an explanation of an experiment by Prof. James D. Forbes which had been advanced as in opposition to the author's theory.

He referred at the outset to the fact pointed out by Mr. Faraday in 1850 *, that two pieces of moist ice, when placed together in contact, will unite together, even when the surrounding temperature is such as to keep them in a thawing state. Mr. Faraday had attributed this phenomenon to a property which he supposed ice to possess, of tending to solidify water in contact with it, and of tending more strongly to solidify a film or a particle of water when the water has ice in contact with it on both sides, than when it has ice on only one side.

Dr. Tyndall had subsequently adopted this fact as the basis of a theory by which he proposed to explain the viscidity or plasticity of ice, or its capability of undergoing change of form, which had previously been known to be the property in glaciers in virtue of which their motion down their valleys is produced by gravitation. Designating Mr. Faraday's fact under the term “regelation," Dr. Tyndall, in the theory referred to, described the capability of glacier ice to undergo changes of form, as being not true viscosity, but as being the result of vast numbers of successively occurring minute fractures, changes of position of the fractured parts, and regelations of those parts in their new positions. The terms fracture and regelation had then come to be the brief expression of Dr. Tyndall's idea of the plasticity of ice.

The author, Professor James Thomson, considered, on the contrary, that if, in a material having no inherent property of plasticity independent of fracture, any steady force applied (such as the force of gravity acting on a glacier) be sufficient to cause fracture, the substance must go down suddenly until a position of repose is attained, and that the addition of a principle of reunion (such as "regelation") cannot have a tendency to reiterate the fractures after such position of repose has been attained. His own theory, he stated, might be sketched in outline as follows:-If to a mass of ice at its melting-point, pressures tending to change its form be applied, there will be a continual succession of pressures applied to particular parts-liquefaction occurring in those parts through the lowering of the melting-point by pressureevolution of the cold by which the so melted portions had been held in the frozen state-dispersion of the water so produced in such directions as will afford relief to its pressure-and recongelation, by the cold previously evolved, of the water on its being relieved from this pressure: and the cycle of operations will then begin again; for the parts recongealed must in their turn, through the yielding of other parts, receive pressures from the applied forces, thereby to be liquefied, and then to go through successive processes as before. He thus considered that the plasticity consists not of fracture and regelation, but essentially of melting by pressure and recongelation on relief from pressure.

Professor James D. Forbes † had adopted the view, that the dissolution of ice is a gradual, not a sudden process, and so far resembles the tardy liquefaction of fatty bodies, or of the metals which in melting pass through intermediate stages of softness or viscosity. He thought that ice must be essentially colder than water in contact with it; and that, between the ice and the water, there is a film having its temperature varying from side to side, which may be called plastic ice, or viscid water; and that through this film heat must be constantly passing from the water to the ice, and the ice must be wasting away, though the water be what is called icecold. Professor Forbes had stated afterwards, as a modification of this supposition, that if a small quantity of water be enclosed in a cavity in ice, it will undergo a gradual "regelation," or that the ice will in this case be increased instead of wasted. In reference to this, Professor J. Thomson put forward the case of a medium quantity of water, in contact with a medium quantity of ice, without addition or abstraction of heat; and stated that, were the idea of Professor Forbes on this subject correct, the result in this case ought to be that the water and ice should ultimately pass into the state of uniform viscidity; for Professor Forbes's own words distinctly deny the permanence of the water and ice in contact in their two separate states, as he says, "bodies of different temperatures cannot continue so without interaction. The water must give off heat to the ice, but it spends it in an insignificant thaw at the surface." Thus then it would follow from the admission of Professor Forbes's views, * See Faraday's Researches in Chemistry and Physics, 1859.

+ See Forbes On the Recent Progress and Present Aspect of the Theory of Glaciers,' forming the introduction to a volume of Occasional Papers on the Theory of Glaciers, 1859.

that viscid water could be produced in any large quantities desired, like as it is supposed to be produced in small quantities in the hypothetical thin film at the surface of hard ice-an inference which is plainly contrary to all experience, as no person has ever, by any peculiar application of heat to, or withdrawal of heat from, a quantity of water, rendered it visibly and tangibly viscid, so that it could be poured in a thick state like honey. We even know that water may be cooled much below the ordinary freezing-point, and yet remain fluid.

Professor Forbes, however, although, in his recent writings, maintaining the views just alluded to, had not rejected the author's theory as altogether unfounded. He had rather admitted that it points out some of the causes which may impart to a glacier a portion of its plasticity; and also that it meets with verification to some extent in the moulding of ice subject to rapid alternations of pressure under the Bramah's press.

