SIR WILLIAM THOMSON, President, in the Chair.

The following Communications were read:

1. The Annual Periods of Thunder (with Lightning), Lightning (only), Hail, and Snow, at Oxford. By Mr Buchan. During the twenty-one years ending 1873, the maximum period of thunder with or without lightning, at Oxford, extended from about 9th April to the end of October, the middle of the period being the first week of July; the three highest days taken consecutively being those immediately preceding the summer solstice. During the five months from November to March, only thirteen cases had occurred during the period. Lightning, on the other hand, had its maximum period from May to November,particularly during August, September, and October. The maximum period of hail was during the first six months of the year; whereas, during the second half of the year, very few cases occurred. The snow period extended from the middle of October to the middle of May,-most falling from December to March,the absolutely highest month being March.

Thus, thunder with lightning, at Oxford, closely follows the sun, the middle of the period being only about ten days after the

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summer solstice; lightning (only) has its maximum period during that time of the year when the humidity of the air is at its maximum; hail is most frequent during that period of the year when the temperature is rising, or when the vertical layers of the atmosphere is in most unstable equilibrium; and snow during the coldest months of the year, with this striking peculiarity, that the maximum period is not in the depth of winter, but in March, in the end of winter; immediately after which the curve abruptly falls.

The intimate connection of the thunderstorm with summer rainfall, and the important bearing of the whole four curves on climatology, was referred to.

2. Note on the Origin of Thunderstorms. By Prof. Tait.

This Note does not refer so much to those great thunderstorms which extend over hundreds of miles in each direction, as to those small local storms which are often seen of from two to five or six miles only in diameter.

It refers particularly to those which are seen, in summer and autumn, to pass down the Tay valley. They almost invariably come from the westward, and I am told each is almost always accompanied by a storm of similar dimensions passing eastwards down the valley of the Forth. So far as I can ascertain, they seem both to commence almost abruptly somewhere in the district about Ben Ledi and Ben Lomond.

Seen from St Andrews, which they frequently pass at a few miles distance to the northward, they usually appear to be in a state of rotation about a vertical axis. It is not very easy to judge of the relative distances of the various clouds, so as to ascertain the sense of the rotation; but, in one case which I observed carefully last autumn, the rotation appeared to be in the positive direction,―i.e., opposite to that of the hands of a watch whose face is turned upwards.

If this be generally the case, and if it should be found that the direction of rotation of the companion storm in the Forth valley is negative, it would seem that their common origin may be explained on the following very simple hypothesis, which has the

additional recommendation of easily accounting for certain other singular phenomena.

It is known from balloon ascents that, in general, the atmosphere is arranged in horizontal strata of considerable depth or thickness, alternately moist and dry,-temperature diminishing steadily with increase of height in the moist, and remaining nearly constant throughout the dry, strata. These strata have usually horizontal velocities, differing (sometimes considerably) both in magnitude and direction. Thus near the common boundary of two such strata, fluid friction will in general tend to produce vortex motion, the vortex columns being at first nearly horizontal, with their ends at the boundary, which is a surface of discontinuity.

A complete investigation of the possible circumstances would show four quite different cases :—

The half vortex-ring thus formed tends, so far as it can, to become semicircular. It may thus extend downwards to the earth or upwards into the higher regions of the atmosphere.

If it extend downwards nearly to the earth, the lower portion will soon be destroyed by friction, and we shall have a couple of vertical vortex columns, with their ends respectively in the surface of discontinuity, and on the ground. They will of course rotate in opposite directions about the vertical, and their mutual influence will tend to cause them to progress in directions parallel to one another, the motion of each being in the same direction as that of the rotatory motion of the side which it at the moment turns to the other. This is exactly the presumed case of the little storms in the Tay and Forth valleys above referred to; the south side of the Tay column (that turned towards the Forth), moving eastward about the axis, while the axis itself moves to the east.

This theory is evidently capable of at once explaining the apparently sudden occurrence of such storms (of which waterspouts must be looked upon as small but quickly rotating examples), when the lower atmosphere has for hours been in a dead calm.

The disturbance has, in fact, its origin above the lower stratum, and works its way downwards into it.

It is also competent to explain the production of similar rotating storms in the higher regions of the atmosphere-many miles above the earth's surface-and thus to account for that by no means small number of cases of so-called summer-lightning," which obviously cannot be explained by the occurrence of an ordinary thunderstorm at such a distance as to be below the spectator's horizon.

I have already explained to the Society that a possible source of at least a large part of the electric charge of a thunder-cloud is the contact-electricity of water-vapour and air. Thus while the precipitation of the vapour develops heat, the water particles precipitated are strongly electrified. And the aggregation into one of a number of equal little drops all charged to the same potential may increase the potential in any ratio whatever. Thus the charge on each drop in a large cloud may become so great that the electricity is driven entirely to the particles at its surface. This is supplementary to, and does not interfere with, Sir W. Thomson's explanation of the process by which the vapour is condensed.

It is possible that taking place in greatly larger spaces of air, but to a much smaller extent in each cubic foot, this sudden production, and as sudden scattering in all directions, of considerable quantities of electricity, may account for some of the main phenomena of the Aurora.

3. An Application of Professor James Thomson's Integrator to harmonic Analyses of Meteorological, Tidal, and other Phenomena, and to the Integration of Differential Equations. By Sir W. Thomson.

A first rough Model of Professor J. Thomson's Integrator was shown.

4. Note on the Thermo-Electric Position of Cobalt. Professor Tait.

By