tric current, will depend entirely upon the amount of resistance offered by the magnetic element as an outlet to the electric force. If the iron is hard, and the resistance consequently great, the amount of work will be but small; but if the iron is soft and the resistance offered small, then the amount of force transformed into magnetism and available for mechanical purposes will be greater.

In a paper read before the Chemical Society in March last, the author showed that the same principle holds true also in regard to heat. When heat is applied to a solid or a liquid body, a portion of the heat goes to raise its temperature, and another portion is consumed in internal molecular work against cohesion. The rising of the temperature and the separation of the molecules are the two paths or outlets for the force, and the relative proportion which passes through each is determined here in like manner by the resistance offered by each to the passage of the force. Hence the reason why the specific heat of bodies increases as their temperature rises; for the resistance offered by cohesion decreases with rise of temperature, thus allowing a greater proportion of the heat applied to become latent in internal molecular work. It was stated as a general principle that, other things being equal, the more easily fused a body is the greater is its specific heat. This was shown also experimentally to be the case.

In conclusion, in the production of molecular work by heat or mechanical work by means of electro-magnetism, there exists no fixed relation between the amount of heat applied and the work performed, for in both cases the quantity of work varies with the molecular resistance offered.

On Electric Cables, with reference to Observations on the Malta-Alexandria Telegraph. By Dr. ERNEST ESSELBACH.

The three sections of this cable touching the shore at Tripoli and Benghazi represent three condensers of 75,000 to 150,000 feet square, which, on account of their size, disclosed several important facts in regard to the nature of the dielectric. They allowed, in the first instance, a clear separation of the residual charge from the resistance test. Dr. Esselbach arrived thereby not only at the true resistance of gutta percha, but attained a new and entirely different test for insulation (electrification test), by which the absence of electrolytic action in the covering could be distinctly ascertained. These observations further afforded proof that the residual charge on Leyden jars was not a penetration of electricity like that of heat in a metal, but an increase of the specific inductive capacity of the material, and merely a function of time, analogous to certain corresponding phenomena of torsion and magnetism. The absolute quantity of charge, as ascertained in Dr. Esselbach's previous paper, showed that an increase in inductive capacity of one per cent., under the influence of electric tension, was sufficient to account for what appeared to the galvanometer as a change in resistance amounting sometimes to as much as 50 per cent.

Dr. Esselbach further showed his diagrams on earth-currents, extending over one month's observation, indicating the great advantage which two lines of 500 and 600 miles from east to west, and one from north to south, in one continuation, offer, and the facility and precision with which they are observed by Wheatstone's bridge.

The cable is taken roughly as being 2000 times better than the old Atlantic cable; and whereas in this latter at least 80 per cent. of the strength of current was lost in the transit, more than 99 per cent. actually arrives in the present case at the other end. The speed of a signal through this cable has been ascertained in different ways, and in the most perfect way by Captain Spratt, C.B., incidentally, upon a comparison between the longitude of Malta and Alexandria. The time for one signal through the whole length of 1300 miles approaches one second nearly. The author drew attention to the fact that the question of practical speed, after having first been brought into prominence by Mr. Latimer Clark's experiments, had remained in abeyance since Professor Thomson's researches at the time of the laying of the Atlantic cable, after which all interest had been absorbed by the insulation question, and very rightly, since it was first necessary to establish communication, and with certainty, before trying to precipitate it. This appearing now assured by

a great and deserved success of manufacturers, attention could freely be turned to experiments on speed, as entered upon by Messrs. Jenkin and Varley; and he mentioned that applications had been made to Government from the first authorities to take advantage of the Malta-Alexandria Telegraph for the purpose.

On an Experimental Determination of the Absolute Quantity of Electric Charge on Condensers. By Dr. ESSELBACH.

This quantity having first been approximately ascertained by Faraday, had been afterwards established by the researches of Weber, Thomson, and Joule; but the application of these results to submarine cables requiring intermediate reductions, the author undertook a direct determination, for which the means had since become available.