Mr. Faraday, in his recently published Researches in Chemistry and Physics,' had adhered to his original mode of accounting for the phenomenon he had observed, and had developed farther the explanation of his ideas on the subject, and adduced examples of the action of numerous other substances in passing from the liquid to the solid, or from the solid to the liquid state, and also in passing from the liquid to the gaseous state. Professor J. Thomson, however, considered that the general bearing of all the phenomena adduced, is not to sustain the view of Mr. Faraday, but to show that the particles of a substance, when existing all in one state only, and in continuous contact with one another, or in contact only under special circumstances with other substances, experience a difficulty of making a beginning of their change of state, whether from liquid to solid, or from liquid to gaseous, or probably also from solid to liquid. He did not admit that anything had been adduced showing a like difficulty as to their undergoing a change of state when the substance is present in the two states already, or when a beginning of the change has already been made. He believed that when water and ice are present together, their freedom to change their state on the slightest addition or abstraction of heat is perfect. He therefore could not admit the validity of Mr. Faraday's mode of accounting for the phenomena of so called “regelation.'

Thus the fact of "regelation," which Professor Tyndall had taken as the basis of his theory for explaining the plasticity of ice, did, in the author's opinion, as much require explanation as the plasticity of ice which it was applied to explain. The two observed phenomena, namely, the tendency of two separate pieces of ice to unite when placed in contact, and the plasticity of ice, are, he believed, cognate results of a common cause, and are explained by the theory he had himself offered. The experiment by Professor Forbes adduced in opposition to the author's theory was to the following effect :

Two slabs of ice, having their corresponding surfaces ground tolerably flat, on being suspended in an atmosphere a little above the freezing-point, upon a horizontal glass rod passing through two holes in the plates of ice, so that the plates may hang vertically, and in contact with one another, were found in a few hours to be united so as to adhere strongly together. This Professor Forbes had supposed would prove that mere contact without pressure is sufficient to produce the union of two pieces of moist ice. The author, on the contrary, explained the fact by the capillary forces of the film of interposed water as follows:-First, the film of water between the two slabs-being held up against gravity by the capillary tension, or contractile force of its free upper surface, and being distended besides, against the atmospheric pressure, by the contractile force of its free surface round its whole perimeter-except for a very small space at bottom, from which water trickles away, or is on the point of trickling away--exists under a pressure which, though increasing from above downwards, is everywhere, except at that little space at the bottom, less than atmospheric pressure. Hence the two slabs are urged towards one another by the excess of the external atmospheric pressure above the internal water pressure, and are thus pressed against one another at their places of contact by a force quite notable in amount.

Secondly, the film of water existing, as it does, under less than atmospheric pressure, has its freezing-point raised in virtue of the reduced pressure; and it would therefore freeze even at the temperature of the surrounding ice, namely, the freezingpoint for atmospheric pressure. Much more will it freeze in virtue of the cold

given out in the melting by pressure of the ice at the points of contact, where, from the first two causes named above, the two slabs are urged against one another.

The freezing of ice to flannel, or to a worsted glove on a warm hand, was, in his opinion, to be attributed partly to capillary attraction acting in similar ways to those just stated; but he considered that, in many of the observed cases of this phenomenon, there are also direct pressures from the hand, or from the weight of the ice, or from other like causes, which must be supposed to increase the rapidity of the moulding of the ice to the fibres of the wool.

On Electrical "Frequency." By Professor W. THOMSON, LL.D., F.R.S. Beccaria found that a conductor insulated in the open air becomes charged sometimes with greater and sometimes with less rapidity, and he gave the name of "frequency" to express the atmospheric quality on which the rapidity of charging depends. It might seem natural to attribute this quality to electrification of the air itself round the conductor or to electrified particles in the air impinging upon it; but the author gave reasons for believing that the observed effects are entirely due to particles flying away from the surface of the conductor, in consequence of the impact of non-electrified particles against it. He had shown in a previous communication that when no electricity of separation (or, as it is more generally called, “frictional electricity," or "contact electricity") is called into play, the tendency of particles continually flying off from a conductor is to destroy all electrification at the part of its surface from which they break away. Hence a conductor insulated in the open air, and exposed to mist or rain, with wind, will tend rapidly to the same electric potential as that of the air, beside that part of its surface from which there is the most frequent dropping, or flying away, of aqueous particles. The rapid charging indicated by the electrometer under cover, after putting it for an instant in connexion with the earth, is therefore, in reality, due to a rapid discharging of the exposed parts of the conductor. The author had been led to these views by remarking the extreme rapidity with which an electrometer, connected by a fine wire with a conductor insulated above the roof of his temporary electric observatory in the island of Arran, became charged, reaching its full indication in a few seconds, and sometimes in a fraction of a second, after being touched by the hand, during a gale of wind and rain. The conductor, a vertical cylinder about 10 inches long and 4 inches diameter, with its upper end flat and corner slightly rounded off, stood only 8 feet above the roof, or, in all, 20 feet above the ground, and was nearly surrounded by buildings rising to a higher level. Even with so moderate an exposure as this, sparks were frequently produced between an insulated and an uninsulated piece of metal, which may have been about th of an inch apart, within the electrometer, and more than once a continuous line of fire was observed in the instrument during nearly a minute at a time, while rain was falling in torrents outside.

