184 



THE SOLAR ECLIPSE OF 1854. 



[1855. 



comparatively steady and permanent character, being subjected 

 only to variations due to changes in the state of that atmos- 

 phere; whereas, on all occasions where a blunted horn has been 

 observed, it has been only a transient phenomenon. Thus, in 

 the eclipse of 1842, M. Arago notices that each cusp was 

 repeatedly blunted and recovered its natural form. Again, 

 there appears to be no evidence of any prolongation of the bright 

 segment, which would certainly be produced by the existence 

 of such an atmosphere. And, what is perhaps the most con- 

 clusive evidence of all, eclipses have been repeatedly observed 

 without the cusps being seen to be blunted at all; as was the 

 case in the eclipse of 1854, the cusps having appeared perfectly 

 sharp, and free from all distortion throughout the eclipse to the 

 observers at Prescott, Kingston and Toronto. We may, there- 

 fore, safely ascribe the phenomenon observed by Dr. Smallwood 

 to the existence of a considerable mountain on the poi'tion of 

 the moon's disc forming the extremity of the cusp. 



II. At 4/t. 49m. 15s. a minute bright spot was seen at King- 

 ston on the surface of the moon, near the eastern cusp. Seve- 

 ral theories have been suggested to account for the appearance 

 of such spots, which have been frequently observed. The first 

 mention of them is by Ulloa, who, in the total eclipse of 1778, 

 "saw in the N. W. region of the moon a luminous point, 

 shining successively as brightly as a star of the 4th, 3rd and 

 2nd order." This phenomenon received from him a very 

 strange explanation, viz., that the moon is penetrated by a sort 

 of long tunnel, through which the sun's disc could be seen. 

 And the same hypothesis, slightly modified, was adopted by M. 

 Valz, who observed the eclipse of 1842 at Slarseilles. It seems 

 strange that such hypotheses could be gravely set forth. It is 

 obvious that, if such were the real cause of the appearance of 

 these luminous spots, instead of being momentary and variable 

 phenomena, they would retain their brilliancy at least for some 

 considerable period ; and, moreover, if such gaps existed, they 

 could scarcely fail to be observed as sliadoivs on the full moon, 

 considering the magnitude of the astronomical instruments now 

 employed. It has also been supposed that the appearance of 

 such bright spots might be due to the existence of active vol- 

 canoes on the moon's surface, which several astronomers have 

 fimcied they have seen. Thus Hevelius asserted that the 

 mountain known as Aristarchus "appeared reddish and seemed 

 to burn " ; and several other astronomers — among whom we 

 may mention Sir W. Herschel — have recorded similar appear- 

 ances. It seems, however, probable that the volcanic charac- 

 ter of these mountains cannot be maintained. The flickering 

 appearance of the light — on which the hypothesis materially 

 depends — seems wholly due to atmospheric causes, as it is not 

 observed when the sky is clear, and the air still. The other 

 grounds on which the mountain in question has been conjec- 

 tured to be a volcano, are, that it has been distinctly seen 

 during a lunar eclipse, and that it is often very conspicuous at 

 the time when the moon is nearly new, and when the portion 

 of her disc not illuminated by the sun is seen by aid of the 

 light reflected from the earth. In the latter case, if we suppose 

 the mountain to have a smooth table land at its summit, and 

 the sides to be rugged and broken, it is easy to understand that 

 the plateau at the top would reflect more light than the sur- 

 rounding regions, and so appear brighter. But it seems almost 

 impossible to account for the continued brightness during a 

 lunar eclipse, when there is actually no light falling upon the 

 moon, except by supposing the spot in question either self- 

 luminous, or, at any rate, capable of giving out during the 

 darkness the light previously absorbed. On -either supposition 

 the appearance of a bright spot on the moon during a solar 



eclipse might be accounted for, supposing it to have been 

 observed by several persons, and for some considerable time. 

 But it seems that whenever such appearances have been 

 observed, they have been temporary, and that they have been 

 noticed only by a few observers. Thus, in the eclipse of 1842, 

 neither Mr. Airy, nor Mr. Baily, nor M. Arago perceived any 

 such bright spots ; and, though they were observed by others, 

 the observations do not present accordance either in time or 

 position — which evidently suggests the enquiry, whether these 

 appearances may not be optical illusions. That a person might 

 be deceived in this respect is evident from the fact that one of 

 the observers at Prescott repeatedly thought he saw such a spot, 

 but as on moving the telescope the spot moved with it, it was 

 at once evident that it was due to some particles of dust or 

 accidental inequality on the object-glass scattering the sun's 

 rays. 



III. None of the obssrvers of the eclipse of 1854 succeeded 

 in seeing the portion of the moon^s disc exterior to that of the 

 sun. It was seen by M. Arago during the eclipse of 1842, 

 and is thus described by him : — " About 40 minutes after the 

 commencement of the eclipse of July 8th, at 5h. 35m. by our 

 clock, I saw the outline of the moon delineated upon the 

 heavens. It formed accurately the prolongation of the dark 

 circular arc which another portion of the same limb traced on 

 the surfiice of the sun, and joined it at two points on the bright 

 limb of the latter body." The same appearance is thus noticed 

 by M. Flaugergues, who observed the eclipse at Toulon. " To- 

 wards the middle of the increase of Ihe eclipse, the disc of the 

 moon was visible about 25° beyond each of the points of 

 intersection of the circumferences. When the eclipse amounted 

 to eleven digits, all the disc of the moon became visible." 



In the Instructions printed by order of the Institute, it was 

 pointed out that this phenomenon would certainly escape ob- 

 servation, if the lenses of the telescope were not perfectly po- 

 lished and scrupulously clean, and it was probably chiefly to 

 the fact of the former condition not being fulfilled, that we 

 must ascribe the failure of the observers in this respect. To 

 explain why these precautions are essential to the success of 

 the observation, it will be necessary to point out the mode in 

 which the moon's disc does become visible under these circum- 

 stances. 



The explanation which would at first ofier itself, and which 

 was assumed to be the time one by some observers in 1842, is 

 that the moon's disc might be rendered visible by the twice- 

 reflected light of the sun, which is called by French astrono- 

 mers, " la laumifere cendr6e, " and which has been by some 

 English writers termed " earth-shine." That this light is, 

 under certain circumstances, sufficiently strong to render the 

 moon's disc visible, is evident to any one who has remarked 

 the appearance of the moon when vei-y young, when not only 

 the small crescent illuminated by the sun's direct rays is visible, 

 but we can also see the remainder of the disc of a peculiar 

 grey colour. This is owing to the fact that at that time the 

 earth as seen from the moon is nearly full, and that the light 

 reflected from the earth to the moon is strong enough, when 

 reflected again to us, to render visible the part of the moon not 

 illuminated by the direct rays of the sun. Is it possible, then, 

 that when the eclipse has begun, the portion of the moon ex- 

 terior to the sun should be rendered visible by this light? In 

 order to answer this question, it will be necessary to ascertain 

 under what conditions an object becomes visible, either by the 

 naked eye or through a telescope. It is evident that we have 

 to take info account something besides the actual amount of 



