1More strictly, it plays the same part as a glass screen before a glowing fire. When the heat of the fire falls on such a screen (through which light passes readily enough), it is received by the glass, warming the glass up to a certain point, and the warmed glass emits in all directions the heat so received; thus scattering over a large space the rays which, but for the glass, would have fallen directly upon the objects which the screen is intended to protect.

2The case here imagined is not entirely hypothetical. We examine Mercury and Venus very nearly under the conditions here imagined; for we can obtain only spectroscopic evidence respecting the existence of water on either planet. In the case of Mars we have telescopic evidence, and no one now doubts that the greenish parts of the planet are seas and oceans. But Venus and Mercury are never seen under conditions enabling the observer to determine the colour of various parts of their discs.

I may add that a mistake, somewhat analogous to that which I have described in the cases of an imagined observer of our earth, has been made by some spectroscopists in the case of the planets Jupiter and Saturn. In considering the spectroscopic evidence respecting the condition of these planets’ atmospheres, they have overlooked the circumstance that we can judge only of the condition of the outermost and coolest layers, for the lower layers are concealed from view by the enormous cloud masses, floating, as the telescope shows, in the atmospheric envelopes of the giant planets. Thus the German spectroscopist VÖgel argues that because in the spectrum of Jupiter dark lines are seen which are known to belong to the absorption-spectrum of aqueous vapour, the planet’s surface cannot be intensely hot. But Jupiter’s absorption-spectrum belongs to layers of his atmosphere lying far above his surface. We can no more infer the actual temperature of Jupiter’s surface from the temperature of the layers which produce his absorption-spectrum, than a visitor who should view our earth from outer space, observing the low temperature of the air ten or twelve miles above the sea-level, could infer thence the actual temperature of the earth’s surface.

3In “Other Worlds than Ours,” I wrote as follows:—“The lines of hydrogen, which are so well marked in the solar spectrum, are not seen in the spectrum of Betelgeux. We are not to conclude from this that hydrogen does not exist in the composition of the star. We know that certain parts of the solar disc, when examined with the spectroscope, do not at all times exhibit the hydrogen lines, or may even present them as bright instead of dark lines. It may well be that in Betelgeux hydrogen exists under such conditions that the amount of light it sends forth is nearly equivalent to the amount it absorbs, in which case its characteristic lines would not be easily discernible. In fact, it is important to notice generally, that while there can be no mistaking the positive evidence afforded by the spectroscope as to the existence of any element in sun or star, the negative evidence supplied by the absence of particular lines is not to be certainly relied upon.”

4Dr. Draper remarks here in passing, “I do not think that, in comparisons of the spectra of the elements and sun, enough stress has been laid on the general appearance of lines apart from their mere position; in photographic representations this point is very prominent.”

5The word “ignited” may mislead, and indeed is not correctly used here. The oxygen in the solar atmosphere, like the hydrogen, is simply glowing with intensity of heat. No process of combustion is taking place. Ignition, strictly speaking, means the initiation of the process of combustion, and a substance can only be said to be ignited when it has been set burning. The word glowing is preferable; or if reference is made to heat and light combined, then “glowing with intensity of heat” seems the description most likely to be correctly understood.

6It would be an interesting experiment, which I would specially recommend to those who, like Dr. Draper, possess instrumental means specially adapted to the inquiry, to ascertain what variations, if any, occur in the solar spectrum when (i.) the central part of the disc alone, and (ii.) the outer part alone, is allowed to transmit light to the spectroscope. The inquiry seems specially suited to the methods of spectral photography pursued by Dr. Draper, and by Dr. Huggins, in this country. Still, I believe interesting results can be obtained even without these special appliances; and I hope before long to employ my own telescope in this department of research.

7In 1860, a year of maximum sun-spot frequency, Cambridge won the University boat-race; the year 1865, of minimum sun-spot frequency, marked the middle of a long array of Oxford victories; 1872, the next maximum, marked the middle of a Cambridge series of victories. May we not anticipate that in 1878, the year of minimum spot frequency, Oxford will win? [This prediction made in autumn, 1877, was fulfilled.] I doubt not similar evidence might be obtained about cricket.

