that, for all practical purposes, the tables quoted above are quite accurate enough, though for great heights, they contain an error; the correction to be applied is small, and additive. The instruments with which Messrs. de Schlagintweit made their observations, were expressly made for the purpose; they ranged from 100° Cels. to 78° Cels., but had a length of 1 foot 9 inches, so that it was possible to divide each degree into fifty parts directly. M. de Schlagintweit drew attention to the accuracy of the results obtainable by these delicate thermo-barometers, which, as far as his experience goes, he considered to be quite comparable to the barometer, if used with all the necessary precautions, though for daily variations the barometer is preferable on account of the greater facility of reading. The circumstance that the thermo-barometer is much less liable to get out of order, makes it a most valuable instrument for travellers.

Messrs. de Schlagintweit's Comparison of Boiling-points,

Group I.-100° to 96°: Correction -0°09 of the Thermo-barometer*.

Group II.-95.99 to 94: Correction -0°10 of the Thermo-barometer.

95.85

1856, April 12

93.00

585-7 92.88 -0.12

Tríchŭgi Naráin..

1855, Sept. 24

Group III.-93.99

92.96 585.2 92.86 -0.10

to 92: Correction-0°-11 of the Thermo-barometer.

Group IV.-91.99 to 90: Correction-0° 12 of the Thermo-barometer.

Jbi Gámin

Jánti Pass

Jánti Pass

Jbi Gámin Pass

1855, June 14..

1855, July 7

497.8 88.59 -0.08 477-2 87.49 -0.17 86.70 459.6 86.53 84.72 425.9 84.58

-0.17

-0-14

Group V.-89.99 to 81: Correction -0°13 of the Thermo-barometer.

*The corrections of the thermo-barometer, given at the different groups, are values obtained by fundamental determinations. These determinations will be given in detail in vol. ii. of Messrs. de Schlagintweit's 'India and High Asia,' 1861.

On Practical Experience of the Law of Storms in each Quarter of the Globe. By Captain W. PARKER SNOW.

From practical experience in several parts of the globe, Captain Snow confirmed the soundness of the theory of the Law of Storms brought forward by Admiral FitzRoy. In one place, well known to the Admiral, viz. the tempestuous seas about Cape Horn, he, Capt. Snow, had cruised for two years without the smallest damage to his vessel, and this owing to the attention he ever paid to those laws of nature in connexion with wind movements.

On another occasion, off the coast of Australia, he preserved his little ship, by similar attention, in a terrific gale, when at the same time several other vessels were wrecked. He well remembered how his chief officer derided the idea of any storm theory being true, and, when referred to Reid and other stormists, said he had never heard of them, and did not believe in what they might say. But that same night the mate was convinced when the gale took the turn predicted by the captain.

One more occasion Capt. Snow would refer to. He was coming home as passenger in a sailing ship; it was in the end of 1856. Within a few days' sail from England a cyclone came down upon them. Clearly it was passing ahead, and Capt. Snow advised his brother captain to adopt those measures which prudence then suggested, and allow the centre to go by. After some argument this was done; but, to convince the master of the ship, Capt. Snow said that the correctness of his view of the case then, would be proved by the direction of the wind at that time in the South of England, which should be opposite to what they had it. Four days afterwards they took a pilot, and ascertained it to have been exactly so.

NOTE. The above is merely the substance of remarks made when Admiral FitzRoy read his paper. They form the outline of what was read on a following day, by Captain Snow, when he entered upon details.

Results of an Investigation into the Phenomena of English Thunder-storms during the years 1857-59. By G. J. SYMONS.

This paper contained an analysis of 1889 observations made in various parts of England during the three years ending December 31st, 1859.

The average number of days on which thunder-storms occurred at one or more stations was 121, the number of days in each month being—

The effect of thunder-storms on the various meteorological instruments was examined and described, special attention being drawn to those sudden oscillations of the barometer which occur during the height of a storm, and which have been found by Mr. Eaton, of Little Bridy, to be contemporaneous with tidal disturbances. The shape, colour, and disruptive force of lightning were also treated in detail, the frequency with which it assumes a globular form being proved by the number of cases in which it is so described: the determination of colour was not perfectly satisfactory; the returns taken generally show that forked lightning is usually blue, sheet lightning being, on the contrary, white. Mr. Symons, however, supposes that the colour may vary with the distance of the discharge from the observer, with the density of the air through which it passes, and with the existence or non-existence of other sources of illumination; but the subject is quite open to investigation: the disruptive force was shown to be often equal to a dead weight of 500 tons.

The advantage of employing the gutters and rain-water pipes of private houses as lightning conductors, by establishing perfect communication with the earth, and at the same time carrying a short rod from the gutter up the side of the chimneys, was discussed; the author's opinion being that, although far from a perfect arrangement, it would determine the discharge to the outside of the house rather than inside, its most frequent course; and from its trifling cost it seems more adapted for general use than the expensive forms hitherto employed.

Notes on Atmospheric Electricity.

By Professor WILLIAM THOMSON, LL.D., F.R.S.

