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placements, and those strains which are actually observed in glacier ice, and which must, therefore, be present in every part of the glacier.
Mr. Crookes has contributed a paper “ On the Measurement of the Luminous Intensity of Light," in which he shows that the measurement of this intensity is a problem which, though repeatedly attempted, has not been as successfully accomplished as in the case of other radiant forces. The problem being clearly susceptible of division into the absolute and the relative, what we want to obtain is the first, but, at present, we are apparently far from procuring or constructing a photometer analogous to a thermometer in fixity of standard and facility of observation. The probable course towards the attainment of this object is shown in the observations of M. Becquerel, Sir John Herschel, and others, on the chemical action of the solar rays, and on the production thereby of a galvanic current capable of measurement by a delicate galvanometer. The measurement of a chemical beam of light is as distinct from photometry proper as is the thermometric registration of the heat-rays constituting the other end of the spectrum. What we want is a method of measuring the intensity of those rays which are situated at the intermediate parts of the spectrum, and which produce in the eye the sensation of light and colour. Experiments made some years since convinced Mr. Crookes that it is not merely the ultra-violet invisible rays which are valuable for photography, but that some of the most highly luminous rays of light are capable of exerting chernical action ; and this position was ultimately proved by him by means of a combination of certain chemical compounds.
It is very likely that the further carrying out of these experiments may lead to the construction of a photometer capable of measuring the luminous rays; it being remembered that the proportion of red, yellow, green, and blue rays is always invariable in white light (for, if this were not so, the light would not be white, but coloured), from which it follows directly that a satisfactory method of measuring one set of the components of white light will give all the information we want, just as in an analysis of a definite chemical compound the chemist is satisfied with an estimation of one or two constituents only, and from these is able to calculate the others. Methods based on the previous considerations would supply us with what may be termed an absolute photometer, the indication of which would be always the same for the same amount of illumination, and would require no standard light for comparison. A relative photometer is one in which the observer has only to determine the relative illuminating powers of two sources of light, one of which is kept as uniform as possible, the other being the light whose intensity is to be determined. The first thing to be aimed at is an absolutely uniform source of light, and this is most difficult to obtain. In the ordinary process of photometry the standard used is a candle, defined by Act of Parliament as a “sperm candle of six to the pound, burning at a rate of 120 grains per hour :” hence the meaning of such terins as “12-candle gas," “14-candle gas.” The difficulty is to obtain candles truly made, containing refined sperm mixed with a small portion of wax, and wicks of the best cotton, each made of three cords plaited, and each cord itself again of seventeen strands. Again, according to the quality of the sperm in richness or hardness, so will also vary the plaiting and number of the strands; and, further, experience that, supposing all the previous conditions to be satisfactorily attained, the illuminating power of the candle will be found to vary with the temperature of the place where it has been kept, the time which has elapsed since it was made, and
the temperature of the room where the experiment is tried. The “Parliamentary candle,” therefore, may be pronounced a failure wherever accurate results are required. The general principle on which the illuminating powers of different substances have hitherto been tested depends on the optical law that the amount of light which falls upon a given surface varies inversely with the square of the distance between the source of light and the object illuminated. In practice, however, this method is not sufficiently accurate to be used, except for the roughest approximations. The rest of Mr. Crookes' paper is devoted to details of a process he has invented to get rid of the “ Parliamentary candle.” This is, however, too abstruse and minute for extraction here.
The Earl of Rosse, F.R.S., has communicated a paper “On the Radiation of Heat from the Moon,” which is too mathematical for a popular notice. The general result, however, is to show conclusively that the moon's heat is capable of being detected with certainty by the thermopile; no inconsiderable quantity of heat reaching the earth by radiation from the moon. The points to be determined were:-1. The heat which, coming from the interior of the moon, does not vary with the phase. 2. That which falls from the sun on the moon's surface and is at once reflected regularly and irregularly. 3. That which, falling from the sun on the moon's surface, is afterwards radiated as a heat of low refrangibility.
Mr. Ellery, of the Observatory, Melbourne, in a letter to the President of the Royal Society, gives an interesting account of the safe arrival of the great telescope we noticed at some length last year, and of the means which have been already taken to set it up and to make it available for its intended use. He states it had, on the whole, travelled perfectly round the Cape, his words being that “the principal or more delicate portions of the instrument came out in good order ; the specula are still in their coats of varnish, and their surfaces appear to be in perfect good order. Some of the large castings and portions of the gearing had got rusted, but not to an injurious extent. The piers were completed on New Year's morning, and form a magnificent piece of masonry; the stone employed being the grey basalt so common here (called “blue stone'), in blocks from one to three tons in weight each. The building we have finally decided on is built of stuccoed brick-work, eighty feet long by forty wide. Forty feet in length is taken up by the telescope-room, which is covered by a ridged roof of iron travelling on rails on the walls, and moving back on the other forty feet of the building, leaving the telescope in the open air. The back forty feet is covered by a fixed roof lower than the moveable one, and will contain a polishing and engine-room, a capacious library, and an office for the observer. The cost of piers, building, and roof will be 17001. The Government, with hard economy in all other directions, have acted very liberally about this work.”