8It must be understood that this remark relates only to the theory that by close scrutiny of the sun a power of predicting weather peculiarities can be obtained, not to the theory that there may be a cyclic association between sun-spots and the weather. If this association exists, yet no scrutiny of the sun can tell us more than we already know, and it will scarcely be pretended that new solar observatories could give us any better general idea of the progress of the great sun-spot period than we obtain from observatories already in existence, or, indeed, might obtain from the observations of a single amateur telescopist.

I think it quite possible that, from the systematic study of terrestrial relations, the existence of a cyclic association between the great spot period and terrestrial phenomena may be demonstrated, instead of being merely surmised, as at present. By the way, it may be worth noting that a prediction relative to the coming winter [that of 1877–78] has been made on the faith of such association by Professor Piazzi Smyth. It runs as follows:—

“Having recently computed the remaining observations of our earth-thermometers here, and prepared a new projection of all the observations from their beginning in 1837 to their calamitous close last year [1876]—results generally confirmatory of those arrived at in 1870 have been obtained, but with more pointed and immediate bearing on the weather now before us.

“The chief features undoubtedly deducible for the past thirty-nine years, after eliminating the more seasonal effects of ordinary summer and winter, are:—

“1. Between 1837 and 1876 three great heat-waves, from without, struck this part of the earth, viz., the first in 1846·5, the second in 1858·0, and the third in 1868·7. And unless some very complete alteration in the weather is to take place, the next such visitation may be looked for in 1879·5, within limits of half a year each way.

“2. The next feature in magnitude and certainty is that the periods of minimum temperature, or cold, are not either in, or anywhere near, the middle time between the crests of those three chronologically identified heat-waves, but are comparatively close up to them on either side, at a distance of about a year and a half, so that the next such cold-wave is due at the end of the present year [1877].

“This is, perhaps, not an agreeable prospect, especially if political agitators are at this time moving amongst the colliers, striving to persuade them to decrease the out-put of coal at every pit’s mouth. Being, therefore, quite willing, for the general good, to suppose myself mistaken, I beg to send you a first impression of plate 17 of the forthcoming volume of observations of this Royal Observatory, and shall be very happy if you can bring out from the measures recorded there any more comfortable view for the public at large.

“Piazzi Smyth,
“Astronomer-Royal for Scotland.”

If this prediction shall be confirmed [this was written in autumn, 1877], it will afford an argument in favour of the existence of the cyclic relation suggested, but no argument for the endowment of solar research. Professor Smyth’s observations were not solar but terrestrial.

[The prediction was not confirmed, the winter of 1877–78 being, on the contrary, exceptionally mild.]

9The reader unfamiliar with the principles of the telescope may require to be told that in the ordinary telescope each part of the object-glass forms a complete image of the object examined. If, when using an opera-glass (one barrel), a portion of the large glass be covered, a portion of what had before been visible is concealed. But this is not the case with a telescope of the ordinary construction. All that happens when a portion of the object-glass is covered is that the object appears in some degree less fully illuminated.

10It may be briefly sketched, perhaps, in a note. The force necessary to draw the earth inwards in such sort as to make her follow her actual course is proportional to (i) the square of her velocity directly, and (ii) her distance from the sun inversely. If we increase our estimate of the earth’s distance from the sun, we, in the same degree, increase our estimate of her orbital velocity. The square of this velocity then increases as the square of the estimated distance; and therefore, the estimated force sunwards is increased as the square of the distance on account of (i), and diminished as the distance on account of (ii), and is, therefore, on the whole, increased as the distance. That is, we now regard the sun’s action as greater at this greater distance, and in the same degree that the distance is greater; whereas, if it had been what we before supposed it, it would be less at the greater distance as the square of the distance (attraction varying inversely as the square of the distance). Being greater as the distance, instead of less as the square of the distance, it follows that our estimate of the sun’s absolute force is now greater as the cube of the distance. Similarly, if we had diminished our estimate of the sun’s distance, we should have diminished our estimate of his absolute power (or mass) as the cube of the distance. But our estimate of the sun’s volume is also proportional to the cube of his estimated distance. Hence our estimate of his mass varies as our estimate of his volume; or, our estimate of his mean density is constant.

