measuring its globe; but the moon, meanwhile, told the astronomer who remained at home a tr

he celestial sphere. You direct a telescope towards the point named, and lo! at the given instant, the promised orb sweeps across the field of view. Each year there is36 issued a thick octavo volume c

measure of confidence to calculations of past events. They know that astronomy is justly named the most exact of the sciences, and they can see that there is nothing, in the nature of things, to render retrospection more difficult than prevision. But there are hundreds who have no such experience of the exactness of modern astronomical methods-who have, on the contrary, a vague notion that modern astronomy is merely the successor of systems now exploded; p

on in their courses for ages with none to note them. Gradually, observant men began to notice and record the more remarkable phenomena. But such records, made with ve

ipse referred to. Long intervals elapse between successive total eclipses visible at the same place on the earth's surface, and even partial eclipses of noteworthy extent occur but seldom at any assigned place. Very early, therefore, in the history of modern astronomy, the suggestion was ma

to grapple with it. Contented with Newton's fame, they had suffered their Continental rivals to shoot far ahead in the course he had pointed out. But the best Continental mathematicians were def

t mathematician Laplace. A full exposition of his views would be out of

y variable. In winter, when the earth is nearest to the sun, his influence is greatest. The lunar month, accordingly (though the difference is very slight), is longer in winter than in summer. This variation had long been recognised as the moo39n's 'annual equation;' but Laplace was the first to point out that the variation is itself slowly varying. The earth's orbit is slowly changing in shape-becoming more and more nearly circular year by year. As the greater axis of her orbit is unchanging,

satisfactorily accounted for, it may well be believed that there was triumph in the mathematical camp. But this was not all. Other mathema

e the commencement of the long conflict maintained by mathematicians with this difficulty), that it is not the moon travelling more quickly, but our earth rotating40 more slowly, which causes the observed d

esult, also, which seemed to flow from the discussion-the constancy of the earth's rotation-movement-was accepted;

t removes only half the difficulty which had perplexed mathematicians. It would be quite impossible to present in brief space, and in form suited to these pages, the views propounded by Adams. What, for instance, would most of our readers learn if we were to tell them that, 'when the variability of the eccentricity is taken into account, in integrating the41 differential equations involved in the problem of the lunar motions-that is, when the eccentricity is made a function of the time-non-periodic or secular terms appear in the expression for the moon's m

efence of Laplace. The contest swayed hither and thither for a while, but gradually the press of new arrivals on Adams's side began to prevail. One by one his antagonists gave

was besieged by the Persians, an eclipse of the sun took place, so remarkable in its effects (and therefore undoubtedly total), that the Median defenders of the town threw down their arms, and the city was accordingly captured. And Hansen has shown that a certain estimate of the moon's motion makes the eclipse which occurred on August 15, 310 B.C., not only total, but central at Nimroud. Some other remarkable eclipses-as the celebrated sunset eclip

ce as fast as it should do according to the calculations of Adams. But before our readers run away with the notion that astronomers have here go

hundred years would elapse before our moon would fairly begin to show in advance. She would, in that time, have brought only one one-hundred-and-fiftieth part of her breadth from behind the false moon. At the end of another century she would have gained four times as much; at the end of a third, nine times as much: and so on. She would not fairly have cleared her

train in motion, intense heat is generated in the substance of the brake. Now, the force employed by the brakesman is not equivalent to the heat generated. Where, then, is the balance of force expended? We all know that the train's motion is retarded, and this loss of motion represents the requisite expenditure of force. Now, is there any process in nature resembling, in however remote a degree, the application of a brake to check the earth's rotation? There is. The tidal wave, which sweeps, twice a day, round the earth, travels in a direction contrary to the earth's motion of rotation. That this wave 'does work,' no one can doubt who has watched its effects. Th

hour and a quarter, and she is losing now at the rate of one second in twelve weeks. In other words, the length of a day is now more by about one eighty-fourth part of a second than it was two thousand years ago. At this rate of change

ysical condition of our globe. Instead of the recurrence of ever-varying, closely intermingled cycles of fluctuation, we see, now for the first time, the evidence of cosmical decay-a decay which, in its slow pro

rnal, October 12, 18