« AnteriorContinuar »
applied to the edges of the plates of rock, and after a few failures the pitch did its work sufficiently well. I regard the matrix of pitch as almost indispensable to success when the experiment of lateral pressure is performed.
By the process just described slabs 9 in. long have been bent until they rose in. in the centre. More conspicuous results may be expected hereafter. The operations required for the production of apparently inconsiderable deflection are tedious and slow, but patience rather than fresh methods seems to be needed. The contortions which we would imitate were not made in a day. Nature is as superior to us in resources of time as of power. Completer and more varied experiments than these are to be desired. Larger specimens of rock should be tested, and the exposure should be longer than thin plates require. With appropriate apparatus a series of observations as detailed and exact as those instituted by Fairbairn in the case of iron and steel might be carried out, greatly to the advantage of geologists, physicists and engineers.
While occupied with this subject of the mechanical properties of rocks a number of examples of unintentional or natural contortion have come under my notice.
Not long ago I saw some small casts of the Elgin marbles prepared in the form of long strips of plaster of Paris 24 in. broad. These casts had been laid aside for some years and had warped visibly. In one case the deflection (estimated as a rectilinear angle) amounted to 6o. This led to some experiments on plaster of Paris. On submitting dry plates of an inch in thickness to the knife-edge machine a deflection of 8° was obtained in six weeks, and I soon found that this material is indefinitely plastic if the strain be gradually applied. It would probably be easier to bend a flat plate of plaster of Paris into a cylinder than a plank of deal.
Walking one rainy day past the burying-ground attached to a country chapel, I found some gravestones supported horizontally upon corner pedestals. The flagstone of which the monuments were constructed had yielded towards the unsupported centres, and there were pools of water standing in the hollows. The sculpturing of the inscriptions was too sharp to admit the supposition of extensive weathering. The stones were quite smooth, and the method of rubbing down the surface must have rendered them quite level before erection.
Shortly afterwards, I saw a flagstaff in a public park resting upon a broad flagstone 24 inches in thickness and supported in an upright position by iron ties fixed in the ground at a short distance. The weight of the mast I guessed to be about two tons. The flagstone at its base was visibly curved, as if it had bent beneath the weight of the pole. That this was actually the case appeared from the raising of the free edges of the slab above the surrounding pavement, which was elsewhere fairly level. But all doubt on this point was removed by subsequent measurement. A year and five months later the centre of the slab had sunk of an inch more, relatively to the ends.
Again, a friend communicated to me the experience of slaters, who find that when by yielding of the timbers the surface of an old roof has become bowed, the tiles are distorted to such an extent that they will not lie flat upon a new roof. Old stone tiles (I do not know whether the same is true of slates and brick tiles) are often rendered perfectly useless in this way, however sound they may be. It is perhaps unnecessary to cite other similar cases. Every observant architect and engineer can give from his own experience facts of interest in reference to the influence of long-continued pressure upon an unsupported edge of stone.
The magnificent instances of contortion sometimes displayed in coast sections are certainly more impressive, but perhaps less wonderful in reality, than the cases on record of distorted pebbles. The unlimited effects of long pressure are nowhere so clearly demonstrated as in the bending of round or oval masses of small size. Instances of pebbles elongated in the direction of planes of cleavage occur at Llyn Padarn, near Llanberis, in the Lake District and elsewhere. But the most remarkable cases of alteration of figure effected by pressure are those described by Dr. Hitchcock, Mr. G. L. Vose, and others, as occurring in New England. In Vermont, Maine, Massachusetts and Rhode Island, are found conglomerates where sometimes for hundreds of square feet every pebble, whether of granite, sandstone, schist or quartz, has been flattened. Occasionally one pebble has been driven into another, so as to indent it or squeeze it into a semicircular form, yet without fracture. Some of the examples figured resemble soft cakes jammed together into one mass with unyielding stones, so freely do they curve round in layers and adapt their shape to the various lines of force. Yet plasticity in any ordinary sense of the word is out of the question. These very pebbles are water-worn and some of them cleaved. Not a few are rolled fragments of plutonic rock. (See Plate LXXX. Figs. 2, 3.)
