of the most tremendous problems in the working of our universe. Here we find Lagrange proving that the system of our sun and planets is self-regulating, so that in spite of all its infinite changes, there is no real irregularity or changeableness in its machinery, but all moves in one perfect and constant round. Laplace shows the reason of those irregularities which seemed to contradict Newton's law of gravitation, and proves that they are all explained by that law, thus completing the work of the great astronomer. Then Herschel takes up the story, and after discovering a new planet, he studies the cloudy nebulæ, and points out the probable formations of new suns going on now in far-distant regions; he pictures our own sun rushing through space at the rate of 150,000,000 miles a year, carrying with it our earth and all the other planets; and above all he traces the law of gravitation into the distant star-world, and shows it there holding suns together and causing them to revolve round each other. And so, out into space as far as the mind can reach, we find everlasting order reigning through out the visible universe. List of Works consulted.-Herschel's 'Astronomy;' Arago, 'Vie et travaux de Herschel,' 1843; Proctor's 'Essays on Astronomy,' 'The Universe,'- 'Other Worlds than Ours'; Grant's History of Physical Astronomy;' Arago, 'Eloge of Laplace;' Airy's 'Astronomy;' Encyclopædia Britannica' -'Astronomy;' 'Orbs of Heaven,' Mitchell. CH. XXXI. ASTRONOMY. 287 CHAPTER XXXI. SCIENCE OF THE NINETEENTH CENTURY. Difficulties of Contemporary History - Discovery of Asteroids or Minor Planets between Mars and Jupiter-Dr. Olbers suggests they may be fragments of a larger Planet-Encke's Comet, and the correction of the size of Jupiter and Mercury-Biela's Comet noticed in 1826-It divides into two Comets in 1845-Irregular movements of Uranus-Adams and Leverrier calculate the position of an unknown Planet-Neptune found by these calculations in 1846—A Survey of the whole Heavens made by Sir John Herschel - His work in Astronomy-Comets and Meteor-systems. WE have now arrived fairly at the beginning of our own century, and shall have to speak of events which happened as it were but yesterday, and of men whom our grandfathers, or even perhaps our fathers, have seen and known. How are we to find our way through the mass of discoveries which have been made in every science during the last seventy years, or to make our choice among the number of famous men whose names we meet with every day? We must begin at once by recognising that it is impossible to mention all, even of the leading points, of the science of our time, and then we may try to learn a few of them, if we do so with a clear understanding that we are leaving important gaps unfilled. There are two special difficulties which we must encounter in the history of this century; first, we cannot avoid mentioning the work of some living men, while at the same time we omit others who are equally eminent; and secondly, we must speak of many subjects which are still, as it were, on their trial, and which will not be finally settled till they can be judged dispassionately by future generations. I have tried, however, to follow as far as possible the plan I adopted in the earlier centuries, of mentioning only a few great men whose work you can understand and follow; and stating on doubtful subjects what is the opinion of those who are best able to judge from the evidence. Therefore you must constantly bear in mind that this last portion of the book cannot be said to contain a history of the science of the nineteenth century, but only an account of a few of the leading discoveries and theories of our times and of the men who made them. Advance in Astronomy.-The science of Astronomy, in particular, has spread far beyond our power to follow it. We have seen that astronomers up to the end of the eighteenth century were always striving to work out the laws which govern the movements of the heavenly bodies. The key to this problem was found by Newton, and the work was completed when Laplace and Lagrange showed that even those planets which seem to have the most irregular orbits are really governed by the force of gravitation. From that time astronomy became really an exact science, and men had only to make their calculations with perfect accuracy in order to be able to foretell what was going to happen; or if they failed, then they knew there must be some other unknown heavenly body (such as Neptune, p. 294) causing the irregularity. Therefore, the science of astronomy in our century has been chiefly occupied in discovering new planets, stars, and star-clusters; at every step giving us new proof that gravitation rules throughout the visible universe. And |