Science

CAPTAIN ERICSSON has recently published, in “Nature,” a paper containing the tabulated results of some very interesting observations for determining the amount of temperature due to direct solar radiation at any given parallel of latitude. The method employed was the same as that which was adopted by Newton in order to obtain the data for estimating the temperature of the solar surface. In the latitude of London, for example, at midsummer, the average temperature of dry earth is 85° F. in the shade, while in direct sunlight it is 150° F. Now, as dry earth is a very poor conductor, but an excellent absorber and radiator, it follows that no appreciable proportion of the heat acquired by a given patch of earth, when exposed to direct sunlight, can be due to conduction from the earth surrounding it ; and, as there can be no radiation laterally, but only upwards in the very direction from which the heating rays proceed, it is evident that the 65 degrees of difference between the shaded and exposed earth must represent the heating power of the rays. The only disturbing circumstance is the possible interference of currents of air blowing over neighboring patches of earth which are hotter or colder ; but this possible source of error may be got rid of, either by averaging the results of a large number of observations, or by confining the patch of earth within a vacuum. Rough as this method of observation may at first sight appear, the results obtained by it do not materially differ from those which are deduced from more elaborate researches with the actinometer or with incandescent radiators. Knowing the aphelion distance of the Sun in midsummer, and knowing the increase of temperature produced by its rays in dry earth, we arc enabled to estimate the temperature of the source from which the rays proceed. In this way Newton obtained for the sun’s surface a temperature of nearly three million degrees Fahrenheit, or about fifteen thousand times the temperature of boiling water, — a result which is in harmony with modern observations.

Captain Ericsson’s object, however, is not so much the verifying of Newton’s deduction, as the ascertaining of the precise increment of temperature due to solar radiation in different latitudes and at different seasons of the year. His tables show a remarkable constancy in the figures which express the amount of heat received from the sun on any given day of the year. On the first day of January, when, being in perihelion, we receive the greatest amount of heat, the excess over the amount received on the first day of July is 5.88° F.; and the rate at which this surplus diminishes goes through a regular cycle of variations until the first day of the next January is reached. This series of results shows to how slight an extent the annual variations in terrestrial temperature directly depend on the variations in the intensity of solar radiation from day to day. When perihelion occurs in summer, as it will in one hundred and five centuries from the present time, the excess of heat actually received on July 1st over that now received on the same day will be but 11.76° F. ; neverthe° less, the cumulative effect of such an increment, when taken in connection with the effects due to the slight change in the number of summer and winter days, cannot but be considerable. It may be noted in passing that, if the eccentricity of the earth’s orbit were to increase to such figures as it is known to have attained several times within the past three million years, these differences between the amounts of heat received in summer and in winter would accumulate to a formidable amount. As it has now become probable that the glaciation of extensive portions of the earth’s surface has been due in past times to such changes in the position of our planet, the application of Captain Ericsson’s figures to the results given in Mr. Croll’s table of variations of the earth’s eccentricity would be very instructive.

While the amount of solar intensity at noon is least at the south pole, when the earth is in aphelion, it is not greatest at the equator, but at the tropic of Cancer, — a result which might be anticipated from the northerly position of the sun at midsummer. It may be asked how the amount of temperature produced by solar radiation at one of the poles can be determined, the poles being inaccessible. The result is deduced from the known zenith distance of the sun at the pole in midsummer. At this time the zenith distance at the Arctic circle is 43°, so that the temperature is obviously 60.57° F. at that point, that being the temperature which coincides with the zenith distance of 43° in other places at other seasons of the year.

Whether the north pole is likely forever to remain inaccessible is a question suggested by the failure of the recent German expeditions sent on routes planned by Dr. Petermann of Gotha. Working upon materials drawn from that “ moral consciousness ” out of which his mythical compatriot so triumphantly constructed a camel, this eminent geographer had persuaded himself of the existence of an open sea between Spitzbergen and Nova Zembla, through which the pole might be reached in sailing-vessels. The two expeditions commanded by Captain Koldewey, the second of which returned home last year, have demonstrated the incorrectness of this supposition ; so that the only route which can now be regarded as practicable is the one which has usually been pursued by English and American explorers,—through Baffin’s Bay and Smith’s Sound. By this route it is possible to approach within about nine hundred and fifty miles of the pole in a sailing vessel ; and the sledge journey which remains does not exceed in length the distances frequently traversed in sledges in the course of the search for Franklin.