Remarks on the Discharge of a Coiled Electric Cable.
By Professor W. THOMSON, LL.D., F.R.S.

Mr. Jenkin had communicated to the author during last February, March, and April a number of experimental results regarding currents through several different electric cables coiled in the factory of Messrs. R. S. Newall and Co., at Birkenhead. Among these results were some in which a key connected with one end of a cable, of which the other end was kept connected with the earth, was removed from a battery by which a current had been kept flowing through the cable and instantly pressed, to contact with one end of the coil of a tangent galvanometer, of which the other end was kept connected with the earth. The author remarked that the deflections recorded in these experiments were in the contrary direction to that which the true discharge of the cable would give, and at his request Mr. Jenkin repeated the experiments, watching carefully for indications of reverse currents to those which had been previously noted, It was thus found that the first effect of pressing down the key was to give the galvanometer a deflection in the direction corresponding to the true discharged current, and that this was quickly followed by a reverse deflection generally greater in degree, which latter deflection corresponded to a current in the same direction as that of the original flow through the cable. Professor Thomson explained this second current, or false discharge, as it has since been some

times called, by attributing it to mutual electro-magnetic induction between different portions of the coil, and anticipated that no such reversal could ever be found in a submerged cable. The effect of this induction is to produce in those parts of the coil first influenced by the motion of the key, a tendency for electricity to flow in the same direction as that of the decreasing current flowing on through the remoter parts of the coil. Thus, after the first violence of the back flow through the key and galvanometer, the remote parts of the cable begin, by their electro-magnetic induction on the near parts, to draw electricity back from the earth through the galvanometer into the cable again, and the current is once more in one and the same direction throughout the cable. The mathematical theory of this action, which is necessarily very complex, is reserved by the author for a more full communication, which he hopes before long to lay before the Royal Society.

On the Necessity for incessant Recording, and for Simultaneous Observations in different Localities, to investigate Atmospheric Electricity. By Professor W. THOMSON, LL.D., F.R.S.

The necessity for incessantly recording the electric condition of the atmosphere was illustrated by reference to observations recently made by the author in the island of Arran, by which it appeared that even under a cloudless sky, without any sensible wind, the negative electrification of the surface of the earth, always found during serene weather, is constantly varying in degree. He had found it impossible, at any time, to leave the electrometer without losing remarkable features of the phenomenon. Beccaria, Professor of Natural Philosophy in the University of Turin a century ago, used to retire to Garzegna when his vacation commenced, and to make incessant observations on atmospheric electricity, night and day, sleeping in the room with his electrometer in a lofty position, from which he could watch the sky all round, limited by the Alpine range on one side and the great plain of Piedmont on the other. Unless relays of observers can be got to follow his example, and to take advantage of the more accurate instruments supplied by advanced electric science, a self-recording apparatus must be applied to provide the data required for obtaining knowledge in this most interesting field of nature. The author pointed out certain simple and easily-executed modifications of working electrometers, which were on the table before him, to render them self-recording. He also explained a new collecting apparatus for atmospheric electricity, consisting of an insulated vessel of water, discharging its contents in a fine stream from a pointed tube. This stream carries away electricity as long as any exists on its surface, where it breaks into drops. The immediate object of this arrangement is to maintain the whole insulated conductor, including the portion of the electrometer connected with it and the connecting wire, in the condition of no absolute charge; that is to say, with as much positive electricity on one side of a neutral line as of negative on the other. Hence the position of the discharging nozzle must be such, that the point where the stream breaks into drops is in what would be the neutral line of the conductor, if first perfectly discharged under temporary cover, and then exposed in its permanent open position, in which it will become inductively electrified by the aerial electromotive force. If the insulation is maintained in perfection, the dropping will not be called on for any electrical effect, and sudden or slow atmospheric changes will all instantaneously and perfectly induce their corresponding variations in the conductor, and give their appropriate indications to the electrometer. The necessary imperfection of the actual insulation, which tends to bring the neutral line downwards or inwards, or the contrary effects of aërial convection, which, when the insulation is good, generally preponderate, and which in some conditions of the atmosphere, especially during heavy wind and rain, are often very large, are corrected by the tendency of the dropping to maintain the neutral line in the one definite position. The objects to be attained by simultaneous observations in different localities alluded to were,-1. to fix the constant for any observatory, by which its observations are reduced to absolute measure of electromotive force per foot of air; 2. to investigate the distribution of electricity in the air itself (whether on visible clouds or in clear air) by a species of electrical trigonometry, of which the general principles were slightly indicated. A portable electrometer, adapted for balloon and mountain observations, with a burn