11Only very recently an asteroid, Hilda (153rd in order of detection), has been discovered which travels very much nearer to the path of Jupiter than to that of Mars—a solitary instance in that respect. Its distance (the earth’s distance being represented by unity), is 3·95, Jupiter’s being 5·20, and Mars’s 1·52; its period falls short of 8 years by only two months, the average period of the asteroidal family being only about 4½ years. Five others, Cybele, Freia, Sylvia, Camilla, and Hermione, travel rather nearer to Jupiter than to Mars; but the remaining 166 travel nearer to Mars, and most of them much nearer.

12Even this statement is not mathematically exact. If the rails are straight and parallel, the ratio of approach and recession of an engine on one line, towards or from an engine on the other, is never quite equal to the engines’ velocities added together; but the difference amounts practically to nothing, except when the engines are near each other.

13I have omitted all reference to details; but in reality the double battery was automatic, the motion of the observing telescope, as different colours of the spectrum were brought into view, setting all the prisms of the double battery into that precise position which causes them to show best each particular part of the spectrum thus brought into view. It is rather singular that the first view I ever had of the solar prominences, was obtained (at Dr. Huggins’s observatory) with this instrument of my own invention, which also was the first powerful spectroscope I had ever used or even seen.

14It varies more in some months than in others, as the moon’s orbit changes in shape under the various perturbing influences to which she is subject.

15It may seem strange to say that one hundred and twenty years after the passage of a comet which last passed in 1862, and was then first discovered, August meteors have been seen. But in reality, as we know the period of that comet to be about one hundred and thirty years, we know that the displays of the years 1840, 1841, etc., to 1850, must have followed the preceding passage by about that interval of time.

16The D line, properly speaking, as originally named by Fraunhofer, belongs to sodium. The line spoken of above as the sierra D line is one close by the sodium line, and mistaken for it when first seen in the spectrum of the coloured prominences as a bright line. It does not appear as a dark line in the solar spectrum.

17Since this was written, I have learned that Mr. Backhouse, of Sunderland, announced similar results to those obtained at Dunecht, as seen a fortnight or so earlier.

18Here no account is taken of the motions of the stars within the system; such motions must ordinarily be minute compared with the common motion of the system.

19Eight pictures of nebulÆ were exhibited in illustration of this peculiarity.

20Sir John Herschel long since pointed to the variation of our sun as a possible cause of such changes of terrestrial climate.

21During these journeys the Atlantic was sounded, and Scoresby’s estimate of the enormous depth of the Atlantic to the north-west of Spitzbergen was fully confirmed, the line indicating a depth of more than two miles. It was found also that Spitzbergen is connected with Norway by a submarine bank.

22It is far from improbable that a change has taken place in the climate of the part of the Arctic regions traversed by Koldewey; for the Dutch seem readily to have found their way much further north two centuries ago. Indeed, among Captain Koldewey’s results is one which seems to indicate the occurrence of such a change. The country he explored was found to have been inhabited. “Numerous huts of Esquimaux were seen, and various instruments and utensils of primitive form; but for some reason or other the region seems to have been finally deserted. The Polar bear reigns supreme on the glaciers, as the walrus does among the icebergs.” Not improbably the former inhabitants were forced to leave this region by the gradually increasing cold.

23Dr. Emile Bessels was tried at New York in 1872, on the charge of having poisoned Captain Hall, but was acquitted.

24The phenomena here described are well worth observing on their own account, as affording a very instructive and at the same time very beautiful illustration of wave motions. They can be well seen at many of our watering-places. The same laws of wave motion can be readily illustrated also by throwing two stones into a large smooth pool, at points a few yards apart. The crossing of the two sets of circular waves produces a wave-net, the meshes of which vary in shape according to their position.