A cable of certain description was charged (and discharged) by 100 Daniell's 400,000 times in 14h 30; this quantity of electricity deposited in four several voltameters 12.9 mgr. of silver. The determination was repeated under different conditions. The absolute quantity can hence be calculated for any other cable by means of the well-known formula for determining their relative capacities. The quantity of charge on the whole Malta-Alexandria cable by 20 cells (the ordinary speaking power) is accordingly equivalent to 0·013 mgr. of silver, a quantity which is furnished in 0.964 second by the battery in a closed circuit of 2500 units (one Daniell by 1000 mercury units depositing 401 mgr. of silver per hour). This would therefore be the maximum speed with this battery, as far as merely the quantity of electricity is concerned. During the investigation of the method which preceded the experiment, Dr. Esselbach found the charge and discharge influenced by the resistance to sufficient extent to admit of verifying experimentally the second case of Professor Thomson's theory of discharge, which is of practical importance for the question of velocity.

Account of an Electromotive Engine. By G. M. GUY.

The author explained the difficulty of obtaining, by any of the methods heretofore suggested, a sufficiently rapid motion within the small spaces through which magnets or electro-magnets acted with sufficient energy, and chiefly in consequence of the rapid diminution of that energy as the distance of the poles increased, even by very minute quantities. He exhibited and explained to the Section a working model of the engine.

METEOROLOGY.

Suggestions on Balloon Navigation. By ISAAC ASHE, M.B.

The author proposed a simple contrivance by means of which the opening of the escape-valve should depend, when desirable, on the relaxation of voluntary exertion on the part of the aeronaut, so that in the event of insensibility supervening at great altitudes, the valve should open spontaneously by means of a weight attached to its rope, thus causing a descent of the balloon to safer altitudes, and obviating the danger to life incurred by Messrs. Glaisher and Coxwell during their recent scientific ascent from Wolverhampton in consequence of their becoming insensible.

Dr. Ashe also proposed the adaptation of screw propulsion to balloons, suggesting a very light screw, capable of being elevated and depressed through an angle of about 150°, so as to be capable of being hoisted while the balloon should be on the ground, of being used horizontally as a propeller, or vertically underneath the car to cause a temporary ascent, as for the purpose of crossing a mountain-range without loss of ballast, which would involve remaining at the elevation so gained, or, on the other hand, by reversing the action of the screw, to effect a descent without loss of gas. Such a screw he considered could be worked at small elevations (2000 feet) by the exertions of the aëronaut; and its advantages would consist in the conferring, to

a certain degree, of definite direction, and also of steering-power, and in obviating the objection to hydrogen balloons, which consisted in the expense of this gas, as a descent could be effected without loss of gas; hence smaller and much more manageable balloons might be constructed than those now in use, and propulsion by means of a screw would be so much easier.

Steering-power being obtained, Dr. Ashe hoped that a modification of shape might be found practicable, so as to present a minimum of resistance to propulsion by the screw. He proposed to steer by means of two small screws connected by a cranked axle placed at right angles to the action of the propeller, and situated in front of the car, so as not to interfere with the hoisting of the propeller; these steeringscrews should have their spirals turned in the same direction, and by revolving them in one direction, or the reverse, the balloon might be made to rotate on its vertical axis as might be desirable. The disagreeable rotation incident to balloons might also thus be obviated. Dr. Ashe suggested the employment of balloons in the investigation of aerial currents and circular storms, and for the exploration of unknown continents: water, that great desideratum in such explorations, could be observed from an elevation when it would otherwise be passed by unobserved, and a descent being effected by the screw, its position might then be taken by observation, and marked for the guidance of foot explorers. Similar remarks would apply to the discovery of the easiest routes by means of balloon observations.

On some Improvements in the Barometer. By ISAAC ASHE, M.B.

The author suggested a contrivance by which a water-barometer might be constructed, having a tube of not more than 33 feet in length, with a range in the height of the column of liquid equal to about 39 inches. Though correct in theory, this contrivance seemed to have some defects which would practically interfere with its accuracy.

