observations to commence a fortnight before and to terminate a fortnight after the Opposition of the planet. In consequence of the great eccentricity of the orbit of Mars, this method is only applicable to those Oppositions during which the planet is nearly at its least possible distance from the Earth. Airy pointed out the several advantages of this method, viz. :-that Mars may then be compared with stars throughout the night; that it has 2 observable limbs, both admitting of good observation; that it remains long in proximity to the Earth; and that the nearer it is, the more extended are the hours of observation; in all of which matters Mars offers advantages over Venus for observations of displacement in Right Ascension. Airy also entered into some considerations relative to certain of the forthcoming Oppositions, and named those of 1860, 1862, and 1877, as favourable for determining the parallax in the manner he suggested".

Le Verrier announced in 1861 that he could only reconcile discrepancies in the theories of Venus, the Earth, and Mars, by assuming the value of the solar parallax to be much greater than Encke's value of 8.571". He fixed 8.95" as its probable value, though, as Stone pointed out, this conclusion taken by itself rests on a not very solid foundation 8.

The importance of a re-determination was thus rendered more and more obvious, and Ellery, of Williamstown, Victoria, succeeded in obtaining a fine series of meridian observations of Mars, at its Opposition in the autumn of 1862, whilst a corresponding series was made at the Royal Observatory, Greenwich. These were reduced by Stone, and the mean result was a value of 8-932′′ for the solar parallax, with a probable error of only 0-032". This result was singularly in accord with Le Verrier's theoretical deduction. Winnecke's comparison of the Pulkova and Cape observations of Mars yielded 8.964".

• Month. Not., vol. xvii., pp. 208-21. May, 1857. Some practical hints on the conduct of observations are given by A. Hall in Ast. Nach., vol. lxviii., No. 1623, Jan. 16, 1867.

Annales de l'Observatoire Impérial,

h

vol. iv., p. 101. Paris, 1861.

• Month. Not., vol. xxvii., p. 241. April 1867.

h Month. Not., vol. xxiii., p. 185, April 1863.

The Opposition of 1877 was observed under favourable circumstances by Gill at the Island of Ascension, and his observations yielded as their final result a parallax of 8.78", with a probable error of 0.012". This implies a mean distance of the Earth from the Sun of 93,080,000 milesi.

Thus, though there may be some uncertainty in the amount of the correction, there is no doubt that the Sun is nearer than was formerly considered to be the case.

The distance amended to accord with a parallax of 8.8" is about 92,890,000 miles, with an error not likely much to exceed 150,000 miles*.

Hansen contributed something towards the elucidation of the matter. As far back as 1854 that distinguished mathematician expressed his belief that the received value of the solar parallax was too small, and in 1863 he communicated to Sir G. B. Airy a new evaluation, derived from his Lunar theory by the agency of the co-efficient of the parallactic inequality. The result was 8.9159′′, a quantity fairly in accord with the other values set forth above1.

Such is a brief statement of the circumstances which caused such special interest to attach to the transits of Venus which were to happen on December 8, 1874, and December 6, 1882: for it was supposed, that, all things considered, transits of Venus were most to be relied on for the purpose of ascertaining the amount of the Sun's parallax. The particular circumstances of the transits in question will come under notice hereafter. Meanwhile it may be stated that Stone has deduced 8.823′′ as the general result of all the British observations of the

1 Mem., R. A. S. xlvi., p. 1, 1881: Month. Not., vol. xli., p. 323. April 1881. * C. A. Young in Sid. Mess., vol. vi., p. 11, Jan. 1887.

1 Month. Not., vol .xxiv., p. 8. Nov. 1863. The amount of the correction to Encke's determination is about equal to the apparent breadth of a human hair seen from a distance of 125ft, or that of a sovereign at a distance of 8 miles. The whole amount of the parallax has been

put as the measurement of a ball one foot in diameter seen from a station nearly 44 miles distant from the ball. Unless the observer can "determine the diameter of the ball so that he shall not be uncertain in his measure to the amount of 003 of an inch, his work will not add anything useful to present knowledge." (Sid. Mess., vol. vii., p. 101, March 1888).

1882 transit. The Brazilian result by Wolf and André is 8-808".

It is almost needless to add that the acceptance of a new value for the solar parallax necessitates the recomputation of all numerical quantities involving the Sun's distance as a unit.

The real mean distance of the Earth from the Sun being ascertained, it is not difficult to determine by trigonometry the true diameter of the latter body, its apparent diameter being known from observation ; and, as the most reliable results show that the Sun at mean distance subtends an angle of 32′ 3.6", it follows that (assuming, as above, a parallax of 8.8") its actual diameter is 866,200 miles. It is generally accepted that there is no visible compression. The surface of this enormous globe therefore exceeds that of the Earth 11,900 times, whilst the volume is 1,306,000 times greater; since the surfaces of two spheres are to each other as the squares of their diameters, and the volumes as the cubes.