The connection between contorting and cleaving force is not quite clear. It would seem that when a certain freedom of extension is allowed even the most intractable substances yield and change their form. But if the compressed mass be wedged up so tightly that change of figure is impossible, the individual particles seem to revolve upon their axes, and arrange themselves, as coins would do, with their principal planes transverse to the line of pressure. If the conditions exclude even such change as this, the indestructible force may develope itself in other ways still less intelligible to us, and reappear as heat, chemical action, or segregation. Into these inquiries we need not now enter. Our W of limestone is explained—that is, brought into an intelligible relation with other observed phenomena.
It would be interesting, though hardly profitable, to pursue this subject yet further into the field of molecular philosophy. Many attempts have been made to resolve various physical states into combinations of certain hypothetical atomic forces. These speculations are sure to recur, and molecular or atomic theory will some day be the basis of all physical science. Newton, as he says, strongly suspected that all the phenomena of cohesion and aggregation, all the phenomena of chemistry and physiology, resulted from the agency of forces varying with the distance of the particles. Boscovitch endeavoured to establish a general theory of cohesion upon the properties of unextended atoms endowed with powers of attraction and repulsion varying not only in degree but kind with the distance, and to such elementary forces he expected ultimately to reduce the peculiar manifestations of chemical and vital change. In our day Sir William Thomson, expanding a suggestion made by Helmholz, has sought to show that all material phenomena may be due to motions created in an incompressible, frictionless, universal fluid, that the ultimate analysis of matter will hereafter give not particles, but vortices. As yet these doctrines remain mere unverified conjectures; the atomic history of the universe is yet to be constructed. We do not accurately know what takes place when a piece of india-rubber is bent or a piece of moist clay squeezed into a new shape. Still we are in the way of progress when we collect and sift facts, arrange them into classes under general propositions, and test those propositions by applying them to fresh cases. Hereafter it will be possible to apply one principle to explain at once the fluidity of water and the contortion of rocks. Even now we can group together the past and the present, the great and the small. We can show that the forces which curved round the Silurian rocks of Wales are still operative, and that the same forces can be displayed and recognised in the laboratory of the student.
EXPLANATION OF PLATE LXXX. Fig. 1. Contortion apparatus. a. Cylinder containing shot. b. Rod connecting a with the vertical plate c. d, d. Frame in which the plate c slides. e. Knife-edge, hinged on c. f. Iron block, sliding in groove h, to which the lamina k is fixed by the
screw g. i. Apparatus for setting the index l at zero in any position of the vertical
plate. 1. Index moving over a graduated arc. m. Index moving backwards and forwards to show the deviation of e from
the perpendicular. Scale nearly of the actual machine.
All the parts are iron or steel, except the cylinder a, the rod b, the indices and the scale, which are brass.
Figs. 2, 3. Examples of distorted pebbles in conglomerate at Newport, Rhode Island, reduced from Memoirs of the Boston Society of Natural History, vol. i. pl. 17, 18.
PSYCHIC FORCE AND PSYCHIC MEDIA.
BY J. P. EARWAKER, MERTON COLLEGE, OXFORD.
TN our former article published in the October number of this 1 REVIEW, we examined the first experiments of Mr. Crookes on Psychic Force as described in the “ Quarterly Journal of Science” for July 1871, and we essayed to show that those experiments, whilst ostentatiously called scientific, were in reality very far from being so. We drew attention to the errors of observation with which the accounts abounded, the gross inaccuracy of the details, the ambiguous way in which they were described, and in conclusion, showed how so many obvious causes of error had neither been allowed for nor even considered. Whilst that article was in the press further investigations on Psychic Force were in progress, and the October number of the same journal contained a second instalment of experiments on this subject. This last article of Mr. Crookes' is a somewhat remarkable one in many respects. With genuine modesty he begins by comparing himself to Galvani, and also draws a parallel between his own investigations and those of the great discoverer of Galvanism, and the first reception accorded to each. But no one, we think, whose mind was free from the bias of foregone conclusions, would see much resemblance between the two series of experiments--Galvani, who experimented on the physiological effects produced by means of the contact of two dissimilar metals, and those of Mr. Crookes, who investigates the phenomena which are produced by a well-known spiritualistic medium. Are the two inanimate and dissimilar metals of Galvani to be compared to the ever-versatile and changeable Mr. Home? Are the results, which, under all circumstances, can be obtained by those who repeat his experiments, to be compared for one moment to those which depend on Mr. Home's state of being at any particular time? Can we, in fact, compare certainties with uncertainties, facts with hypothetical assumptions, or the rigid laws of science with the