Dr. Radcliffe, M.D., has contributed a very able paper, entitled “Researches in Animal Electricity,” containing a description of certain instruments now employed for the first time in researches of this kind, the chief subjects of inquiry being the electrical phenomena which belong to nerve and muscle in a state of rest; those which mark the passing of nerve and muscle from a state of rest into that of action; the motor phenomena ascribed to the action of the “inverse” and “ direct” voltaic currents, and electrotonus. The instruments used were Sir W. Thomson's reflecting galvanometer, Latimer Clarke's potentiometer, and some new electrodes devised by the author. The last, which is an ingenious adaptation of the idea on which Wheatstone's bridge is based, is an extremely delicate instru
ment for the measurement of tension. The new electrodes are simply pieces of platinum wire, flattened and pointed at the free ends, and having these free ends freshly tipped with sculptor's clay at the time of an experiment. Living nerve and muscle supply currents to the galvanometer (the nerve current and the muscular current, so called) which are not supplied by dead nerve and muscle. These currents, when the tissues supplying them are fresh and at rest, show that the surface composed of the sides of the fibres, and the surface composed of the ends of the fibres, are in opposite electrical conditions, the former surface being positive, the latter negative. Nerve and muscle, and the animal tissues generally, oppose a very high resistance to the passage of a common voltaic current, so high, indeed, as to justify the inference that muscles and nerves may be looked upon as non-conductors rather than as conductors. Again, in considering the electrical phenomena which mark the passing of nerve and muscle from a state of rest into that of action, the more the evidence is considered the more it seems to justify the conclusion, that the passing of nerve and muscle from the one state to the other is marked by a discharge of electricity analogous to that of the torpedo. Again, when experiments are made as to the “motor phenomena ” ascribed to the action of the “inverse and direct voltaic currents,” it seems probable that, ordinarily at least, the sheaths of the fibres are charged positively at their exterior, and negatively at their interior. The resistance of the animal tissues to electrical condition, it is assumed, is sufficient to keep the two opposite electricities apart-an assumption, be it remarked, which is not a little borne out by the fact that the resistance which the voltaic current encounters in the hind limbs of a frog, when its course is up one limb and down the other, is sufficient to keep the two limbs in opposite electrical conditions as regards discharge. The general conclusion seems to be that muscular relaxation is associated with a state of charge, and muscular contractions with a state of discharge. It would even seem as if all the evidence so far gave countenance to the conclusion that the state of charge may cause muscular relaxation by keeping the molecules of the muscle in a condition of mutual repulsion, and that the state of discharge may lead to muscular contraction by doing away with that state of electrical tension which prevents the molecules of the muscle from yielding to the attractive force inherent in their physical constitution, and which is ever striving to bring them together. The experiments on electrotonus, and the results deducible from them, are of too technical a character for popular statement.
From Mr. Robert H. Scott, the Director of the Meteorological Office, we have a paper of some note, “On the connexion between oppositely disposed Currents of Air and the Weather subsequently experienced in the British Islands.” In this paper he states that Mr. Meldrum at the Mauritius, and he himself in England, had had their attention directed to remarkable storms which appeared to be connected with the previous existence at the earth's surface of the two windcurrents, polar and equatorial, in close proximity to each other. This was specially noticeable in a gale of January 22, 1868, when the atmospherical conditions over these islands were very remarkable. Easterly winds were prevalent over the central and northern portions of this country, while in France there were strong westerly gales. The channels of the currents were so close to each other, that while at Yarmouth there was a strong easterly gale, there was a westerly gale at Portsmouth. The contrast exhibited by the two currents as regards temperature was very remarkable, and a dense fog was experienced in London. Barometrical readings were very low over the regions which separated the districts of
the respective currents. Next day pressure rose very rapidly; and this was the precursor of an equally sudden diminution of the amount, and of the advent of the equatorial current which swept with great violence over these islands, producing a very serious southerly gale on the 24th of January.
Other able papers to which we can only now refer, briefly as possible, are-a notice by Professor Maskelyne, “On the Mineral Constituents of the Breitenbach Meteorite ;” by Professor Abel, “On the History of Explosive Agents ;” by Dr. Archibald Smith, “ On the Causes of the Loss of the iron-built sailing-ship ‘Glenorchy’;" by Mr. H. F. Blanford, “On the Origin of a Cyclone;" by H. C. Sorby, whose paper on precious stones we have already noticed with some detail, “ On Jargonium, a new elementary substance associated with Zirconium ;" by Dr. R. Norris, “ On the Laws and Principles connected with the Aggregation of Bloodcorpuscles both within and without the Vessels ;” “On some Experiments with the great Induction Coil at the Royal Polytechnic,” by J. H. Pepper ; “On the mechanical description of Curves,” by W. H. L. Russell ; On the Thermodynamic Theory of Waves of Finite Longitudinal Disturbance," by W.J.Macquorn Rankine, F.R.S. ; "On a Group of varieties of the Human Neck, Shoulder, and Chest, with their Transitional Forms and Homologies in the Mammalia,” by John Wood, Esq., F.R.C.S., Examiner in Anatomy to the University of London ; “On the Cavern of Bruniquel and its Organic Contents,” by Professor Owen; “On a Comparison of the Granites of Cornwall and Devonshire with those of Leinster and Munster,” by Professor Haughton; and “On Luteine and the Spectra of yellow Organic Substances,” by Dr. Thudicum.
