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same quantity of air must successively fill a space 3, 5, 7, 9 times as great as at first, and must hence diminish its density and consequently its pressure so as to become from to of what it was in the centre of the circle. The pressure of the air which passes through the tube into the space between the disks is thus, although originally greater than the pressure of the atmosphere, gradually becoming less while radiating to, and escaping at the edge, and becomes actually less than the atmospheric pressure. It is true that the velocity of the air-particles decreases from the centre to the edge, and the diminution of pressure is not quite so great as would appear from our calculation, because the external air opposes a considerable resistance to the escaping current, but the ultimate effect is that the pressure over the greater portion of the space between the disks is less than the external pressure of the atmosphere : hence the remarkable result that the external air presses the suspended disk against the current blown from the tube, and moves it close to the fixed disk, and not until the current of air diminishes in strength will the disk fall back again.”

The general principles of wave-propagation, both when the vibrations of the particles of the medium are transversal and also when they are longitudinal, are clearly explained, and a careful account given of the vibrations of strings, rods, and plates, the laws obeyed by them, the theory of overtones, &c. Occasionally, however, the explanations are by no means adequate : for instance, on page 382 we find the following, relative to the action of a flue organ-pipe : A current of air is directed through a narrow slit and strikes the opposite edge of the aperture or mouth' of the instrument." This is all: here the description comes to a sudden conclusion, while the reader is left to explain for himself as best he can what becomes of the air thus striking against the sharp edge, and how it sets in vibration the air-column in the pipe. A little further on, the pitch of an organ-pipe is spoken of as depending on its length only, the lengths of two open pipes which yield notes whose vibration-frequencies are as 2 to 1 being as 1 to 2. It might at least have been mentioned that the pitch is a function also of the cross section of the pipe.

On page 347 pitch and tone are used synonymously.

These, however, are small matters. On the whole the chapter on Acoustics is good, the portions relating to the organ of voice and the difference of vowel sounds of the same pitch as depending on the presence of various overtones, differing in intensity, combined with the fundamental, being especially well worked out.

In the Optics, after some well-considered directions have been given for the construction and silvering of mirrors, several simple cases of reflection are described and geometrically solved. There is here the same absence of general formule which we noticed in the case of the mechanics. The student is instructed to work out each example separately, and to determine geometrically the relative sizes of an object and its image and their distances from the mirror. No mention is made of spherical aberration (which the

experimenter cannot fail to notice and be perplexed by), nor, in the case of lenses, of chromatic aberration.

The student is taught how to make a spectroscope for himself which, though rough, is all that is absolutely necessary to show the distinctive spectrum-lines of such elements as sodium, lithium, calcium, &c. The description of this little apparatus, and of the method of using it, occupies many pages and is too long to quote here; it may, however, be stated that it consists of a single disulphide-of-carbon prism, which is made from the wider part of a lamp-cylinder cut obliquely at both ends and closed by glass plates, with an aperture on the upper side for pouring in the liquid. This is placed at one end of a long wooden box with square section, the further end of which is closed by a thin plate of wood with a narrow vertical slit in it. Following the directions given, the student will have no difficulty in arranging apparatus for exhibiting the reversion of the bright lines in the spectrum of an element, and in apprehending the cause of such reversion.

We find nothing about the phenomena of Interference or Diffraction. This is scarcely to be wondered at, since experiments on these subjects demand the use of more elaborate instruments than the author contemplates his students to be in possession of. His plan is throughout to confine himself to the exposition of such phenomena and principles as can be reproduced and exemplified by the student with such apparatus as he can construct for himself. The student is not expected to make measurements of any he is not supposed to have arrived at such a state of proficiency in his subject. Indeed no measuring-instruments are described or even mentioned in any part of the book. We think, however, it would not have been inconsistent with the plan of the work had some of the elementary experiments on the polarization of light been given.