ing match, regulated by a spring so as to give a cone of fire in the open air, in a definite position with reference to the instrument, was exhibited. It is easily carried, with or without the aid of a shoulder strap, and can be used by the observer standing up, and simply holding the entire apparatus in his hands, without a stand or rest of any kind. Its indications distinguish positive from negative, and are reducible to absolute measure on the spot. The author gave the result of a determination which he had made, with the assistance of Mr. Joule, on the Links, a piece of level ground near the sea, beside the city of Aberdeen, about 8 A.M. on the preceding day (September 14), under a cloudless sky, and with a light north-west wind blowing, with the insulating stand of the collecting part of the apparatus buried in the ground, and the electrometer removed to a distance of 5 or 6 yards and connected by a fine wire with the collecting conductor. The height of the match was 3 feet above the ground, and the observer at the electrometer lay on the ground to render the electrical influence of his own body on the match insensible. The result showed a difference of potentials between the earth (negative) and the air (positive) at the match equal to that of 115 elements of Daniel's battery, and, therefore, at that time and place, the aerial electromotive force per foot amounted to that of thirty-eight Daniel's cells.

On the Cause of Magnetism. By G. V. TOWLER.

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On Changes of Deviation of the Compass on Board Iron Ships by “heeling,” with Experiments on Board the City of Baltimore,' 'Aphrodite,'' Simla,' and Slieve Donard. By JOHN T. TOWSON.

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The author explained the manner in which the Compass Committee was first formed, in accordance with the advice of the Section, and stated that two reports had been drawn up, which, with the advice of the Astronomer Royal, had been printed and "presented to both Houses of Parliament by command of Her Majesty." He thanked the Astronomer Royal for his valuable advice and support. There were matters of consideration which the Compass Committee deemed incomplete the one was the change which took place in iron ships in proceeding to the opposite hemisphere; the other, the change that was produced by what is technically denominated heeling,' that is, when the deck of a vessel leaned over, through the action of the wind or otherwise: if, when looking towards the bow, it slanted downwards to the right, it was said to heel starboard; if to the left, to heel port. The first question was undertaken by the late respected Rev. Dr. Scoresby, who proceeded to Australia in the 'Royal Charter,' and whose exertions in the pursuit of this branch of the inquiry shortened a most valuable life. The second question was the subject of his present report. Having described the principles on which his graphic illustration was constructed, the author pointed out the unexpected amount of deviation which this source of disturbance (heeling) brought about, amounting in most instances, when the ship's head was in the position to produce the maximum effect, to two or three points in the standard compass, and to a greater amount so far as the steering compass is concerned. He remarked on several particulars connected with this investigation. Generally the north end of the compass was drawn to the upper side of the ship-the case with seven out of nine compasses on board the City of Baltimore;' but in the two steering compasses the needles were drawn in a contrary direction. He explained the theory on which this disturbance arose, partly from subpolar magnetism below the compass, and partly from the disturbance of the inductive magnetism of the ships. In such ships as those under consideration, the following empirical rule held good with respect to compasses favourably placed. When the vertical force, as determined either by vibration experiments or torsion on board the ship, maintained the ratio, as compared with the vertical force on shore, of nine to fourteen, little or no effect was produced by heeling in the same hemisphere and latitude. And in the case of the Simla' this plan of predicting the amount of error was adopted: a moveable upright magnet was applied so as to produce the before-named vertical force, when it was found, "with magnet in," no error was produced, although “with magnet out" it amounted to 24° from changing a heel of 10° starboard to 10° port. There appeared to be another remarkable result. He believed that when a ship was

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