body moving more slowly at E would not pass clear of the Sun at s, so also, tracing its path back, it could not have arrived along a course clear of the Sun's globe ; and the only interpretation of its motions would be that it had been projected angularly from the Sun's surface with sufficient velocity to reach the Earth. It is not difficult to calculate the least velocity with which a body could travel when at the Earth's distance on an orbit just grazing the Sun's surface.* This velocity is

* There are many ways of attacking the problem. The best, perhaps, for our present purpose is the following:—

It is obvious that if a body be projected tangentially to the Sun's surface with such velocity as to just reach the Earth's orbit when at its greatest distance from the Sun, it would continue to revolve in such an orbit as we are inquiring into. Now, in order to determine the velocity of projection requisite for such a result, we may apply the formula in the preceding note, remembering that if a body projected vertically upwards from the Sun would reach to a height H, a body projected horizontally with the same velocity would travel in an orbit having a mean distance equal to (H+R) where R is the Sun's radius; so that we require our body to be projected tangentially with such velocity as would be required to project a body vertically to a height equal to the Earth's mean distance D (say) from the Sun's centre, since the required orbit is to have a mean distance equal to formula for v in the preceding note we have

(D+R). Hence, by the

(approximately);

whence we find the required velocity of tangential projection equal to 359 miles per second; and we note in passing that such a velocity as this would be required to project a body from the Sun to the Earth's distance. Now, this is the perihelion velocity of our projectile, when at a distance of 425,000 miles from the Sun's centre. In aphelion, its distance is 91,500,000 miles (approximately); hence from Kepler's first law its velocity there will be

or about 1.85 miles per second, which is almost exactly one-tenth of the Earth's velocity in her orbit, but considerably exceeds the Moon's.

about one-tenth of that with which the Earth moves in her orbit, or 1.85 miles per second; and no body can possibly reach the Earth with a smaller real velocity than this, unless actually projected from the Sun. A smaller relative velocity might very well be observed however. For example, a meteor travelling very little faster than the Earth might overtake her, and so seem to enter her atmosphere very slowly. And observations seeming to indicate that the real velocity of a meteor was less than 1.85 miles per second, would need to be very strongly confirmed before they would be accepted by astronomers; since to reach the Earth a body must be projected from the Sun with a velocity of about 360 miles per second.

Here, having already passed the limits I had proposed to myself in dealing with the subject of this chapter, I draw it somewhat regretfully to a close. It would have been easy to extend the chapter so as to have occupied a volume twice the size of the present, and yet to have discussed no well-worn facts. The whole subject of the Sun's rule over the space surround

This velocity would, however, be somewhat increased by the Earth's attraction. It is to be noticed, that the Moon's velocity in her orbit being about ths of a mile per second, the greatest velocity the Earth can control at the Moon's distance is but about ths of a mile per second; so that no body moving to the Earth's neighbourhood under the influence of the Sun's attraction can by any possibility be compelled by the Earth to travel in an orbit around her unless it comes much nearer than the Moon. The maximum velocity which the Earth can control close by her surface is, however, about seven miles per second; so that bodies having the aphelion of their orbit nearly at the Earth's distance and the perihelion close to the Sun's surface, could, if they happened to come close by the Earth, be compelled to circuit in an orbit round her.

ing him has remained in a sense almost untouched, until lately its significance began to be recognised by Mayer, Thomson, Waterston, and others. Even these, however, have dealt rather with the limits of activity possessed by bodies close to the Sun's surface than with the velocities which measure his influence at greater distances. For my own part, I find a wonderful interest in the ideas suggested by the Sun's activity throughout the whole range of his wide domain. The velocities to which I have referred in this chapter as those which the Sun can control are also those which he can generate. We have to think of him, therefore, as capable of drawing towards himself all such cosmical matter as comes under his exclusive attraction either by leaving the domain of some other star, or on account of his own motion through space. In so drawing cosmical materials towards himself, he imparts to them velocities such as we have been considering in the present chapter. The vaster the distances from which they come, the greater the velocities he imparts to them. As they sweep onward in their course they are subject to the influences of the planets-the patrols of the solar system-and under such disturbing influences large numbers must be compelled to follow either temporarily or permanently true orbits (that is, closed curves) around the Sun. But the majority of such visitants, whether comets or meteors, must return to the sidereal depths after once paying their respects -in the full rush of their perihelion swoop-to the giant ruler of our system. It seems probable that in

this continual rush of matter, this continual interchange of attendants on suns and stars, we may recognise the progress of processes exercising a most important influence on the welfare of planetary systems. It is still more probable that the bodies which are finally drawn into the solar domain perform highly important functions in the economy of our own particular planetary scheme. But unless I mistake, the real significance of the considerations we have dealt with in the present chapter lies in their bearing on the past history of the solar system. The rush of matter which we now recognise affords perhaps but the faintest indication of the amazing conflicts in which our system had its birth. Tracing back the history of that system, we seem to recognise a time when the Sun's supremacy was still incomplete, when the planets struggled with him for the continually inrushing materials from which his substance as well as theirs was to be recruited. see him by the mighty energy of his attraction clearing a wide space around him of all save such relatively tiny orbs as Venus and the Earth, Mars, Mercury, and the asteroids. With more distant planets the struggle was less unequal. The masses which flowed in towards the centre of the scheme swept with comparatively slow motion past its outer bounds, so that the subordinate centres there forming were able to grasp a goodly proportion of material to increase their own mass or to form subordinate systems around them. And so the giant planets, Jupiter and Saturn, Uranus and distant Neptune, grew to their present dimensions; and became

We can

records at once of the Sun's might as a ruler-for without his overruling attraction the material which formed these planets would never have approached the system -and of the richness of the chaos of matter from which his bulk and theirs were alike evolved. Nor is the consideration without a mysterious attraction that in thus looking back at the past history of our system we have passed after all but a step towards that primal state whence the conflict of matter arose. We are looking into a vast abysm, and as we look we fancy we recognise strange movements, and signs as if the depths were shaping themselves into definite forms. But in truth those movements show only the vastness of the abysm, those depths speak to us of far mightier depths within which they are taking shape. Lo! these are but a portion of His ways; they utter but a whisper of His glory.'