43 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



was that of moonlight, but not of the full moon z ex- 

 cepting the brilliant light close to the moon's limb, 

 which is equal to the brightest moonlight, and I 

 think its action on the sensitive plate confirms this 

 opinion. 



A point of much interest to be noticed is, that the 

 light of the corona had been considered to be much 

 less active than it really is ; eight seconds were suf- 

 ficient to produce on the plate an effect of light 

 extending beyond the moon's limb, at least one and 

 a half million of miles. 



I leave it to others to account for the cause of the 

 great gaps or rifts in the corona : also their identity 

 in position with those shown in thephotograph taken 

 by the American photographers at Cadiz. The iden- 

 tity of one of the rifts is absolutely fixed by the two 

 prominences between which it appears in the photo- 

 graphs, and this one gives the relative places of the 

 others. 



When the two photographs are compared, there is 

 an apparent difference in the places of the rifts with 

 respect to their angular position on the moon's cir- 

 cumference. How this difference arises I am not 

 prepared to say, as I have no information as to how 

 the American picture was taken, and there is no 

 mark on the transparency, which has been lent to me 

 by Prof. Young, to indicate the north point. In the 

 engraving from my photograph the top is the north. 



It is perhaps necessary to say that it is quite im- 

 possible to represent in an engraving on wood the 

 delicate detail of the corona. The cut fairly gives 

 the main features, but it is hard when compared with 

 the original : the contrast should not be so great ; 

 the ground should not be perfectly black ; and the 

 effect should not be produced by lines. No wood- 

 cut has ever yet accurately represented the phenom- 

 ena of the eclipsed sun. 



"When the photograph No. 5 is combined in the 

 stereoscope with the one taken about one minute 

 earlier, stereoscopic relief is produced the corona is 

 distinctly seen beyond the moon. It may be thought 

 that this is merely the effect of contrast, but I be- 

 lieve it is really due to the change in the position of 

 the moon. No such relief is seen when two copies 

 of the same photograph are combined stereoscopi- 

 cally. 



In order to see the woodcut with the best effect, it 

 should be placed at a few feet distance from the ob- 

 server, so as to lose all trace of the lines of the en- 

 graving ; the effect is then very accurately given of 

 the corona as seen by the unaided eye. 



In a second contribution to Nature, Mr. 

 Brothers gives outlines of the photograph 

 taken at Syracuse, the American photograph 

 at Cadiz, and Prof. Watson's pencil-sketch at 

 Carlentini. These exhibit a marked agree- 

 ment, as to the large rifts, between the two 

 photographs, and a general resemblance in the 

 same respect in the pencil-sketch. The coin- 

 cidence between the outlines of the corona in 

 the two photographs is very striking. 



See also Sun, Eclipse of. 



Supposed Changes in the Moon. At the 

 August meeting of the British Association, 

 Mr. W. R. Birt reported on behalf of the Com- 

 mittee on Lunar Objects suspected of Change. 

 The report states that, much attention having 

 of late years been given to lunar objects, the 

 purpose for which the committee had been 

 appointed would be best carried out by con- 

 fining the discussion to the observations of a 

 small but well-known portion of the moon's sur- 

 face. The area of Plato in which the spots exist 

 measured about 2,700 square miles; as many 

 as thirty-seven spots had been observed, but he 



wished it to be particularly understood that 

 the whole had never been seen together ; the 

 greatest number observed on any one occasion 

 was twenty-seven, the mean or average num- 

 ber being not more than eight. With the aid 

 of diagrams drawn on the black-board, he 

 showed that the mean number seen at inter- 

 vals of twelve hours of the lum-solar day 

 varied during the progress of the day, so much 

 so as to indicate that the number of spots visi- 

 ble at any given interval does not depend upon 

 the angle at which the sun's light falls upon 

 the floor of Plato. Some spots, he said, had 

 been seen more frequently at about sixty 

 hours after sunrise upon the floor of Plato than 

 at any other portion of the luni-solar day ; 

 the positions of these spots on the floor were 

 pointed out, and it was remarked that they 

 were situated in the western part of the crater, 

 and they agreed in having been more frequent- 

 ly observed in August. 1869, than at any other 

 period of the observations. Other spots were 

 observed more frequently at a later period 

 than in August, 1869, and they had been seen 

 more frequently at a later hour of the day, or 

 after the sun had passed the meridian. Day- 

 light at the moon is equal to fourteen terres- 

 trial days and nights. These facts Mr. Birt 

 argued were incompatible with the assump- 

 tion that variations of aspect were entirely 

 dependent upon variations of illumination, 

 and rather pointed to the existence of activity 

 on the moon's surface, the exact nature of 

 which required farther observations to elu- 

 cidate. 



In observing the moon shortly before the last 

 quarter in August, Mr. Holden, of London, 

 found the unilluminated portion unusually 

 bright with earth-shine. A few prominent cra- 

 ters could be traced, and the whole of the dark 

 outlines of the Mare Serenitatis were easily 

 recognized. The darkest object was the Mare 

 Crisium,, which appeared almost black. Mr. 

 Holden suggests that the depth of color in the 

 Mare Crisium and other planes may be due to 

 a covering of alluvial earth, to which vegeta- 

 tion may at times give the greenish tinge occa- 

 sionally observable. 



The Moon's Mass inferred from Tides. Mr. 

 William Ferrel, of the IT. S. Coast Survey, at 

 the April meeting of the National Academy of 

 Sciences, presented the results of his investi- 

 gation of tidal movements with reference to 

 determining the mass of the moon. For this 

 purpose he employed a series of observations 

 taken for the Coast Survey during the full 

 lunar cycle of nineteen years, and a similar 

 series made at Brest, France, from 1812 to 

 1831 inclusive. The following is a synopsis 

 of his paper : 



"Without going into the mathematical form of the 

 investigation, he endeavored to show that the moon's 

 mass must be mainly inferred from the ratio -which 

 the spring and neap tides bear to the constant or 

 average tides. This ratio, however, does not depend 

 entirely upon the moon's mass, but varies greatly foi 

 different ports, the heights and times of the tide be- 