25It is a pity that men of science so often forget, when addressing those who are not men of science, or who study other departments than theirs, that technical terms are out of place. Most people, I take it are more familiar, on the whole, with eyelids than with palpebrÆ.

26This nautical expression is new to me. Top-gallants—fore, main, and mizen—I know, and forecastle I know, but the top-gallant forecastle I do not know.

27The instrument was lent to Mr. Huggins by Mr. W. Spottiswoode. It has been recently employed successfully at Greenwich.

28Thus in Christie Johnstone, written in 1853, when Flucker Johnstone tells Christie the story of the widow’s sorrows, giving it word for word, and even throwing in what dramatists call “the business,” he says, “‘Here ye’ll play your hand like a geraffe.’ ‘Geraffe?’ she says; ‘that’s a beast, I’m thinking.’ ‘Na; it’s the thing on the hill that makes signals.’ ‘Telegraph, ye fulish goloshen!’ ‘Oo, ay, telegraph! geraffe’s sunnest said for a’.’” “Playing the hand like a telegraph” would now be as unmeaning as Flucker Johnstone’s original description.

29Not “to represent the gutta-percha,” as stated in the Times account of Mr. Muirhead’s invention. The gutta-percha corresponds to the insulating material of the artificial circuit; viz., the prepared paper through which the current along the tinfoil strips acts inductively on the coating of tinfoil.

30I must caution the reader against Fig. 348 in Guillemin’s Application of the Physical Forces, in which the part cd of the wire is not shown. The two coils are in reality part of a single coil, divided into two to permit of the bar being bent; and to remove the part cd is to divide the wire, and, of course, break the current. It will be seen that cd passes from the remote side of coil bc, Fig.6, to the near side of coil de. If it were taken round the remote side of the latter coil, the current along this would neutralize the effect of the current along the other.

31The paper is soaked in dilute ferrocyanide of potassium, and the passage of the current forms a Prussian blue.

32Sir W. Thomson states, in his altogether excellent article on the electric telegraph, in Nichol’s CyclopÆdia, that the invention of this process is due to Mr. Bakewell.

33It is to be noticed, however, that the recording pointer must always mark its lines in the same direction, so that, unless a message is being transmitted at the same time that one is being received (in which case the oscillations both ways are utilized), the instrument works only during one-half of each complete double oscillation.

34It seems to me a pity that in the English edition of this work the usual measures have not been substituted throughout. The book is not intended or indeed suitable for scientific readers, who alone are accustomed to the metric system. Other readers do not care to have a little sum in reduction to go through at each numerical statement.

35Hanno’s Periplus—the voyage of Hanno, chief of the Carthaginians, round the parts of Libya, beyond the Pillars of Hercules, the narrative of which he posted up in the Temple of Kronos.

36I may mention one which occurred within my own experience. A mastiff of mine, some years ago, was eating from a plate full of broken meat. It was his custom to bury the large pieces when there was more than he could get through. While he was burying a large piece, a cat ran off with a small fragment. The moment he returned to the plate he missed this, and, seeing no one else near the plate, he, in his own way, accused a little daughter of mine (some two or three years old) of the theft. Looking fiercely at her, he growled his suspicions, and would not suffer her to escape from the corner where his plate stood until I dragged him away by his chain. Nor did he for some time forget the wrong which he supposed she had done him, but always growled when she came near his house.

37It may be suggested, in passing, that the association which has been commonly noticed between prominent eyeballs and command of language (phrenologists place the organ of language, in their unscientific phraseology, behind the eyeballs) may be related in some degree to the circumstance that in gradually emerging from the condition of an arboreal creature the anthropoid ape would not only cease to derive advantage from sunken eyes, but would be benefited by the possession of more prominent eyeballs. The increasing prominence of the eyeballs would thus be a change directly associated with the gradual advance of the animal to a condition in which, associating into larger and larger companies and becoming more and more dependent on mutual assistance and discipline, they would require the use of a gradually extending series of vocal signs to indicate their wants and wishes to each other.

