tained results not differing very greatly from these. He calculated that an interval somewhat greater than 2h. 13m. would be required to melt a layer of ice one inch thick.* Expressing his result according to a somewhat different method, he states, again, that if the Sun's heat were distributed uniformly over the Earth's surface it would in one year suffice to liquefy a layer of ice 100 feet thick, or to heat an ocean of fresh water sixty miles deep from the temperature of melting ice to the boiling point.'

Yet this enormous annual supply of heat is but the 1-2,138,000,000th † part of that which the Sun actually radiates into space in the course of a year. All the planets of the solar system are able to intercept but about the 227-millionth part of the heat actually emitted by the Sun. There is a fine passage in Herschel's Outlines of Astronomy' which shows how enormous is the amount of heat deduced by increasing in the ratio above indicated the supply of heat actually received from the Sun by the Earth :- Supposing a

The actual relation between Pouillet's and Herschel's results may be thus expressed. Sir John Herschel deduced 43:39 feet as the thickness of ice which the Sun is capable of melting per minute, supposing the ice continually applied to the Sun's surface (and the water produced by its fusion continually carried off). Pouillet deduced 387 feet per minute. Sir John Herschel says that 40 feet may be regarded as a probable mean. (It will be noticed that, with characteristic modesty and generosity, he places the mean much nearer to Pouillet's value than to his own.)

† It is singular how persistently the number 2,300,000,000, calculated by Mayer, maintains its ground in scientific treatises. This number was correctly deduced from the old value of the Sun's distance. But the above is the true value, according to the best modern estimates of the solar parallax.

cylinder of ice forty-five miles in diameter to be continually darted into the Sun with the velocity of light, the heat now given off constantly by radiation would then be wholly expended in its liquefaction on the one hand, while on the other the actual temperature at the Sun's surface would undergo no diminution.'

The luminosity of the Sun's surface is more readily estimated than the heat, since the intrinsic brilliancy of a self-luminous substance is in no way affected by distance; and we have only to take into account the effect which our own atmosphere may have in diminishing the apparent brightness of the Sun in order to form an accurate estimate of the intrinsic brilliancy of the Sun's light. Comparisons have been instituted directly between the light of the Sun and that of known terrestrial lights. It has been found that the most intense light we can produce appears absolutely black by comparison with the brightness of the solar orb. It has been estimated that the intrinsic brilliancy of the Sun's surface exceeds more than 146 times the brilliancy of the lime-light, and 32,700 times that of a sperm candle. In order to conceive the real amount of light to which a body close by the Sun-within a foot, say, of the photosphere-would be exposed, we must conceive the amount of light we receive in the full splendour of a summer's day increased in the same proportion that the whole hemisphere of sky exceeds the solar disc-besides, of course, a further addition corresponding to the proportion in which the brilliancy of the solar disc, if viewed without the interposition of

any atmosphere, would exceed the actual brilliancy observed on a summer day.

Of the chemical activity of the solar rays, it is not in our power to speak with so much confidence, since we have not as yet measured the Sun's power in this respect in a way which enables us to pronounce on its real extent. We can compare the intensity of the Sun's chemical action with that of terrestrial lights; but we have not yet found a means of determining its value as compared with those forces which the chemist more ordinarily employs to produce chemical changes.

It is worthy of notice, as respects the last two forms of solar activity, how large a share of the force we derive from the Sun is obtained through their action. This will be apparent when we remember the important bearing of the processes of vegetation on the wants of the human race. Nature,' says Mayer,

has proposed to herself the task of storing up the light which streams earthward from the Sun--of converting the most volatile of all powers into a rigid form, and thus preserving it for her purposes. To this end she has overspread the Earth with organisms, which, living, take into them the solar light, and by the consumption of its energy incessantly generate chemical forces. These organisms are plants. The vegetable world constitutes the reservoir in which the fugitive solar rays are fixed, suitably deposited, and rendered ready for useful application. With this process the existence of the human race is inseparably connected.

And even if we regard the effect of the Sun's heat as exerted upon the oceans and continents of our globe, we find that a large proportion of that which is eventually utilised by man, in one way or another, is first made subservient to the processes of vegetation. When the Sun's rays are poured down upon the ocean, or on parts of the Earth's surface in which water is abundant, the heat raises into the atmosphere large quantities of aqueous vapour. And this vapour, rising by reason of its extreme lightness, reaches eventually a region where it is condensed into clouds. Again, the heat of the Sun producing various effects, according to the nature of the regions on which it falls, gives rise to those differences of temperature which result in the generation of winds. By the agency of winds the clouds are transferred from the place of their formation to regions which require to be nourished by copious showers. And thus winds and clouds combine to support vegetation. The winds convey the clouds from place to place, and the clouds themselves, in the expressive language of Scripture, drop fatness on the earth.'

It is worthy of notice, too, that beside the action of the Sun in supporting vegetation at the present time, it was the same form of action exerted in longpast ages which resulted in storing up for our use those vast supplies of energy which are contained within our coal-mines. In other words what may be called our force-principal is as fully due to the Sun's action (direct or indirect) in promoting vegetation as

that force-interest which we derive each year from the Sun's seasonal action.

And here I may be permitted to dwell on considerations which, though bearing rather on the economy of our Earth than on the general subject of solar physics, yet illustrate in a significant manner the work which the Sun has been appointed to do. I may premise, indeed, that we have no means of determining what the Sun's influence on the other planets may be, however clear it may appear to us that we are not the only, nor even the chief, recipients of those stores of force he lavishes so abundantly. It is on this account that while I give to this treatise a title indicating the Sun's position in the solar system, I deal only in this chapter-the sole one bearing on the Sun's office— with his position as our fire, light, and life. If in the considerations I am about to urge the Earth only seems concerned, it is none the less probable that results affecting the economy of the whole planetary scheme are in truth illustrated.

We are accustomed to look upon the Earth as an inexhaustible storehouse whence all our wants may be supplied. Year after year we till the soil, and still there is no lack in the growth of all the vegetable productions needed by man; nor do our flocks and herds diminish, notwithstanding the enormous supplies of flesh-meat we are continually consuming. Taking the whole Earth, it is probable that the yearly produce of agricultural and pastoral labours increases at even a higher rate than that at which the human race is in