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viscous condition. The gravitating attraction of the Sun's mass is ever tending to generate a downward velocity in the gaseous molecules, located in any part of it, towards its centre. At the same time, the radiation of heat at its surface diminishes the upward counterbalancing pressure. This allows the molecules to be actually more or less drawn inwards. Their individual average velocities are, upon the whole, increased. Their knocks against each other become more violent, and generate an increase of temperature. And the result, previously stated, which might at first seem to be almost paradoxical, is explained, viz. that the Sun, as its surface radiates heat away (which so far is a cooling process), may nevertheless, through its consequent contraction, generate a higher temperature by the fiercer clashings together, or knocks, of its molecules, as they are drawn inwards. This may enable it to send forth a more intense heat and light than before.

Therefore we said just now that a gaseous Nebula formed by one of the greater knocks of astronomy—the terrific knock of two great globes—after first showing a certain temporary brilliancy, and then, perhaps, becoming only faintly luminous while in a highly heated gaseous condition, might, in its subsequent slow contraction into a Central Sun, manifest a great increase of light and warmth. This result of an increased vigour in the knocks of the gaseous molecules would, however, only follow so long as they should retain their freedom to fly about and knock against each other. a globe of gas should begin to solidisy, it would then enter upon a stage of very much slower contraction, and its supply of heat-radiation, due to the increased velocity of its molecules, would soon practically fail.

As regards the Sun, it is believed that the heat evolved by such contraction as may be taking place in it at the present time keeps it very nearly at a constant temperature, and is just about enough to counterbalance its loss of heat by radiation. It has, however, been calculated that such evolution of heat by contraction cannot, in all probability, maintain the Sun's temperature sufficiently to support life as at present upon the Earth for much more than ten millions of years to come; nor that a similar supply can have been kept up for a period variously calculated at from ten to eighteen millions of years past. In any case, however, it is very interesting that the maintenance of the Sun's temperature depends upon the knocks of its molecules, as affected by its radiation and gravitation, during the present existence in it of a condition chiefly gaseous. Apart from these knocks the Earth would almost immediately become a frozen lifeless waste.

Of other astronomical knocks we must say but little. They doubtless abound in the head of a Comet, between the meteorites which in all probability form it, or in other groups of meteorites which may Vol. XXXVIII— No. 222

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be coursing through space or revolving around Suns. They may from time to time occur in the case of some of the myriad Satellites which form the Rings of Saturn. They may give some help to the evolution of gas, or of electricity, in Comet, Nebula, or Star. But in regard to most of the phenomena observed in all these denizens of space, and especially in connection with those of the heads and tails of Comets, in the glowing splendours and fiery jets of the heads, and in the immensely rapid repulsion of the tails, we believe that the molecular knocks are by far the most important.

The knocks and vibrations of electrical action, or of light and heat radiation, are, in consequence of their numbers and their vigour, of far more moment, in spite of the minuteness of the atoms and molecules involved, than any greater knock of one meteorite against another. These lesser knocks, which evoke, in an all-pervading ether, the undulations or vibrations by which the effects of light and heat and electricity are transmitted through space to affect our eyes, our brains, and our nerves, depend upon entities which are almost indescribably minute. Yet in astronomical observations they reveal to us the most brilliant luminaries, the greatest distances, the intensest temperatures, yet known.

In that branch of Astronomy which depends upon spectrum analysis there is, however, something even more wondrous. When one far-distant body after another tells us by its spectrum of what gases or substances it is made; when Comet, or Nebula, or Star reveals its own bright or dark lines, isolated in certain special localities in the spectrum, which localities are invariably the same for each gas or vapour to which they belong, and exactly correspond to the greater or less rapidity of the vibrations involved; why does each gas, in thus revealing its presence, produce only its own appropriate lines and rates of vibration, and no others? We cannot certainly say. It may be because the atoms or molecules of each gas, although almost indescribably minute, are capable of certain special individual vibrations of their own, and thereby impose certain vibrations, and no others, upon the ether within which they move. They may do so by their internal movements, or in some other way. Perhaps by the effect of their shapes as they move, or by a certain electrical action. Whatever the actual process may be, recent investigations indicate that the vibrations produced in the ether often form a beautiful harmonic series.

In any case, these molecular effects thus seen in the glittering lines of the spectrum of Sun or Nebula, of Comet or Meteorite, take us one step further into the mysterious recesses of the exceedingly small. These most minute actions of the very smallest things of which we know, tell us of the constitution of the vastest, the most distant, the most glorious.