Two water-dropping collectors for atmospheric electricity were prepared, and placed, one at a window of the Natural Philosophy Lecture-room, and the other at a window of the College Tower of the University of Glasgow. A divided ringelectrometer was used at the last-mentioned station; an electrometer adapted for absolute measurement, nearly in the form now constructed as an ordinary house electrometer, was used in the lecture-room. Four students of the Natural Philosophy Class, Messrs. Lorimer, Lyon, M'Kerrow, and Wilson, after having persevered in preliminary experiments and arrangements, from the month of November, devoted themselves with much ardour and constancy during February, March, and April to the work of observation. During periods of observation, at various times of day, early and late, measurements were completed and recorded every quarter-minute or every half-minute; the continual variations of the phenomenon rendering solitary observations almost nugatory. During several hours each day simultaneous observation was carried on on this plan at the two stations. A comparison of the results manifested often great discordance, and never complete agreement. It was thus ascertained that electrification of the air, if not of solid particles in the air (which have no claim to exclusive consideration in this respect), between the two stations and round them, at distances from them not very great in comparison with their mutual distance, was largely operative in the observed phenomena. It was generally found that after the indications had been negative for some time at both stations, the transition to positive took place earlier by several minutes at the tower station (upper) than at the lecture-room (lower). Sometimes during several minutes, preceded and followed by positive indications, there were negative indications at the lower, while there were only positive at the upper. In these cases the circumambient air must have contained negative (or resinous) electricity. A horizontal stratum of air several hundred feet thick overhead, if containing as much positive electricity per cubic foot as there must have been of negative per cubic foot of the air about the College buildings on those occasions, would produce electrical manifestations at the earth's surface similar in character and amount to those ordinarily observed during fair weather.

Beccaria has remarked on the rare occurrence of negative atmospheric indications during fair weather, of which he can only record six during a period of fifteen years of very persevering observation by himself and the Prior Ceca. On some, if not all of those occasions, there was a squally and variable wind, changing about rapidly between N.E. and N.W. On several days of unbroken fair weather in April and May of the present year the atmospheric indication was negative during short periods, and on each occasion there was a sudden change of wind, generally from N.E. to N.W., W., or S.W. For instance, on the 3rd of May, after a warm, sunny, and very dry day, with a gentle N.E. breeze and slight easterly haze in the air, I found about 8.30 P.M. the expected positive atmospheric indication. After dark (nearly an hour later) it was so calm that I was able to carry an unprotected candle into the open air and make an observation with my portable electrometer. To my surprise I found a somewhat strong negative indication, which I observed for several minutes. Although there was no sensible wind in the locality where I stood*, I perceived by the line of smoke from a high chimney at some distance that there was a decided breeze from W. or S.W. A little later a gentle S.W. wind set in all round, and with the aid of a lantern I found strong positive indications, which continued as long as I observed. During all this time the sky was cloudy, or nearly so. That reversed electric indications should often be observed about the time of a change of wind, may be explained with a considerable degree of probability, thus :

The lower air up to some height above the earth must in general be more or less electrified with the same kind of electricity as that of the earth's surface, since this reaches a high degree of intensity on every tree-top and vegetable fibre, and must therefore cause always more or less of the phenomenon, which becomes conspicuous as the " light of Castor and Pollux," known to the ancients, or the "fire of St. Elmo" described by modern sailors in the Mediterranean, and which consists of a

* About six miles south of Glasgow

flow of electricity of the kind possessed by the earth into the air. Hence, in fair weather the lower air must be negative, although the atmospheric potential, even close to the earth's surface, is still generally positive. But if a considerable area of this lower stratum is carried upwards into a column over any locality by wind blowing inwards from different directions, its effect may for a time predominate, and give rise to a negative potential in the air and a positive electrification of the earth's surface. If this explanation is correct, a whirlwind (such as is often experienced on a small scale in hot weather) must diminish, and may reverse the ordinary positive indication. Since the beginning of the present month I have had two or three opportunities of observing electrical indications, with my portable electrometer, during day thunderstorms. I commenced the observation on each occasion after having heard thunder, and I perceived frequent impulses on the needle which caused it to vibrate, indicating sudden changes of electric potential at the place where I stood. I could connect the larger of these impulses with thunder heard some time later, with about the same degree of certainty as the brighter flashes of lightning during a thunder-storm by night are usually recognized as distinctly connected with distinct peals of thunder. By counting time I estimated the distance of the discharge, not nearer on any occasion than about four or five miles. There were besides many smaller impulses, and most frequently I observed several of these between one of the larger and the thunder with which I connected it. The frequency of these smaller disturbances, which sometimes kept the needle in a constant state of flickering, often prevented me from identifying the thunder in connexion with any particular one of the impulses I had observed. They demonstrated countless discharges, smaller or more distant than those that gave rise to audible thunder. On none of these occasions have I seen any lightning. The absolute potential at the position of the burning match was sometimes positive and sometimes negative; and the sudden change demonstrated by the impulses on the needle were, so far as I could judge, as often augmentations of positive or diminutions of negative, as diminutions of positive or augmentations of negative. This afternoon, for instance (Thursday, June 28), I heard several peals of thunder, and I found the usual abrupt changes indicated by the electrometer. For several minutes the absolute potential was small positive with two or three abrupt changes to somewhat strong positive, falling back to weak positive, and gathering again to a discharge. This was precisely what the same instrument would have shown anywhere within a few yards of an electrical machine turned slowly so as to cause a slow succession of sparks from its prime conductor to a conductor connected with the earth.

