Ocr. 3, 1884.] 



♦ KNOWLEDGE ♦ 



275 



made over the whole surface of the earth without proving 

 ^inything beyond this. But the observations made by 

 astronomers luive attained to great a degree of delicacy as 

 to show how far the e;irth's shape departs from the figure 

 of a true s|ihere. Slight differences in the rate of elevation 

 or depression of tiie pole as tlie north and south journeys 

 have been carried out, have shown that the curves P a P', 

 P b V, ic. (Fig. 1), are not true circles ; though differing so 

 little from the circular figure, that if their true shape were 

 drawn down on pajer, a very sharp eye indeed would be 

 required to detect the difference between their figure and a 

 perfect circle. They are slightly elliptical, P P' being the 

 shorter axis of each. In other words, the earth is somewhat 

 ilattened at the poles. 



My aim is, however, to give an exact account of the 

 simpler features of the earth's figure, rather than to deal 

 with minor peculiarities, and as the departure of the earth's 

 figure from the true sphere belongs to the latter order, I 

 need not discuss this feature farther. 



I conclude this chapter by mentioning certain phenomena 

 which afford tolerably clear evidence of the globe figure of 

 the earth, though I may remark, in passing, that nothing 

 but the complete survey described above can be held to be 

 a satisfactory proof. 



Every one who has ever been at the sea side on a fine 

 day, has noticed how clear-cut the horizon-line is, how it 

 forms a well-marked line against the sky. How this cir- 

 cumstance rightly understood gives very forcible evidence 

 that the visible part of the earth's surface is globular. If, 

 ■on the contrary, it were plane, then, however little the eye 

 were raised above the surface of that plane, yet every part 

 of the plane wiuld be visible; and the appearance of a 

 well-defined hoiizon-line would imply that we were look- 

 ing at the actual limits of the plane in that direction, which 

 is obviously absurd. If the plane had no limits within the 

 range of vision, the appearance presented would be that of 

 a gradual fading away up to the horizon level. 



When, however, we find further that at the sea side 

 ships )iass that apparently well-defined borderline, and 

 gradually seem to sink out of view, no doubt can remain 

 that the visible surface is not plane. For we see di-tinctly 

 the clear-cut water-line across the equally well defined 

 masts and sails of the ship. We know, therefore, that the 

 ship is beyond the water-line, and as we know that it has not 

 ■crossed a limit of any sort, no other explanation is available 

 than that it has sailed over a oontinuoubly curved surface. 



The conviction thus arrived at hardly admits of being 

 strengthened, but those who have ever used a powerful 

 telescope at the sea side find a certain new interest in 

 observing this well-known phenomenon. It may seem a 

 simple amusement for a student of astronomy, but often 

 when I lived by the sea side, I used to watch with my -tin. 

 achromatic the gradual disap|)earanoe of a ship beyond the 

 sharply cut border-line of the sea wiih much interest and 

 pleasure. Souietimes the air would be so clear that a tall 

 -ship would sink until her upper masts would be out of 

 focus when the sea horizon was brought into focus, and 

 vice versd. A slight motion of the tocussing rack-work 

 would bring either the sea-waves or the masts and ropes 

 into focus, as might be wished : a very pretty and simple 

 proof, by the way, of the relative distances of the sea 

 horizon and the ship, and a supplementary evidence of the 

 «arth's globular figure, which so far as I am aware has 

 never yet been noticed.* 



* Perhaps I should not have had my attentiou called to the fact 

 that the distance of the visible sea-horizon is measurable in this 

 simple way, had it not been for the blatant assertions of a well- 

 .known charlatan, that a telescope will bring a ship into sight which 

 lias passei hryond the visible horizon. Certainlj- this man never 



Another proof of the globular figure of the earth has 

 been derived from the shape of the earth's shadow as seen 

 during a lunar eclipse. 'This proof is not perhajis very 

 striking, because the curvature of the earth's shadow as 

 seen on the moon is by no means so well marked as many 

 suppose. The shadow has not a well-defined edge, the 

 circle it belongs to is much larger than the moon, and 

 finally the moon's surface is marked with so many varia- 

 tions of brilliancy as to confuse the border of the umbra. 



Tlie real evidence derived from lunar eclipses is, how- 

 ever, much more striking when it is i)roperly studied. Let 

 it be remembered that the astronomer calculates the minute 

 at which a lunar eclipse begins, and the place at which the 

 earth's shadow at first makes its appearance on the lunar 

 disc, on the hypothesis that, the earth being a globe, the 

 section of the shadow-cone where the moon crosses it is a 

 circle of a certain size, and in no single instance has his 

 tiust in this hypothesis been confounded by the eclipse 

 either occurring at the wrong time, or commencing at the 

 wrong part of the moon's disc. 



Fig. 2. 



A peculiarity noticed by balloonists seems at first sight 

 strikingly opposed to the theory that the earth is globular. 

 When a balloon has risen high above the earth on a 

 clear day, the latter appears to the aii'ronauta to be shaped 

 like a basin beneath them, the sky assuming also an 

 appearance of concavity (more flattened than usual), 

 facing the earth's apparently hollow surface. Many bal- 

 loonists have been so struck by this appearance, which 

 is represented in section in Fig. 2, as to think there 

 must be something wrong about the theory of the earth's 

 globular figure. In reality, however, the appearance is 

 precisely what was to have been expected. The balloonist, 

 even after rising four or five miles, has scarcely caused the 

 visible horizon to appear appreciably below his level It 

 is, in fact, lowered some 3i degrees, but the unaided eye 

 cannot recognise so small a depression, whereas the vast 

 gap which separates the observer from the earth beneath 

 him is very obviously presented to the eye. Therefore the 

 effect is as though the horizon were on a level with the 

 eye, and the earth's surface under the observer far below 

 that level. 



Of course if the peculiarity proved anything, it would 

 show that the earth is basin-.-^haped, certainly not that it is 

 plane. As yet, however, no one has claimed for the earth 

 the figure of a basin. 



{To be continued.) 



did a bolder thing than in coming to a sea neighbonrhood, and 

 there making an assertion so easily disproved. He told the Ply- 

 mouth people that he could show them the Eddystoue Lighthouse 

 as clearlv from the foot as from the summit of the Hoe, and on a 

 certain clear, calm morning in autnmn he carried out the experi- 

 ment. Of course, he tailed. But if that were all I should have 

 left the story untold. The best is to come. One might suppose he 

 would have blamed his telescope, though it had shown the Eddy, 

 stone nicelv from the summit of the Hoe. He was too sharp for 

 this, however. It happened that the atmospheric refraction (which 

 is well known to be variable) was unusually weak that morning, 

 and so the lantern of the lighthouse was altogether out of sight, 

 when, according to the Admiralty instructions (calculated for 

 average refractive effects), it should have been partly visible. 

 Here was a means of escape. " I promised to show you the Eddy- 

 stone," he argued, " though the Admiralty says all but the lantern 

 would be ought of sight. Well, you see the Admiralty is wrong; 

 not even the lantern can be seen. Is it not obvious that the 

 received theory is false .' " I am told he even published this argu- 

 ment in a book. 



