Nov. 16, 1883.] 



♦ KNOWLEDGE ♦ 



303 



nothing should be attempted that cannot be well mastered 

 in a single observation. 



Then, again, something has to be said as to the necessity 

 of a certain degree of proficiency in the art of correct 

 drawing. An eye sufficiently accurate to comprehend the 

 proportions of what lies before it is by no means of con- 

 stant occurrence ; nor a hand adequately trained to transfer 

 them faithfully to paper. Artistic taste is not much con- 

 cerned in the matter. We do not want a pleasing picture, 

 but a faithful likeness ; and the two are not always as 

 well balanced as they are in the portraits, alike beautiful 

 and accurate, given by Green in his views of the planet 

 Mars. Practice, however, will do much to improve both 

 eye and hand ; and especially a resolute determination not 

 to be discouraged by early failure. 



Again, it is an indispensable qualification to be well 

 acquainted with " relief," as shown by light and shade, 

 without which we may be needlessly surprised, or even 

 hopelessly perplexed, in the interpretation of what we see. 

 In this important respect we may especially recommend 

 the carefid examination of a somewhat irregular white sur- 

 face, such as a sheet of plaster of Paris, or even smooth 

 mortar, spread on a board, on which elevations or cavities 

 may be formed ; the oblique exposure of this to the light of 

 the sun, or a lamp, from either side, and at varying angles, 

 will readily show what strange differences of aspect may 

 result from so simple a cause. With certain forms, and in 

 certain positions, it will soon be seen how great a propor- 

 tion those diflerences bear to a comparatively slight change 

 in the direction of the incident ray; what "giant-like" 

 dimensions may be assumed by the shadows of mode- 

 rate elevations, what pointed spires of blackness 

 may be formed, perhaps for a short time only, by 

 blunt or rounded summits, and what singular deforma- 

 tions may take place when they are thrown across or 

 terminate in irregular ground, The effect also must be 

 noted of a slight variation in the direction of the illumina- 

 tion, not only, as is constantly in progress, from W. to E., 

 or the reverse, but on certain occasions also from IST. lo S. 

 Such a variation actually e.xists in consequence of a slight 

 inclination of the Moon's axis to the plane of its orbit, and 

 though very limited in amount, it may, under some condi- 

 tions, be appreciable enough to produce changes that would 

 otherwise be unintelligible ; a straight ridge, for instance, 

 running E. and W. with a similar inclination on either 

 hand, might from this cause cast a shadow now on one, 

 then on the other side, or become imperceptible from want 

 of shade, according to the variation in the lunar azimuth 

 of the rising or setting sun. Or a group of luminous 

 points, the summits of some lofty and complex mountain 

 mass, just touched beyond the terminator by the rays of 

 the rising or setting sun, may appear to vary in form or 

 number in different lunations, from the ssme cause, which 

 is equivalent to a very sliglit change of seasons on the 

 Moon. 



Again, we must not leave out of account the change of 

 perspective foreshortening due to the effect of libration, 

 which is constantly varying, from night to night, the 

 position of objects as referred to the limb, or, what is of 

 course the same thing, the ajiparont centre of the lunar 

 disc. It is not necessary in this place to enter into the 

 cause or the limits of this disturbance in the optical presen- 

 tation of the moon's visible hemisphere to the earth ; but 

 the attentive observer will soon be sensible of its existence, 

 from the changing proportions of the ellipses into which the 

 circular formations are projected, and the varying visibility 

 of minute details, especially on slopes receding from the 

 eye. The perspective alterations arising from this cause 

 increase, of course, rapidly with the increasing distance 



from the centre of the disc ; and towards the limb they 

 become so exaggerated, and often puzzling, that it would 

 be mere waste of time for a beginner to attempt the deli- 

 neation of those regions ; excepting, indeed, in one case — 

 that of actual representation in profile. For it occasionally 

 happens near the time of full moon that some huge moun- 

 tain mass, such as Leibnitz or Dorfel, is thrown up in 

 imposing relief against the dark sky beyond the regular 

 circle of the disc ; or that the points of the crescent are 

 carried on in detached patches — islands, as it were, of light 

 — along the dark limb. Such projections lend themselves 

 readily to the pencil, and offier an outline worthy of 

 preservation. 



The facility of delineation is increased by the unifor- 

 mity of tone in the shadows, which, in the absence of a 

 reflective atmosphere, are of unbroken blackness. Their 

 edges, too, are sharply defined ; for though they must be 

 attended, like every terrestrial shadow, by a penumbra, or 

 dusky border, arising from the breadth of the star-disc, 

 yet this, excepting in very oblique illumination close to 

 the terminator, subtends too small an angle at the distance 

 of the moon to be worthy of notice ; in fact, it would not 

 be perceptible with such instruments or magnifying powers 

 as are now under consideration. 



It is scarcely necessary to say that in such work a loose, 

 undecided, sketchy manner is especially to be avoided. 

 Such designs may possibly recall what was actually seen 

 to the mind of the artist himself, but to others they may 

 only be unintelligible or even misleading. The rule of 

 good drawing in ordinary cases is at least equally appli- 

 cable here — that every line should have a meaning. As 

 far as possible, nothing should be left to memory ; and 

 an appro.vimate date should accompany every drawing, 

 together with a specification of the size and power of the 

 telescope employed, and the quality of definition. 



PRETTY PROOFS OF THE EARTH'S 

 ROTUNDITY. 



By Richard A. Pboctor. 



{Concluded from page 270.) 



EVERY one who has been much at sea must have (or 

 rather uuc/ht to have) noticed how very markedly 

 even a slight change of elevation affects the apparent 

 position of the sea horizon as seen athwart the upper masts 

 of a distant ship. Where the sea horizon crosses the lower 

 masts the effect is not so marked ; but when the air is 

 clear so that the sea horizon is sharply defined and the ship 

 beyond it clearly seen, the change is obvious enough in this 

 case also. I have often been struck by the singular way in 

 which a distant ship seems to rise and sink bodily as you 

 watch it through a good glass, when in reality the change is 

 due to the steady and almost imperceptible pitching of the 

 ship from which you view her. To catch this effect best 

 you should observe of course from the bow or stern, when 

 the ship is crossing a long swell, the swell being very 

 slight and the sea in other respects as still as may be. 

 You rise and sink then through perhaps three or four feet 

 by a slow movement scarce noticeable under any circum- 

 stances, and quite imperceptible when the eyes are directed 

 on a distant hull down ship. It is obvious that as you 

 thus rise and sink the line to the horizon sways o\er the 

 curved surface of the sea, somewhat as a plank might see- 

 saw over the large round trunk of a prostrate tree. We 

 see from Fig. 20 why the effect would be greatest on a 

 distant object. For if a, b, are the positions of tlie eye as 



