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swiftness, are years in reaching our remote sphere, keeping their secret intact for our eyes through all the perils of immeasurable space.

In seeking to give some rapid idea of the remarkable discoveries that have been made of late years in the physical and planetary relations of the central orb of our system, we will first glance at its exterior planetary relations, and close with a description of the new views concerning its surface. The planetary bodies are affected in various ways by their governing centre; by the heat constituent of its beams, to which all their surface forces are primarily due; by the actinic constituent of its light, without which no life could appear, and by its vigorous attractive force, which firmly binds its swift attendants in their ordained orbits.

To a correct measurement of these, particularly the latter power (it being the unit of all astronomical calculation), a close knowledge of the distance of the sun from the earth is necessary. This distance has not yet been satisfactorily determined, and the coming transit of the planet Venus is of peculiar interest to astronomers, as it will afford them the most reliable means of correcting their former measurements. There are several other methods of arriving at this determination, but none of sufficient precision to weigh against the sharp lines drawn by Venus across the sun's disc, twice in a century.

A close calculation has been made of late years of the observations of the transit of the last century, collated with results from these other methods, the supposed distance being reduced from 95,000,000 miles to about 91,370,000 miles, with a probable error of about 300,000. Sir John Herschel says that this correction in the value of the sun's parallax corresponds in value to the breadth of a human hair at one hundred and twenty-five feet, or of a sovereign at eight miles disThis new figure as to the sun's distance will reduce

tance.

his diameter to about 850,000 miles.

The rays which flow from the sun, and a minute portion of which touch the earth in their outward flight, are, as we have said, the chief moving agents of all the evolutions of na

ture upon the terrestrial surface. Not only those evident changes which we have particularized, but the slow degradation of the solid surface which has been the chief agent of geological changes; the force of ocean currents in transporting material wearing away the surface, and, by diminished pressure, permitting volcanic eruptions; the laying up great reservoirs of coal and petroleum, and numerous other less apparent results, have proceeded directly from the incessant action of these beams through vanished myriads of years.

The amount of heat thus emitted by the sun is almost beyond our powers of conception. The beams poured down each hour on an equatorial square mile would melt 26,000 tons of ice. The heat which the earth receives in a year would, if equally distributed, melt a layer of ice 100 feet thick over the entire surface.* Yet to arrive at the total radiated heat of the sun, we must multiply this quantity by 2,138,000,000. All planets absorb but the 227-millionth part of this inconceivable emanation, the great ocean of which rushes through the space beyond our system with marvellous speed, to announce the existence of our solar orb to every star of the visible universe. It has been calculated that if a cylinder of ice, 45 miles in diameter, were darted into the sun with the speed of light (180,000 miles per second), the amount of heat now emitted would be just sufficient to melt it, while the active temperature of the sun's surface would suffer no diminution.‡

A portion of the heat force thus emitted is stored up upon the earth in the forms of vegetation, which, hardened into coal, form the main force agency of the modern world. We now use what Professor Tyndall calls the sun of the carboniferous period.

Efforts are being made to replace the rapid consumption of this stored force by the employment of the active emanations of the sun. Ericsson and several French scientists are experimenting with what they denominate solar engines,

* Herschel's Outlines of Astronomy.

Herschel, p. 231.

Le Soleil, p. 275.

which they seek to move with the direct heat of the sun. Calculation proves this direct heat to be exceedingly vigorous, and if it can be economically employed, it may greatly increase our present sources of power. Proctor calculates that the force expended to produce a day's steady rain over an area equal to that of the county of Middlesex, England, would be equivalent to a mechanical power competent to raise 1,000,000,000 tons three miles high.*

Professor Tyndall draws a still more striking picture of this force, comparing the thundering vehemence of the fierce. stone avalanche of the Alps with the soft descending of the fragile snow crystals. Yet to produce from aqueous vapor a quantity which a child could carry of the snow would consume a force sufficient to gather up the scattered blocks of the largest stone avalanche and hurl them back to twice the height from which they fell.

It is one of the most difficult problems of physical astronomy to discover the source of the vast heat thus emitted, and which has been flowing out with equal energy for uncounted millions of years. There is nothing on earth in consonance with such a condition, and we are forced to pure conjecture to arrive at some conception of its cause.