Astronomy may well claim to be the most wonderful of all sciences, not because its measures are upon a scale that seems immense to beings such as ourselves, but because, in its revelations, the very greatest and the very smallest things—distances measured by billions of miles, masses weighing quadrillions and quintillions of tons, light and heat far surpassing all that is earthly—are intermingled in intimate union with the vibrations and movements of molecules or atoms of which trillions may be found in a single cubic inch of gas. Our view of every Sun, the explanation of its heat maintenance, the knowledge of its constitution, the knock of meteorite against meteorite, or even of star against star-all these in their vastness inextricably involve the knocks of molecules so small that it is difficult to believe that such minuteness can be real.

E. LEDGER.

UNIVERSITY EXTENSION IN AMERICA

(IVITH A PRIZE ESSAY)

No friend of University Extension can visit America without watching with interest and admiration the energy with which the movement has been carried on, or deriving stimulus from the enthusiasm with which the system is being developed. These were the opening words of the address delivered by Mr. M. E. Sadler, secretary to the Oxford University Extension Delegacy, at the first annual meeting of the National Conference, held in Philadelphia, the 29th of December, 1891. A brief account of the manner in which the University Extension movement originated in America, its influence and effect, as far as they can be traced during the period of not quite four years, may not be without interest to English readers.

The work in America of the Society for the Extension of University Teaching is of much more recent date than in England, where, as is well known, it was begun at Cambridge University in 1873, taken up by Oxford in 1878, while, at the present time, every University in England is engaged in it.

In February 1890 a number of leading educators of Pbiladelphia, and some of its most public-spirited and influential citizens, met by the invitation of Dr. William Pepper, then Provost of the University of Pennsylvania, to discuss the movement and the advisability of organising it in that city. It is only just to say, in passing, that it is hardly possible to over-estimate the services of the man who first saw the need of University Extension in the United States, and the great opportunities it offered to the cause of popular education. The plan met the approval of many who were consulted during the months succeeding, when the aid of the neighbouring Universities of Princeton, Lehigh, and Pennsylvania, and of Haverford, Swarthmore and Bryn Mawr Colleges was pledged, and on the 1st of June, 1890, the Philadelphia Society for the Extension of University Teaching was formed, with Dr. Pepper as President.

In order to commence the work with the advantages of an intimate acquaintance with English methods, the secretary was sent to study the system in Oxford, Cambridge and London. On his return he drew up a "Report on the University Extension Movement in England,' in which was indicated a general plan of organisation. In the meantime a communication had been sent to libraries, institutions, and associations of every character, describing what was about to be undertaken, and inviting their co-operation in the formation of local centres.

The active work was opened at Roxborough, a suburb of Philadelphia, on the 3rd of November, 1890, with a course on chemistry. From the very first the progress was rapid, and before the close of the season over forty courses of lectures on literature, history and advanced mathematics were delivered to a total attendance of more than 60,000 persons. Thus in six months the movement which had originated in Philadelphia secured a larger enrolment than might have been expected in years. For this success there were two reasons; one, and by far the most important, the well-developed system of Extension Teaching which had grown up in the course of more than a score of years' experiment in England, and the possibility of transferring it to America; the other, that it was started under favourable conditions, in a city where neighbouring colleges of the highest rank were freely drawn upon for lecturers.

The greatest impetus, however, was given during 1890 and the years following, by the presence and enthusiasm of able and distinguished men from the Universities of Cambridge and Oxford, who, not only by their lectures, but by their thorough acquaintance with every phase of the movement in England, did much to encourage and assist it in America. Its success in Philadelphia no sooner became generally known than requests for assistance in forming local centres came from so many parts of the United States, that the organisation was unable to satisfy them, and a national Society was determined upon; on the 23rd of December, 1890, therefore, the American Society for the Extension of University Teaching was established.

Those who had looked with some indifference on a merely local movement became interested in one of national scope, and those who had been striving for the cause before were cheered by the consciousness that it now seemed to be on the high road to success.

The results of the formation of this Society have been most encouraging; the time of uncertainty is past, University Extension evidently is destined to become a permanent feature in American life. Those who have the fullest knowledge no longer ask themselves whether it has a future, but rather how great that future is to be. Statistics are not convincing to many minds, and, indeed, are frequently of little real value, but when, as the outcome of the movement whose history has been briefly sketched, one can find in almost every State of the Union various societies and organisations labouring so successfully for this cause, the outlook is full of encouragement.

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