One notice we must add here—though of a book rather than of a paper—by that veteran philosopher and man of science, Professor John Phillips of Oxford, who has recently made public an excellent account of “ Vesuvius,” and of his ascent of that mountain. Need we say that we hail this ascent (more than once) of Vesuvius during its recent outbreak by the Professor of Geology at Oxford with somewhat of the same feeling the scientific men of his day must have hailed that of the elder Pliny? only we rejoice where they deplored: our man of science comes back to us unburnt, we believe we may say, unscathed, while the great Roman naturalist remains under the volcanic storm which overwhelmed Pompeii and Herculaneum, and has not, we feel grateful to say, been as yet exhumed-a spectacle for the gaping idiots of fashion who yearly “do” those interesting remains in their own meaningless and unprofitable method. Professor Phillips's work may be termed a history, not a merely scientific essay, for he tells us all the chief facts about the mountain from the time of Pliny to that of the eruption which, commencing in 1861, on December 8, is not even now entirely subdued. His style is at once clear and picturesque, and the following description of his view of the eruption from Naples affords a fair example of it:“One long burning stream," he says, "flowed down the whole north-western slope of the great cone, quite reaching into and spreading across the Atrio del Cavallo. On the top, fitful bursts of clouds of fiery bombs and wide-spread ashesbelow, just where it appeared last night, but now far brighter, and glowing with a full steady eye of light, the second great burst of light and motion. Now it spreads its bright cloud above, then down to the valley ; knots and lines, sometimes double, of sharp white or reddish fire swelling into considerable masses, or broken into many gleaming points. Towards the base, a wild cataract of fire is pouring towards us, and is stretching its red fingers over the elder lava. Now and then a star-like point in advance seems to beckon onward 'der freien Tochter
der Natur.' Finally, on the deepest part of the whole visible horizon, ahorizontal row of fourteen small bright star or gem-like fires marks the conquest of the current over the flat space of the Atrio, and seems to unite again the longseparated masses of Somma and Vesuvius-parent and child—the far-descended progeny of the struggling Titans.”
The British Association for the Advancement of Science met this year at Exeter under the distinguished Presidency of the Rev. G. G. Stokes, Lucasian Professor of Mathematics at Cambridge, and formerly Senior Wrangler. Professor Stokes delivered to the assembled members and their friends an address widely differing from some which the Association has heard in recent years, to some of the leading points of which we shall now briefly refer. Taking first, as was but natural for one of the most eminent of modern mathematicians, the great subject of Astronomy, Professor Stokes showed that, though Newton's discovery of the law of gravitation did practically explain almost all the motions of the heavenly bodies, yet it was fortunate for science that Adams at Cambridge, and Le Verrier in France, were able to reverse the problem ; and, instead of determining the disturbing effect of a known planet, to set themselves to inquire what must be the mass and the orbit of an unknown body which shall be capable of producing by its disturbing force the unexplained deviations from the calculated place of Uranus. Passing on from this, Professor Stokes pointed out in how important a degree astronomy was indebted to the science of optics ; at the same time showing that astronomy well repaid this debt, by settling once and for ever the numerical powers of the velocity with which light travels, and, at the same time, exhibiting the remarkable phenomenon discovered by Bradley, and termed by him, and since his day, “ the aberration of light.” For optics, it may be urged that, though the motions of the heavenly bodies are chiefly revealed to us by astronomical observations, yet that the application of the spectroscope has proved to us the existence in them of various elements already known to us by the chemical examination of the materials of our own earth. It is clear, therefore, that the two sciences of optics and astronomy must be studied together, as the one throws the greatest light upon the other. Again, the science of optics has the highest value when we want to ascertain whether a particular star is approaching us or receding from us, and the chain of reasoning pursued is similar to what happens in the case of the pulsation of a musical wote. The pitch of a note is well known to depend on the number of vibrations which reach the ear in a given time-say a second-so if light be, as we have good reason to believe, a vibrating fluid, the pulsations of this fluid may be reduced to calculation. Now the result of present scientific researches tends to prove that light consists of a tremor or vibrating movement propagated in an elastic medium filling the planetary and stellar places, a medium which thus fulfils for light an office similar to that of air for sound. The professor then went on to describe the value of Professor Kirchkoff's experiments on the lines in the solar spectrum, and demonstrated that though the coincidence of certain dark lines in the solar spectrum with bright lines in certain artificial sources of light had been in one or two instances previously noticed, still it was to Kirchkoff we owe the inference that a glowing medium which emits bright light of any particular refrangibility necessarily (at that temperature at least) acts as an absorbing medium extinguishing light of the same refrangibility. It is curious that in this discovery Kirchkoff was preceded, though unconsciously, by our own countryman Professor Balfour Stewart.