On the Determination of Heights by means of the Barometer. By JOHN BALL. The object of this paper was to direct attention to the serious errors which are involved in the ordinary process of reducing barometric observations taken for hypsometrical purposes. This process involves two assumptions: 1st, that the volume of a column of air unequally heated is nearly the same as that of an equal weight of air of the same mean temperature; 2ndly, that the mean temperature of the column or stratum of air between the stations of observation corresponds to the mean of the readings of thermometers standing in the shade at each station. The error involved in the first assumption is not very considerable; that arising from the second is, on the contrary, highly important.

M. Bravais, who along with M. Charles Martins has contributed largely to our knowledge of the meteorology of the Alps, was the first to propose a practical plan for applying a correction to the assumed mean temperature of the air depending upon the hour of the day and the season of the year at which observations are made, but it is to M. Plantamour, the distinguished astronomer of Geneva, that we owe the fullest investigation of this important subject.

Having ascertained by careful levelling the true height of the Great St. Bernard above Geneva, M. Plantamour finds that the mean of all the barometric observations, made during eighteen years, deviates by fourteen English feet from the true height, and he attributes this deviation, with great apparent probability, to an abnormal depression of the mean temperature of Geneva, owing to the neighbourhood of the lake.

The readings of the barometer and thermometer at the observatories of Geneva and the St. Bernard are taken daily at nine hours or epochs. M. Plantamour assumes that, on an average of a long period of years, the mean of the observations taken at any one epoch in the twenty-four hours should give the true difference of height between the two stations, with an error due to the difference between the mean of the readings of the thermometers at both stations at the same epoch, and the true mean temperature of the air in the intervening stratum. Calculating then the height of the St. Bernard by the elements corresponding to each epoch of the day during the four summer months, from June to September,

he obtains a series of measures differing from the true height-those corresponding to the hottest hours being in excess, and those appertaining to the coldest hours in defect of the true height. He then ascertains the amount of correction which, being applied to the mean sum of the readings of the thermometer at each epoch in each of those months, would bring out the true height. In this manner he obtains a table, showing what he calls the normal correction for each of the nine epochs of the day during the four summer months. There is good reason to believe that, in reducing barometric observations which are to be compared with Geneva and the St. Bernard, the application of the normal correction ascertained in the manner above stated will in general give truer results than those where this is not applied; but as it is obvious that the conditions of temperature at the moment when a given observation is made are constantly varying from the mean of the corresponding day and hour, it follows that a further supplemental correction should be made on this account.

To apply this further correction is a matter of no slight difficulty. The method employed by M. Plantamour is as follows. He obtains from the observations at Geneva and the St. Bernard (by interpolation when necessary) the elements corresponding to the day and hour of the observation which is sought to be reduced, and from these he calculates the height of the St. Bernard. The height so obtained, when compared with the measure which is derived from the mean of the readings for the same day and hour, as shown in his Table of normal corrections, furnishes a criterion by which to judge of the conditions with respect to temperature of the moment when the observations to be reduced were made. M. Plantamour thinks it not difficult to infer from the observations themselves, and from the general state of the weather at the time, whether the moment was one of atmospheric equilibrium or the reverse. In the latter case the observation is treated as one of inferior utility, to which a lower value should be assigned in the final calculation. Supposing, on the contrary, the observations not to betray a disturbance of equilibrium between the two stations, the deviation of the height, as calculated for that particular moment from the height derived from the corresponding means, is the measure of the amount and sign of the supplemental correction corresponding to the moment of observation.

Without entering at present into sundry points of secondary importance, the writer believes that, while it is at present impossible to clear the mode of dealing with this correction of some arbitrary elements, it is easy to adopt a system less cumbrous and less inconvenient, and at least equally accurate with that proposed by M. Plantamour. He finds that many of the observations which appear to M. Plantamour to be clear of anomalies arising from the disturbance of atmospheric equilibrium, show unequivocal traces of such disturbance. These anomalies can be eliminated only by comparing the observations in hand with many different standard stations, such as Milan, Turin, &c.; but, in the absence of direct evidence, the introduction of an empirical correction in the manner proposed is likely to lead to

error.

