to the Moon; and when the Moon is New, the Earth is Full to the Moon. These remarks apply only to those parts of the lunar surface which are turned towards our globe; for a spectator on the opposite side would never see the Earth at all, and spectators located on the apparent borders of the lunar disc would only now and then obtain a glimpse of it in their horizon, for which they would be indebted to the librations in longitude and latitude already noticed.

X

If the whole sky were covered with Full Moons they would scarcely make daylight, for Bouger's experiments give the brilliancy of the full Moon as only 300000 that of the Sun. Wollaston's value is 5010 Zöllner's 1000, and G. P. Bond's 70950.

The Moon's surface is supposed to be much heated, possibly, according to Sir J. Herschel, to a degree much exceeding that of boiling water; yet we are not in a general way conscious of there being any heat at all available for warming the Earth. This need not however excite surprise, for it is probably very small in amount, and what there is of it is doubtless quickly absorbed in the upper strata of our atmosphere. Melloni, in 1846, thought that he detected a sensible elevation of temperature by concentrating the rays of the Moon in a lens 3ft in diameter. C. P. Smyth, in 1856, also thought that he obtained evidence on Teneriffe of the Moon's rays possessing calorific power, but his instrumental appliances were not very perfect. Professor Tyndall has stated that his experiments in 1861 seem to show that the Moon imparts to us, or at least to the Professor's thermometric apparatus," rays of cold." More recently, however, the Earl of Rosse, M. Marié-Davy, and Prof. Langley have conducted experiments which seem to give conclusively affirmative results, and on the whole the balance of evidence leans to this view of the question.

Prof. Langley's summary of his own observations and deductions is as follows:-" While we have found abundant evidence of heat from the Moon, every method we have tried, or that has been tried by others, for determining the character of this heat appears to us inconclusive; and, without questioning that the Moon radiates heat earthward from its soil, we have not yet found any experimental means of discriminating with such certainty between this and reflected heat that it is not open to misinterpretation."

The first astronomer who paid much attention to the delineation of the Moon's surface was Hevelius, who in his well-known Selenographia, published in 1647, gave a detailed description of it, accompanied by one general and some 40 special charts; which, taking into consideration the inferior optical means at his disposal, were very creditable to the industry of the illustrious observer of Dantzig. Four years later Riccioli brought out a map of the Moon, having proper names assigned to many of the principal localities: and this nomenclature, improved and enlarged, is still in general use. J. D. Cassini and T. Mayer of Göttingen published charts in the years 1680 and 1749 respectively, the latter of which was the only one used by observers for many years subsequent to the opening of the present century. In 1791 Schröter published a large work entitled Selenotopographische Fragmente, in which are given diagrams of many of the principal spots. Schröter was an industrious observer, but his descriptions are not always satisfactory.

In 1824, W. G. Lohrmann of Dresden published the first 4 of a series of 25 excellent lunar charts, but was prevented by failing sight from continuing the work. It was, however, taken up by others and completed in 1878. Beer and Mädler's elaborate Mappa Selenographica was published in 1837, and is undoubtedly the best of the kind yet published; but the most generally useful and also most generally accessible map for the class of

d Mem. Nat. Acad. Sciences, vol. iii. p. 42. 1885.

The two engravings on Plate VII are copied from this work; Archimedes from

Plate XVI, and Pico from Plate XXII.

Month. Not., vol. xxxix. p. 267. Feb. 1879. Published by J. A. Barth, Leipzig; price, with book, 50 marks.

readers whom I address is the Rev. T. W. Webb's, reduced from Beer and Mädler's. Undoubtedly, however, the most minutely accurate and elaborate lunar map yet made is the one of 7.67ft in diameter, by Schmidt of Athens, published at the expense of the German Government in 1878. Maps by Russell and by Blunt are in circulation, but they are not of much value as regards details.

The British Association for the Advancement of Science, through a sub-committee, began in 1866 the preparation of an entirely new map of the Moon, but this was eventually abandoned by the Association. The late Mr. W. R. Birt, however, continued it for a time.

A wax model of the whole lunar surface was executed many years ago by a Hanoverian lady named Witte, and Nasmyth has modelled in plaster of Paris several single craters. Photography, too, has been called in by De La Rue, Rutherford, and others, with good results.

In computing the places of the Moon the Tables of Burckhardt, published in 1812, were formerly used, but in 1862 the new and more perfect Tables of Hansen were introduced at the Nautical Almanac office; and these have entirely superseded Burckhardt's. Damoiseau, Plana, Carlini, Pontécoulant, Lubbock, and afterwards Delaunay, in addition to Hansen, did much to improve the theory of the Moon. Delaunay's labours earned for him a foremost place in the rank of geometrical astronomers. More recently still, Sir G. B. Airy has been treating the subject by a new method. His memoir entitled the "Numerical Lunar Theory" was published in 1887. He is understood to be still investigating some points in it which need further elucidation". According to a recent determination by Stone the Moon's mass is that of the Earth.

81

1

To record a tithe of the influences ascribed to the Moon would be a herculean task; nevertheless (in addition to the tides) one

Fig. 65 is from a photograph of one of these. But they are of little value, being very inexact.

h Month. Not., vol. xxxiv. p. 89. Jan. 1874.

deserves notice. Evening clouds at about the period of Full Moon will frequently disperse as our satellite rises, and by the time it has reached the meridian a sky previously overcast will have become almost or quite clear. I first observed this in 1857, and subsequently found that Sir J. Herschel had made the same remark. The idea has been disputed, but I am firmly convinced of its truth. Humboldt speaks of it as well known in South America, and Arago indirectly confirms the theory when he shows that more rain falls at about the time of New Moon (cloudy period) than at the time of Full Moon (cloudless period according to the theory). According to Forster, Saturday new Moons result in 3 weeks of wet weather. He alleged that observations extending over 80 years showed this coincidence'. Bernadin asserts it as a fact that many thunderstorms occur about the period of New or Full Moon. With these possible exceptions it is safe to assert that "changes" of the Moon have no discoverable influence on the weather m.

1 Outlines of Ast., p. 285.

Ellis, Phil. Mag., 4th Ser., vol. xxxiv. p. 61. July 1867.

1 Month Not., vol. ix. p. 37. Dec. 1848. m See Nasmyth and Carpenter, Moon, p. 180.

CHAPTER VIII.

THE ZODIACAL LIGHT.

General description of it.—When and where visible.—Sir J. Herschel's theory.— Historical notices.—Modern observations of it.—Backhouse's Conclusions.

AS

STRONOMICAL writers are not agreed as to the proper head under which to describe and discuss the Zodiacal Light. I deal with it here, because, whatever its origin, it is a matter of terrestrial cognizance, and therefore a description of it may, without any serious incongruity, be associated with what has to be said about the Earth.

The Zodiacal Light is a peculiar nebulous light of a conical or lenticular form ", which may very frequently be noticed in the evening soon after sunset about February or March, and in the morning before sunrise about September. It extends upwards from the Western horizon in the spring and from the Eastern horizon in the autumn, and generally, though by no means always b, its axis is nearly in a line with the ecliptic, or, more exactly, in the plane of the Sun's equator. The apparent angular distance of its vertex from the Sun's plane varies, according to circumstances, between 50° and 70°; sometimes it is more; the breadth of its base, at right angles to the major axis, varies between about 8° and 30°. During its evening apparition it usually reaches to a point in the heavens situated not far from the Pleiades in Taurus. It is always so extremely ill-defined at b Month. Not., vol. xxx. p. 151. March 1870, et infra.

a Lens, a lentil.