Professor Helmholtz, the greatest of contemporary physicists, has just published the second instalment of his series of popular scientific lectures. Besides the famous essays on the conservation of force and the transformations of the different modes of motion, which, when they first appeared, opened a new era in scientific speculation, the work contains four chapters, of which three are devoted to the phenomena of vision. In discussing color-blindness, Professor Helmholtz has advanced what would seem to be the most probable explanation, namely, that in the normal retina there are specialized filaments for the reception respectively of red, green, and violet rays. Such a specialization is indeed no more than might be anticipated from the parallel fact that the auditory nerve, in entering the cochlea, is divided into a large number of filaments, which are capable of vibrating each in response to a particular sound, just as a tuning-fork will vibrate when its own note is sounded near it. And the hypothesis is further strengthened by Dr. Brown-Séquard’s discovery of the number of fibres in the spinal cord which are specialized for the reception of particular sensations.

The most remarkable scientific work published in English during the past year is undoubtedly Mr. E. B. Tylor’s “Primitive Culture : Researches into the Development of Mythology, Philosophy, Religion, Art, and Custom,” 2 vols. 8vo, London, John Murray. Nothing else that has yet been written about savage habits of thinking will bear comparison, with it. That it has not attracted more attention from the general public is surely not owing to the dryness or abstruseness of the subject, for the work is not more abstruse, though far more truly profound, than the works of Max Müller, which have been so popular; nor is it because the author does not write a pleasing style, for his style is remarkably lucid and graceful ; but it is probably because Mr. Tylor is, in a certain sense, a new writer, having previously been little known save among scholars. His earlier work on the “Early History of Mankind,” must be regarded as a masterpiece of scientific and philological investigation ; and the present work marks an era in the study of those primeval modes of thought which, on a superficial view, seem so alien from our own. Properly to epitomize such a work would require a long article. We can only say that the leading conclusion established by Mr. Tylor’s researches is, that myths and customs and beliefs which in an advanced stage of culture seem meaningless find their explanation in a reference to lower stages. Myths, like words, survive their primitive meanings ; and hence it results that the higher culture may be a further development of the lower, while the lower culture cannot be a degradation from the higher.

Mr. Richard A. Proctor, who is perhaps a trifle more self-conscious than a scientific writer ought to be, reminds us that he has written and published twelve books within the past six years. Of these the latest is a collection of short essays entitled “ Light Science for Leisure Hours,” in which nearly all things get discussed, from a transit of Venus to a college boat-race. One of the most interesting papers is that which shows, in admirably lucid language, how the great mathematician Adams has modified the conclusions of Laplace regarding the length of the terrestrial day. The conclusion that the length of the day has not varied by a fraction of a second since the time of Hipparchos is no longer tenable. It has been shown that the tidal wave which the moon draws twice a day around the earth, in the opposite direction to the terrestrial rotation, acts upon the earth like a brake on a carriage-wheel. Owing to this circumstance, the day is now one eighty-fourth part of a second longer than at the beginning of the Christian era. By and by the rate of retardation will diminish, but in the course of a trillion years our day will have become as long as a lunar month. Had he cared to speculate on all the consequences deducible from this result, Mr. Proctor might have brought out some startling subjects for reflection. For example, as the weight of any body on the earth’s surface depends on the intensity of gravity diminished by the centrifugal force, and as the amount of centrifugal force depends on the velocity of the planet’s rotation, does it not follow that in course of time our human bodies will get rather inconveniently heavy, and may it not come true that une Plume pèse à qui soulevait la charrette Fauchelevent? And as the limit to the growth of a race of organisms depends on the fact that the forces to be overcome increase as the cubes of the dimensions, while the muscular power of overcoming them increases only as the squares, must we look forward to being succeeded by a race of Liliputiana ? Or shall we say that in times now immeasurably ancient the iguanodon waddled lightly about and the megatherium raised his huge bulk against a tall tree-trunk the more unconcernedly because of the shortness of his days ? Such questions may be droll, but they are legitimate, if only because they show us how inextricably all manner of seemingly independent phenomena are mixed up with each other.