The writer proposes to deal directly with the correction for temperature upon the best information that is available in regard to each of the stations where observations are recorded. He considers that the deviation of the thermometer at the time of observation from its mean height at the corresponding day and hour, is a tolerably accurate measure of its greater or less deviation at that time from the true temperature of the air freed from surface-radiation, and may therefore be taken with its proper sign for the supplemental correction.

It is important that the comparison between Geneva and St. Bernard, made by M. Plantamour, should be extended to other stations near the base of the Alps, and for this, as well as other reasons, it is highly desirable that the observations at Milan and Turin should be made at hours which correspond with the Swiss observations.

On the Extent of the Earth's Atmosphere.

By the Rev. Professor CHALLIS, M.A., F.R.S., F.R.A.S.

The object of this paper was to show that the earth's atmosphere is of limited

extent, and reasons were adduced, in the absence of data for calculating the exact height, for concluding that it does not extend to the moon. It was argued on the hypothesis of the atomic constitution of bodies, that the upward resultant of the molecular forces on any atom, since it decreases as the height increases, must eventually become just equal to the force of gravity, and that beyond the height at which this equality is satisfied, there can be no more atoms, the atmosphere terminating with a small finite density. It has been generally supposed that the earth's atmosphere is about 70 miles high, but on no definite grounds, and the estimates of the height have been very various. Against the opinion that it extends as far as the moon, it was argued that, as the moon would in that case attach to itself a considerable portion by its gravitation, which would necessarily have some connexion with the rest, there would be a continual drag on the portion more immediately surrounding the earth, and intermediately on the earth itself, which would in some degree retard the rotation on its axis. Hence if, as there is reason to suppose, the rotation be strictly uniform, the earth's atmosphere cannot extend to the moon. The author also stated that if by balloon ascents the barometer and thermometer were observed at two heights ascertained by observation, one considerably above the other, and both above the region in which the currents from the equator influence the temperature, data would be furnished by which an approximate determination of the height of the atmosphere might be attempted.

On the "Boussole Burnier," a new French Pocket Instrument for measuring Vertical and Horizontal Angles. By F. GALTON, F.R.S., F.R.G.S. This instrument is about 3 inches long and inch deep. Its outside is composed of two faces of brass with pear-shaped outlines, separated by vertical sides. In the body of the instrument are two delicate circles placed in parallel planes; at its smaller end is a cylindrical lens, which views the nearer graduations on the rims of the two circles; on the upper face of the instrument are sight-vanes like those of an azimuth compass; on the lower face is a light universal joint, which is used when the instrument is attached to a support, and not held, as it may be, in the hand.

One of the circles is of aluminium, and is borne by a compass-needle; it gives horizontal angles when the instrument is held horizontally. The other is of silvered copper, unequally weighted, and is supported by a delicate axis playing in jewelled holes: it gives vertical circles through the action of gravity when the instrument is held vertically, just as the compass-circle gives azimuthal angles through the action of the magnetic force when the instrument is held horizontally.

The remarkable simplicity and compactness of the Boussole Burnier would make it useful to the traveller, the geologist, and the military engineer. It is the invention of Lieut.-Col. Burnier of the French Engineers, and has been perfected in its details by M. Balbreck, No. 81 Boulevard Mt. Parnasse, Paris.

European Weather-Charts for December 1861. By F. GALTON, F.R.S., F.R.G.S. The author submitted for examination a series of printed and stereotyped charts, compiled by himself, that contained the usual meteorological observations made at eighty stations in Europe, on the morning, afternoon, and evening of each day of December 1861. They were printed partly in symbols and partly in figures, in such a form that each separate group of observations occupies a small label, whose centre coincides with the geographical position of the station where the observations were made. The amount of cloud is expressed by shaded types, the direction of the wind by an equivalent to an arrow, and its force by a symbolical mark. The temperature of the wet and dry thermometers, and the barometric readings (reduced to zero and sea-level) are given in figures. As the charts had been too recently printed to admit of a thorough examination, and as they were ultimately to appear as a separate publication, the author abstained from other deductions than those that were obvious on inspection. Among these, the enormous range and the simultaneity of the wind-changes, testifying to the remarkable mobility of the air, were exceedingly conspicuous.