In the treatment of Electricity the same order is adopted as in the ordinary books on Physics. This portion of the work, not less than the rest, is characterized by copious illustration of principles and by many-sided hints and suggestions for insuring the success of the experiments described. In the main we agree with the writer of the Preface, that though the instructions may sometimes appear to readers unaccustomed to experimental work needlessly minute, yet it will appear in actual trial that the apparently small matters to which attention is sometimes called are just such as make the difference between success and failure. At the same time the thought forces itself upon us as we read through these many pages of instructions, that the author would have done wisely had he sometimes curtailed them, and so left space

for

many useful and important matters which he has been obliged to omit. We should like to have seen explained and illustrated the law of inverse squares as applied to electrical and magnetic attractions and repulsions, something about the torsion-balance, Faraday's theory of induction, specific inductive capacity, thermo-electricity, the tangent-galvanometer, electrical resistance, &c. Of none of these is any mention made.

kind;

The pages which treat of the magnetization of a rod of soft iron by a galvanic current and the attendant phenomena are well worth studying. The author explains clearly by a number of capital illustrations the Ampèrian theory of magnetism and the properties of solenoids. The applications of electromagnetism in telegraphy (Morse's Telegraph) and in the electric bell are well described. Considering, however, how much is omitted, nine pages are rather too many to devote to the electric bell alone.

In concluding this notice we may state our opinion that Weinhold's Introduction to Elementary Physics will prove of very great service to those who have to teach the elementary principles of Mechanics and Physics in schools, or otherwise, and are compelled by reasons of economy to construct their own apparatus ; to these it will have the additional recommendation that the diagrams and pictures of apparatus are all drawn to scale, and the scale is in all cases given. We do not expect it will be largely adopted as a text-book in schools; it is too big and heavy, and too costly also.

XLVI. Proceedings of Learned Societies.

GEOLOGICAL SOCIETY.

[Continued from p. 329.]
June 10th, 1874.—John Evans, Esq., F.R.S., President,

in the Chair. THE following communications were read :

1. “ Notes on the Phenomena of the Quaternary Period in the Isle of Portland and around Weymouth.” By Joseph Prestwich, Esq., F.R.S., F.G.S.

The author remarked that although the physical features connected with the later geological changes in this district were of much interest, they had hitherto attracted little attention. Commencing with the oldest drift-beds, he showed that the remains of one, formerly more extensive, had been found in the Isle of Portland at a height of 400 feet above the sea, that it contained the remains of the Elephas antiquus, Equus fossilis, &c., and that he found in this bed a number of pebbles of sandstone and ironstone of Tertiary age, and of chert from the Greensands—whence he inferred that, as such pebbles could not now pass over the plain of Weymouth, they must have done so before that area was denuded, and when bridged over by the Portland and Purbeck beds ; for the pebbles are derived from beds which are only in situ to the north of the Weymouth district, and at a distance of 8 to 10 miles from Portland. Further this transport must have taken place before the elevation of the north end of Portland, and when the slope from the Bill to the Ridgeway was uniform and gradual. The anticlinal line which has elevated the intermediate area must be of later date than the drift-bed.

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The author next proceeded to notice the raised beach at the Bill of Portland, in which he had, with the assistance of Mr. Jeffreys, determined 26 species of shells, two of them not now living in the British Channel, and one new. This beach contains pebbles of the Devonshire and Cornwall rocks.

The raised beach Mr. Prestwich found to abut against an old cliff that had been swamped at a later geological period by a land-wash, which had levelled it and the old sea-land with the adjacent landsurface. The mass which had thus swamped the cliff and buried the beach consisted of loam and angular débris, the latter being in larger proportion at top. In the loam he found several species of land and freshwater shells and fragments of bones. The angular débris consisted of pieces of the local rocks, together with a number of specimens which by their organic remains were shown to belong to the Middle Purbecks, a part of the series not now existing in Portland. A similar bed, but much thicker, was then described at Chesilton, in the north of the island. It is there 60 feet thick, and contains large blocks of Portland stone and Portland chert, the greater number of which are in the upper part of the deposit, which is here on the sea-level, and 400 feet lower than the Portland escarpment which rises above it. This loam and angular débris the author was disposed to attribute to a temporary submergence of the land to a depth exceeding the height of Portland, by which the land as it emerged was swept and its débris carried down to the lowest levels, with the remains of its land-animals and land and freshwater shells, which latter, where protected by large masses of loam and suddenly entombed, have been preserved uninjured. To this deposit, which is common over the raised beaches on the south coast, the author proposed to apply the term “ Land-wash.”