If the sun were a solid block of coal it would be burnt out in 5,000 years. If it were a heated globe, with the great specific heat of water, it would cool 15,000° in the same period.† We have on earth no sources of constant heat superior to these. Yet we know that no body can yield heat without a positive loss of force, and the question of the origin of the sun's intense and unceasing emanations remains open. M. Mayer ascribes it to a constant fall of cosmical matter on the solar surface, a fierce rain of meteorites whose concussion produces an evolution of heat. We will consider this theory further on, simply premising here that there is great probability of its being one of the elements of the problem.

Professor Helmholtz considers it due to a gradual contraction of the solar mass. According to his calculation a shrink

*The Sun, p. 405.

+ Meyer, Dynamik des Himmels, p. 10.

age of 1-10,000 part of its diameter would produce heat sufficient to cover its emissions for 2,000 years. * The evidence of a contraction from an original nebulous condition lends force to this theory, and each of these ideas, while having objections as yet unexplained, are yet very probable contributions to the unsolved question of the source of the solar heat.

Our sun is but one of millions of similar orbs which the telescope has revealed to us, and of myriads too inconceivably distant to reveal themselves to the utmost powers of optical instruments. Yet their light streams across these mighty intervals of endless space. They are tied by gravitation in bonds of intimate connection, and possess relative motions. The sun and his family of planets are thus drifting through space at the rate of 150,000,000 miles per year, and a like star drift has been observed in many of the stellar orbs, whole families of suns in certain cases floating on with equal speed in parallel lines. This is evident, for instance, in five of the principal stars of Ursa Major, and indicates a connection between suns resembling that between planets.†

But while bound to these sister orbs by the chains of equal attraction, the sun has a vast field of space under his own supreme dominion, an empire with a diameter of many billions of miles, peopled by numberless bodies, varying in size from the smallest meteorite to the size of the majestic planet Jupiter, all of which are his servants, and move in strict subserviency to his attraction.

His vigorous force spreads throughout this immense domain, drawing towards himself all its physical tenants, and causing them to rotate in orbits in conformity with their original speed. If any body starts from a state of rest to fall towards the sun from any distance short of infinity, it will inevitably be forced to assume an orbital revolution. If it fall from an infinite distance, its force of motion will exceed the sun's power of attraction, and it will pass again into infinity. This, of course, supposing that only the sun and the body existed,

* Tyndall's Heat as a Mode of Motion, p. 443. † Nature, March 3, 1870.

and that the universe contained no disturbing elements to such a relation.

Mathematical astronomy readily solves this problem, and arrives at the speed of a body thus falling from infinity, and from less distances, and is thus enabled, by knowing the speed and direction of a body at any fixed distance from the sun, to calculate the shape of its orbit, and the maximum and minimum distances of this orbit. from the centre of attraction. The great solar sphere, with a mass 315,000 times that of the earth, and an attractive vigor so great that a pound weight at its surface would weigh 14 tons, while a man would weigh about 20,000 tons, grasps the huge bodies of the planets with a vigor sufficient to overpower their enormous vis viva, and force them to rotate in fixed orbits.

An increase of the earth's mass, if effective at all, would tend to increase the strength of this bond, and draw it yet nearer the sun. An increase of its speed, however, or a diminution of the sun's mass, would lessen the force of connection, the earth moving away from the sun. If this change were sufficiently great it would leave the sun forever. The earth's mean velocity, in its orbit, is now 18.2 miles per second. Were this velocity increased to a speed greater than 25.7 miles per second, the sun's attractive force would be negatived, and the earth move off, a free tenant of space. The same result would occur were the sun's mass reduced in the ratio of 1,414 to 1,000.

A body revolving in a circle just touching the sun's surface, would need a speed of 268 miles per second to keep from being drawn in by the intense gravitative force. At a speed of 378.9 miles per second it would just graze the surface, and return after proceeding outward to an indefinite distance. If the speed were increased beyond this, it would not return. This, then, is the speed a body would attain in falling from an infinite distance to the sun, and without aid or retardation from exterior influences.

The sun can control a body, at Neptune's distance, having a velocity of 4.7 miles per second, being 1-80th of its controlling power at its surface. At four times the distance of Nep

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