tween 1855 and 1858. The theory of gravitation alone cannot account for such a continued decrease of period; hence the hypothesis has been started that there is a resisting medium filling the space through which the comet passes. This hypothesis is a deus ex machina for explaining this solitary phenomenon, and cannot possess any validity or probability unless it can be shown that other phenomena are deducible from it. Many persons have identified this medium with that through which heat undulations pass, but I am not aware that there is anything in the undulatory theory of light to show that the medium would offer resistance to a moving body. If Professor Balfour Stewart can prove that a rotating disc experiences resistance even in a perfectly vacuous receiver, here is an experimental fact which distinctly supports the hypothesis. But in the mean time it is open to question whether other known agents, for instance electricity, may not be brought in, and I have tried to show that if, as seems highly probable, on other grounds, the tail of a comet is an electrical phenomenon, it is almost a necessary result of the theory of the conservation of energy that the comet shall exhibit a loss of energy manifested in a diminution of its mean distance from the sun and its period of revolution k. If so, the residual phenomenon seems to confirm an hypothesis as to the nature of the comet itself, rather than that of the medium through which it moves.

In other cases residual phenomena have involved important inferences not recognised at the time. Newton showed how the velocity of sound in the atmosphere could be calculated by a theory of pulses or undulations. from the observed tension and density of the air. He inferred that the velocity in the ordinary state of the

k Proceedings of the Manchester Literary and Philosophical Society,' 28th November 1871, vol. xi. p. 33.

atmosphere at the earth's surface would be 968 feet per second, and very rude experiments made by him in the cloisters of Trinity College seemed to show that this was not far from the truth. Subsequently it was ascertained by other experimentalists that the velocity of sound was more nearly 1142 feet, and the discrepancy being no less than one sixth part of the whole was far too much to attribute to casual errors in the numerical data. Newton attempted to explain away this discrepancy by hypotheses as to the relations of the molecules of air, but without success.

Many new investigations having been made from time to time concerning the velocity of sound, both as observed experimentally and as calculated from theory, it was found that each of Newton's results was inaccurate, the theoretical velocity being 916 feet per second, and the real velocity about 1090 feet. The discrepancy therefore remained as serious as ever, and it was not until the year 1816 that Laplace showed it to be due to the heat developed by the sudden compression of the air in the passage of the wave, this heat having the effect of increasing the elasticity of the air and accelerating the motion of the impulse. It is now perceived that this discrepancy really involved the whole doctrine of the equivalence of heat and energy, and the discrepancy was applied by Mayer, at least by implication, to give an estimate of the mechanical equivalent of heat. The estimate thus derived agrees satisfactorily with independent and more direct determinations by Dr. Joule and other physicists, so that the explanation of the residual discrepancy which so exercised Newton's ingenuity is now complete.

As Sir John Herschel observed, almost all the great astronomical discoveries have been first disclosed in the form of residual differences. It is the practice at well

conducted observatories to compare the position of the principal heavenly bodies as actually observed with what might have been expected theoretically. This practice was introduced by Halley when Astronomer Royal, and his reduction of the lunar observations gave a series of residual errors from 1722 to 1739, by the examination of which the lunar theory was improved. Most of the greater astronomical variations arising from nutation, aberration, planetary perturbation were in like manner disclosed. The precession of the equinox was perhaps the earliest residual difference observed; the systematic divergence of Uranus from its calculated places was one of the latest, and was the foundation of the remarkable discovery of Neptune by anticipation. We may also class under residual phenomena all the so-called proper motions of the stars. A complete star catalogue, such as that of the British Association, gives a greater or less amount of proper motion for almost every star, consisting in the apparent difference of position of the star as derived from the earliest and latest good observations. But these apparent motions are often due, as is expressly explained by Baily, the author of the catalogue, to errors of observation and reduction. In many cases the best astronomical authorities have differed as to the very direction of the supposed proper motion of stars, and as regards the amount of the motion, for instance of a Polaris, the most different estimates have been formed. Residual quantities will of necessity be often so small that their very existence will be doubtful. Only the gradual progress both of theory and of accurate measurement will clearly show whether a discrepancy is to be referred to previous errors of observation and theory or to some new phenomenon. But nothing is more requisite for the progress of science than

1 'British Association Catalogue of Stars,' p. 49.

the careful recording and investigation of all such discrepancies. In no part of physical science can we be free from exceptions and outstanding facts, differences and discrepancies of which our present knowledge can give no account. It is among such anomalies that we must look for the key to wholly new realms of facts worthy of discovery. They are like the floating waifs which led Columbus to suspect the existence of the new world.

CHAPTER XXVI.

CHARACTER OF THE EXPERIMENTALIST.

THERE seems to be a tendency to believe that, in the present age, the importance of individual genius is less than it formerly was.

'The individual withers, and the world is more and more.'

Society, it seems to be supposed, has now assumed so highly developed a form, that what was accomplished in past times by the solitary exertions of a single great intellect, may now be gradually worked out by the united labours of an army of investigators. Just as the combination of well-organized power in a modern army entirely supersedes the single-handed bravery of the mediaval knight, so we are to believe that the combination of intellectual labour has superseded the genius of an Archimedes, a Roger Bacon, or a Newton. So-called original research is now regarded almost as a recognised profession, adopted by hundreds of men, and communicated by a regular system of training. All that we need to secure great additions to our knowledge of nature is the erection of great laboratories, museums, and observatories, and the offering of sufficiently great pecuniary rewards to those who can invent new chemical compounds, or detect new species, or discover new comets. Doubtless this is not the real meaning of the eminent men who are now urging upon Government the elaborate endowment of physical