38The word hypothesis is too often used as though it were synonymous with theory, so that Newton’s famous saying, “Hypotheses non fingo” has come to be regarded by many as though it expressed an objection on Newton’s part against the formation of theories. This would have been strange indeed in the author of the noblest theory yet propounded by man in matters scientific. Newton indicates his meaning plainly enough, in the very paragraph in which the above expression occurs, defining an hypothesis as an opinion not based on phenomena.

39I find it somewhat difficult to understand clearly Mr. Mivart’s own position with reference to the general theory of evolution. He certainly is an evolutionist, and as certainly he considers natural selection combined with the tendency to variation (as ordinarily understood) insufficient to account for the existence of the various forms of animal and vegetable existence. He supplies the missing factor in “an innate law imposed on nature, by which new and definite species, under definite conditions, emerged from a latent and potential being into actual and manifest existence;” and, so far as can be judged, he considers that the origin of man himself is an instance of the operation of this law.

40The Middle Tertiary period—the Tertiary, which includes the Eocene, Miocene, and Pliocene periods, being the latest of the three great periods recognized by geologists as preceding the present era, which includes the entire history of man as at present known geologically.

41Closely following in this respect his illustrious namesake Roger, who writes, in the sixth chapter of his Opus Majus, “Sine experientia nihil sufficienter sciri potest.”

42Fibrine and albumen are identical in composition. Caseine, which is the coagulable portion of milk, is composed in the same manner. The chief distinction between the three substances consists in their mode of coagulation; fibrine coagulating spontaneously, albumen under the action of heat, and caseine by the action of acetic acid.

43To this article of the Professor’s faith decided objection must be taken, however.

44Those whose custom it is to regard all theorizing respecting the circumstances revealed by observation as unscientific, may read with profit an extremely speculative passage in Newton’s Principia relating to the probable drying up of the earth in future ages. “As the seas,” he says, “are absolutely necessary to the constitution of our earth, that from them the sun, by its heat, may exhale a sufficient quantity of vapours, which, being gathered together into clouds, may drop down in rain, for watering of the earth, and for the production and nourishment of vegetables; or being condensed with cold on the tops of mountains (as some philosophers with reason judge), may run down in springs and rivers; so for the conservation of the seas and fluids of the planets, comets seem to be required, that, from their exhalations and vapours condensed, the wastes of the planetary fluids spent upon vegetation and putrefaction, and converted into dry earth, may be ultimately supplied and made up; for all vegetables entirely derive their growths from fluids, and afterwards, in great measure, are turned into dry earth by putrefaction; and a sort of slime is always found to settle at the bottom of putrefied fluids; and hence it is that the bulk of the solid earth is continually increased; and the fluids, if they are not supplied from without, must be in a continual decrease, and quite fail at last. I suspect, moreover, that it is chiefly from the comets that spirit comes which is indeed the smallest but the most subtle and useful part of our air, and so much required to sustain the life of all things with us.”

45See my “Science Byways,” pp. 244, 245.

46The following passage from Admiral Smyth’s Bedford Catalogue is worth noticing in this connection:—“We find that both the Chinese and the Japanese had a zodiac consisting of animals, as zodiacs needs must, among which they placed a tiger, a peacock, a cat, an alligator, a duck, an ape, a hog, a rat, and what not. Animals also formed the Via Solis of the Kirghis, the Mongols, the Persians, the Mantshus, and the ancient Turks; and the Spanish monks in the army of Cortes found that the Mexicans had a zodiac with strange creatures in the departments. Such a striking similitude is assuredly indicative of a common origin, since the coincidences are too exact in most instances to be the effect of chance; but where this origin is to be fixed has been the subject of interminable discussions, and learning, ignorance, sagacity, and prejudice have long been in battle array against each other. Diodorus Siculus considers it to be Babylonian, but Bishop Warburton, somewhat dogmatically tells us, ‘Brute worship gave rise to the Egyptian asterisms prior to the time of Moses.’” There is now, of course, very little reason for questioning that Egyptian astronomy was borrowed from Babylon.