148 



• KNOWLEDGE ♦ 



[Sept. 7, 1883. 



in reality somewhat though not perceptibly downwards, but 

 upwards from the direction in which you had first looked 

 expecting to see the horizon. 



This is by many regarded rather as a difficulty than as a 

 proof of the earth's rotundity. Yet no one can look at the 

 sharply defined sea-horizon as seen from a considerable 

 height, under favourable atmospheric conditions, without 

 seeing at once that that sharpness of definition is inconsis- 

 tent with flatness. When far beyond the sea-horizon land 

 is in view, the contrast between the distinctness of the sea- 

 horizon and the indistinct hazy look of the land lieyond 

 affords another striking evidence of the rotundity of the 

 sea-surface, for it shows that there is more haze between 

 the eye and the land than there is between the eye and 

 the part of the sea-surface where the sea-horizon line is 

 formed. 



effective. I have never known any one who has over tried 

 it under good observing conditions without finding that 

 strong though his faith might already have been in the 

 rotundity of the earth, it was much confirmed and 

 strengthened by this particular observation. It makes the 

 observer y>eZ as it were that the sea surface rises in a bold 

 sweep between him and the more distant object seen in the 

 same field of view. 



But this way of recognising the sea's rotundity may be 

 improved upon. Let us suppose that our observer starts 

 from A, to climb up the cliff (along the roadway will do 

 very well) till, after passing the level a, he reaches the 

 summit of the cliff at a'. 



When he is at A the line of sight to the sea-horizon 

 meets the sea curve close by as at p and the clearness of the 

 sea-line, supposing the observation made in good observing 



Thus if a (Fig. 1.5) be the place of an observer, A the 

 sea-level beneath him, and aF b the line of sight touching 

 the sea-horizon at P, and extending onwards to the cliffs at 

 b, we are not merely convinced but feel instantly that P is 

 much nearer than b, when we find that the sea-horizon at 

 P is seen much more clearly than the clifl's at b. We know 

 then that the surface must round itself above the straight 

 line A B in order to have this relative nearness at P. 



r. But an even prettier proof of the rotundity of the 

 water-surface may be obtained by using a powerful tele- 

 scope at a station such as a, and directing it upon the 

 horizon-line at P towards a ship as at s. It will be found 

 that when the telescope is focussed so as to show the sea- 

 horizon distinctly, the masts and sails of the ship s are 

 seen indistinctly. Fig. 1 6 gives an idea of what is seen. 

 To bring the ship sharply into view, the focussing rack- 

 work must be used so as to carry in the eye-tube, 

 as for a more distant object, and then presently the 

 parts of the rigging in view are seen sharply defined as 

 in Fig. 17, while the sea-horizon has become hazy and 

 indistinct. Where a low magnifying power is employed 

 on a large telescope, so that the focal range for different 

 distances is relatively great,'this observation is singularly 



weather, is very striking, as is also the contrast between 

 the sharp definition of the sea surface at ;; and the haziness 

 of the cliff at b' if that is visible at all, as of a ship at s. 

 If the telescope is used as in the experiment just described, 

 at such a station as A, the amount of focussing required to 

 correct from a clear sea-horizon into well defined ship 

 rigging, or vice versd, is much greater than when the tele- 

 scope is set higher above the sea-level. 



When the observer has reached a, the sea-horizon has 

 retreated to P, and is less distinctly seen, though it is 

 very marked against the distant clifi at b. But when, 

 passing onwards to a', he brings the sea-horizon to 

 B or even further from him, so that he sees to the very 

 foot of the cliff, then the sea-horizon (on either side of the 

 cliff Bb b' (which we suppose to be a cape projecting 

 towards him) is no longer seen to be freer from the tflects 

 of haze that the cliflf's face at b. If the observer has 

 noted the sea-horizon from time to time during his ascent 

 he will have seen (presuming the weather remains tolerably 

 constant) that the sea-horizon gets lighter and lighter in 

 tone as he passes higher and higher, showing that it passes 

 farther and farther away and is thus more and more 

 afiected by the presence of any haze that may be in the air 