The paper concluded with a short notice of the drift-beds formed subsequently to the denudation of the Weymouth district, and therefore never on the high-level Portland drift. Among these was one near Weymouth of singular character, consisting almost entirely of subangular fragments of Greensand chert, which could not have been derived from beds nearer than Abbotsbury. The lower drift of the district is the valley-gravel of Upway and Radipole, in which the remains of Elephas primigenius have been found.

2. “ On the Character of the Diamantiferous Rock of South Africa." By Prof. N. Story Maskelyne, F.R.S., F.G.S., Keeper, and Dr. Flight, Assistant, of the Mineralogical Department, British Museum.

In this paper the authors confirmed certain statements made by one of them from a superficial examination of specimens brought to this country by Mr. Dunn. The specimens examined and analyzed by Dr. Flight were obtained from various diggings and from different depths, down to 180 feet in the case of one mass from Colesberg Kopje. Their characters throughout are essentially the same.

The rock consists of a soft and somewhat pulverulent groundmass, composed of a mineral (soapy to the touch) of a light yellowish

colour in the upper, and of an olive-green to bluish-grey colour in the lower parts of the excavations. Interspersed in the mass are fragments of more or less altered shale, and a micaceous-looking mineral of the vermiculite group, which sometimes becomes an important constituent of the rock, which also contains bright green crystals of a ferruginous enstatite (bronzite), and sometimes a hornblendic mineral closely resembling smaragdite. A pale buff bronzite occurs in larger fragments than the green form of the mineral ; and in the rock of Du Toit's pan an altered diallage is present. Opaline silica, in the form of hyalite or of hornstone, is disseminated through the greater part of the rock-masses ; and they are everywhere penetrated by calcite.

The analyses of the component minerals (given in detail in the paper) show that this once igneous rock is a bronzite rock converted into a hydrated magnesium silicate, having the chemical characters of a hydrated bronzite, except where the remains of crystals have resisted metamorphism. Except in the absence of olivine and the small amount of augitic mineral, it might be compared with the well-known Lherzolite rock.

The diamonds are said to occur most plentifully, or almost exclusively, in the neighbourhood of dykes of diorite which intersect the hydrated rock, or occur between it and the horizontal strata through which the igneous rocks have been projected. The authors compare the characters of the diamonds found in different positions, and come to the conclusion that their source is not very remote from that in which they are now found.

The mineral above-mentioned as resembling vermiculite is described by the authors as a new species under the name of Vaalite.

XLVII. Intelligence and Miscellaneous Articles.

ON THE SPECTRUM OF THE AURORA.

To the Editors of the Philosophical Magazine and Journal.

GENTLEMEN, I WISH to point out a slight error in a communication from Mr.

J. R. Capron on the Spectrum of the Aurora, which appears in the last Number of the Philosophical Magazine. Mr. Capron writes as follows:

“From Dr. Watts's “Index of Spectra’ I have extracted the three principal carbon-tube bands or lines; and they compare with Dr. Vogel's oxygen-tube as under :

Yellow. Green. Blue. Dr. Vogel's oxygen-lines

5603 5195 4834 Dr. Watts's carbon-tube bands or lines. 5622 5189 4829

“ The 5622 for the yellow line of the tube must be an error. 5608 seems to me, from my own observations, nearer its place; and I calculate 5193 and 4825 for the other lines."

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