The linear value of 1" of arc at the mean distance of the Sun is about 450 miles.

The Sun's mass, and consequently its attractive power, is 332,260 times that of the Earth, and (approximately) is 749 times the masses of all the planets put together.

By comparing the volumes of the Sun and the Earth and bringing in the value of their masses, we obtain the relative specific gravity or density of the two.

The Sun's volume is to that of the Earth in the ratio of 1,306,000 to 1; the Sun's mass is to the Earth's in the lesser ratio of 332,260 to 1. Therefore the density of the Sun is to the density of the Earth as 332,260 to 1,331,570, or approximately as 1 to 4. Then taking Baily's value of the density of the Earth (5.67 times that of water), the density of the Sun is 1.42 times that of water.

Some interesting points may conveniently be noted here re

m Lindenau in 1809 and Secchi in 1872 propounded some strange ideas about the visible diameter of the Sun being subject

to periodical change, but those ideas met with no favour. (Auwers in Month. Not., vol. xxxiv., p. 22, Nov. 1873.)

specting the consequences which result from the stupendous magnitude and mass of the Sun. At the surface of the Earth a body set free in space falls 16.1ft in the first second of time, with a velocity increasing during each succeeding second. A body similarly set free at the surface of the Sun would start with a velocity 27.4 times as great as that of a body falling at the surface of the Earth. This is equivalent to saying that a pound weight of anything on the Earth would, if removed to the Sun, weigh more than 27. Liais has pointed out a singular consequence of this fact:-" An artillery projectile would have on the Sun but very little movement. It would describe a path of great curvature, and would touch the surface of the Sun a few yards from the cannon's mouth." The centrifugal force due to the rotation of any body diminishes gravity at its surface. At the Earth's equator the total diminution is pait; whilst at the Sun's equator the centrifugal force is only about 100 part of the force of gravity. It would be necessary that the Sun should turn on its axis 133 times quicker than it does, for the force of gravity to be neutralised. In the case of the Earth, however, a speed of rotation 17 times as great as it is would suffice to produce the same result. The insignificance of centrifugal force at the Sun's equator, compared with the amount of the force of gravity, suffices to explain the absence of appreciable polar compression in the case of the Sun's disc.

P

1

A consideration of the comparative lightness of the matter composing the Sun led Sir J. Herschel to think it "highly probable that an intense heat prevails in its interior, by which its elasticity is reinforced, and rendered capable of resisting [the] almost inconceivable pressure [due to its intrinsic gravitation] without collapsing into smaller dimensions "." That the internal pressure exerted by the gases imprisoned within the luminous surface or photosphere of the Sun, must be absolutely stupendous, we have evidence of in the fact of the almost inconceivable velocity (100 to 200 miles per second) of the uprushes of incandescent gas and metallic vapours, which are almost constantly taking place

Outlines of Ast., p. 297.

at various parts of its surface. It would seem all but certain that the Sun is nearly wholly gaseous, and that its photosphere consists of incandescent clouds, in which the aqueous vapour of our terrestrial clouds is replaced by the vapours of metals. These considerations, however, introduce a difficulty of a precisely opposite character to that which Sir J. Herschel essayed to combat; inasmuch as, in the light of our present knowledge, it seems hard to conceive how a mere shell of metallic vapour should be able to confine gases at the incomprehensible pressure at which those which rush out in the form of the now wellknown "Red Flames" (see post) must be confined.

The Sun is to be regarded as a fixed body so far as we are concerned; when therefore we say that the Sun "rises," or the Sun "sets," or the Sun moves through the signs of the zodiac once a year, we are stating only a conventional truth; it is we that move and not the Sun, the apparent motion of the latter being an optical illusion.

The Sun is a sphere, and is surrounded by an extensive and rare atmosphere; it is self-luminous, emitting light and heat which are transmitted certainly beyond the planet Neptune, and therefore more than 2700 millions of miles. Of the Sun's heat, it has been calculated that only 381000000 part reaches us, so that what the whole amount of it must be it passes human comprehension to conceive: like many other things in science. Our annual share would be sufficient to melt a layer of ice all over the Earth 100ft in thickness, or to heat an ocean of fresh water 60ft deep from 32° F. to 212° F., according to Herschel and Pouillet. Another calculation determines the direct light of the Sun to be equal to that of 5563 wax candles of moderate size, supposed to be placed at a distance of one foot from the

• Ganot, Physics, p. 391, 7th Eng. ed. 1875. This was calculated on the old value of the solar parallax.

P To show the great power of the calorific rays of the Sun, it may be mentioned that in constructing the Plymouth Breakwater, the men, working in diving bells, at a distance of 30ft below the sur

face, had their clothes burnt by coming under the focus of the convex lenses placed in the bell to let in the light. And houses have been set on fire by the Sun's rays. Langley puts the thickness of the layer of ice which could be melted at 160ft. (New Ast., p. 95.)