1855.] 



THE SOLAR ECLIPSE OF 1854. 



183 



boundary, boing, in fact, the peaks of bigh mountains on which 

 the sun is shining, although his rays are intercepted from the 

 valleys in the same neighbourhood. Again, when a star is oc- 

 culted by the moon, it is sometimes seen to run along the ex- 

 tremity of the disc, alternately appearing and disappearing as it 

 is successively hidden by the mountains on the edge, and 

 allowed to shine through the hollows between them. These 

 mountains may be much more considerable at one part of the 

 circumference than at another : and although, speaking roughly, 

 the moon always keeps the same face turned towards the earth, 

 yet in consequence of her librations, and the different positions 

 of observers, the portion which forms the boundary of her visible 

 hemisphere will vary, thoagh within small limits. Conse- 

 quently, it is quite possible that the edge of the moon's disc 

 might appear serrated during one eclipse, and smooth during 

 another ; but it is difficult to understand why at the same time 

 the moon's limb should have appeared serrated to one observer, 

 and to another, at a comparatively short distance from the 

 former, perfectly sharp and smooth. This is the more remark- 

 able, because, at Prescott, a phenomenon icas observed which 

 depends upon this mountainous boundary of the moon, viz., 

 " Baily's beads." These have been observed both in total and 

 annular eclipses. In the case of a total eclipse, just at the 

 commencement of the totality, that is, just at the moment when 

 the eastern limb of the moon is coming into contact with the 

 eastern limb of the sun, the very narrow bright segment of the 

 latter body becomes broken up into brilliant irregular portions 

 called, from the person who first observed them, " Baily's 

 beads." The same appearance is also observed at the end of 

 the totality. This phenomenon is certainly a result of the un- 

 eveuness of the lunar surface ; the " beads" being nothing 

 else than gaps between the mountains, through which the sun 

 can still be seen. The Astronomer Koyal thus describes their 

 appearance at the beginning of the totality of the eclipse of 

 1851, pointing out very distinctly the cause to which he con- 

 siders them due. lie says, " I took off the higher power, with 

 which I had scrutinized the sun, and put on the lowest power 

 (magnifying about .34 times). With this I saw the mountains 

 of the moon perfectly well. I watched carefully the approach 

 of the moon's limb to the sun's limb, which my graduated dark 

 glass enabled me to do in great perfection. I saw both limbs 

 perfectly well defined to the last, and saw the line becoming 

 narrower, and the cusps becoming sharper without any distor- 

 tion or prolongation of the limbs. I saw the moon's serrated 

 limb advance up to the sun's, and the light of the sun glim- 

 mering through the hollows between the mountain peaks, and 

 saw these glimmering .spots extinguished, one after the other, 

 in extremely rapid succession." In the case of an annular 

 eclipse, the same phenomena are observed, but in an inverted 

 order. The first appearance of the beads is at the moment 

 when the western edge of the moon is just detaching itself 

 from the western edge of the sun. If the boundary of the disc 

 of the former body were perfectly smooth, we should sec the 

 bright disc of the .sun surrounding it gradually and continu- 

 ously, until the whole surface of the moon became projected 

 upon that of the sun. Instead of this, however, when the an- 

 nularity has very nearly commenced, and consequently only a 

 very small segment of the circumference of the sun is obscured, 

 that remaining segment, instead of being gradually revealed, 

 becomes suddenly bright in a number of points, with dark 

 spaces intervening ; the bright spots increase in size, and seem 

 to run into each other, until the moon's limb is at length 

 wholly surrounded by the bright disc of the sun. A similar 

 appearance recurs at the end of the annularity, when the east- 



ern limb of the moon joins the eastern limb of the sun. These 

 beads were seen by one of the observers at Prescott, distinctly 

 at the beginning of the annularity, and with less clearness at 

 the end ; and the fact that they were visible is sufficient to 

 prove ihat, althought the moon's limb did not ajqiear jagged 

 in passing over the sun's disc, it really was uneven, and that it 

 must have been in consequence of some peculiarity in the in- 

 struments or colored glasses employed that the serrated edge 

 was not previously observed. The beads were not distinctly 

 observed at Kingston, but we have the following remark in the 

 report from that station, " On the first contact of the western 

 limbs of the sun and moon, Baron de Rottenburg remarked a 

 slight appearance of the light as it were running along the line 

 of contact, but not with that degree of brilliancy or certainty 

 which has been recorded on similar occasions." 



Another indication of the unevenness of the surface of 

 the moon appears in the fact, that at one time during 

 the progress of the eclipse, the horn or cusp of the solar disc 

 was observed at Montreal to be slightly blunted, which must 

 have been caused by the existence of a large mountain just at 

 that part of the moon's limb which, at the time in question, 

 appeared to join the sun's limb. It has, indeed, been suggested 

 that this appearance of bluntness in the cusp might be pro- 

 duced by a lunar atmosphere. The effect of such an atmos- 

 phere would certainly be to distort perceptibly the shape of the 

 narrow crescent of light, when the eclipse was considerably 

 advanced. The rays of the sun would be bent inwards by pas- 

 sing through this medium, and thus a small portion of the sun 

 would become visible, which would not be seen if this lunar 

 atmosphere did not exist, and the extremity of the bright seg- 

 ment would, in consequence, appear blunted. The existence, 

 therefore, of a lunar atmosphere would account for such an 

 appearance as that noted by Dr. Smallwood; but we must not 

 infer from the phenomenon that such an atmosphere really 

 does exist. As has been before remarked, the existence of a 

 large mountain at the part of the moon's disc which intersects 

 that of the sun would also account for the appearance : and 

 that this, and not the former explanation, is the true one, is evi- 

 dent from several considerations. In the first place, there is 

 pretty conclusive evidence, from other sources, that no such 

 atmosphere does exist in the moon : no atmosphere, that is to 

 say, capable of sensibly refracting light passing through it. 

 Thus, in the case of the occultation of a star by the moon, if a 

 lunar atmosphere existed the star would continue to be visible 

 by reason of the refraction after the moon's disc was really in- 

 terposed, so that the disappearance of the star would be post- 

 poned in consequence of the existence of such an atmosphere ; 

 and as the time of re-appearance would be anticipated b_y the 

 same amount, the observed time of occultation would be shorter 

 by twice that amount than the time given b}- calculation. No 

 such difference, however, is observed. Again, if the moon 

 were surrounded by an atmosphere containing any appreciable 

 amount of vapour, the extinctinn of a star's light by an occul- 

 tation ought to bo gradual, and very faint stars ought to become 

 invisible before the proper time of occultation, as the moon's 

 atmosphere would be sufficient to intercept their light. This 

 is also contrary to what is observed; for, during an eclipse of 

 the moon (which, owing to the absence of diffused light, is the 

 most favourable time for making such observations), stars of 

 the tenth magnitude are seen smhhiifj/ extinguished by the in- 

 terposition of the moon's disc. And, in additiun to this evi- 

 dence from other sources, it is clear, that in the case of a 

 solar eclipse, any alteration in the shape of the bright crescent 

 produced by an atmosphere about the moon, would be of a 