I have repeatedly observed the electric potential in the neighbourhood of a locomotive engine, at work on a railway, sometimes by holding the portable electrometer out of a window of one of the carriages of a train, sometimes by using it while standing on the engine itself, and sometimes while standing on the ground beside the line. I have thus obtained consistent results, to the effect that the steam from the funnel was always negative, and the steam from the safety-valve always positive. I have observed extremely strong effects of each class from carriages even far removed from the engine. I have found strong negative indications in the air after an engine had disappeared round a curve, and its cloud of steam had dissolved out of sight.

In almost every part of a large manufactory, with steam-pipes passing through them for various heating purposes, I have found decided indications of positive electricity. In most of these localities there was some slight escape of high pressure steam, which appeared to be the origin of the positive indications.

These phenomena seem in accordance with Faraday's observations on the electricity of steam, which showed high pressure steam escaping into the air to be in general positive, but that it was negative when it carried globules of oil along with it.

Note on the Dispersion of the Planes of Polarization of the Coloured Rays produced by the Action of Magnetism. By M. VERDET, Paris.

The researches in which I have been for some years engaged upon the magnetic relations of the plane of polarization, discovered as we all know by Mr. Faraday, have naturally led me to examine how these relations vary with the nature, or, using theoretical language, with the length of the undulation, of the light. The experimental method which I have employed is the general method introduced into science

by MM. Fizeau and Foucault, which consists in decomposing the white light after it has traversed the apparatus in which it has suffered a certain modification; and in examining how this modification varies from one extremity to the other of the spectrum thus obtained, selecting especially, for the numerical measure of the relative effect, the seven principal rays which Fraunhofer has defined, and the length of those undulations he has determined. As the exact determination of the position of a plane of polarization requires that the light shall have a certain intensity, I have been obliged to confine myself to measuring the relations of beams which correspond with five rays, C, D, E, F and G.

As in my previous researches, in order to operate upon certain bodies well defined and easily reproduced, I have always experimented upon liquids, contained in tubes closed at their extremities by transparent plates; placing these tubes in the interior of a strong electro-magnetic coil, and so arranged that their two ends shall sufficiently pass the edges of the coil, to obviate the necessity of taking into account the action which the transparent plates themselves might exercise on polarized light. But under these conditions the employment of a powerful current was rendered absolutely necessary by the feebleness of the phenomena; and the nature of the experiments requiring that each liquid should remain for a long time under observation, an elevation of temperature was produced which easily reached 50° or 60° Centigrade, and which produced a contraction of the rotations observed, which it was very difficult to correct.

The only way of avoiding this source of error, was to place the tube containing the liquid in an annular collar continually traversed by a stream of cold water within the electro-magnetic coil,-a considerable complication to the apparatus.

The coil which I employed was not less than 45 centimetres in length, 15 centimetres in internal, and 30 centimetres in external diameter. It contained more than 80 kilogrammes of copper wire 2:25 millimetres in diameter, and was set in action by a Bunsen's battery of twenty or thirty elements.

I have not yet quite finished my experiments, but I am now in a position to establish one result, which does not appear to me to be unworthy of being communicated to the Association. M. Wiedemann, in a note published in 1851, believed that he might deduce from a small series of experiments, that if one submitted to the action of magnetism a substance capable of itself of turning the plane of polarization, such as the spirit of turpentine or essential oil of lemon, the rotation proper to the substance and the magnetic rotation were proportional to one another through all the colours of the spectrum. In order to submit to a decisive proof this law, which would be, were it true, of immense theoretical importance, I have just examined an extreme case, that of tartaric acid. We know that solutions of this acid induce in the planes of polarization rotations which do not increase from the red to the violet, as in ordinary cases, but which present in the interior of the spectrum a maximum whose exact position varies with the strength of the solution. Were the relations admitted by M. Wiedemann correct, the magnetic rotations of tartaric acid should present the same anomaly. My experiments have, however, proved, on the contrary, that the magnetic rotations in different solutions of this acid always increase from the red to the violet. There is no essential relation between the two orders of phenomena, as there is no analogy between their causes.

On the other hand, my experiments show how the two phenomena may have appeared in some cases proportional. They show, in fact, that in all cases the magnetic rotations of the plane of polarization increase very rapidly from the red to the violet, but that the product of the rotations by the square of the length of the undulations, increase very slowly between the same limits; and one recognizes in this announcement, that which experiment has long ago demonstrated in most of the natural rotatory powers.

Results of Self-registering Hygrometers. By E. VIVIAN, M.A., Torquay. Mr. Vivian reported to this Section a series of observations made with his new self-registering hygrometers, which were first exhibited before the Association at its Cheltenham meeting. One is a combination of the ordinary wet and dry bulb and the differential thermometers, registering the maximum and minimum, or range during