committee hope that they will soon be in a position to decide which method is on the whole most suitable for the purpose. The black glass method has the one great advantage that it works well with the ordinary plates, and as the mirror may be easily removed and replaced a few cloud pictures may be taken during any photographic excursion without the necessity of carrying slides charged with plates of little use for other

purposes.

PHOTOGRAPHS OF LIGHTNING.

The registration of photographs of lightning is beset with difficulty, just such as interfered with the description of clouds. A provisional classification has been issued under the authority of the Thunderstorm Committee of the Royal Meteorological Society. This, however, was premature, and cannot be regarded as satisfactory. Hence your committee have turned their attention rather to the study of lightning than to recording pictures of it.

The phenomena accompanying electric discharges do not seem to have been very perfectly studied, but certain facts are known, and photographs of lightning and of electric sparks point to others. It seems, therefore, that no classification can be generally accepted which ignores existing knowledge of the connection between the electrical conditions and the character of the discharge.

The so-called black flashes have of course been disposed of. The experi ments described two years ago by the Secretary to your committee showed that the appearance is due to reversal produced by some form of diffused light having fallen upon the plate. This conclusion has been subsequently confirmed by Mr. Shelford Bidwell, F.R.S., and again by Mr. Clayden in the photograph numbered 2 B. This was taken at Bath in the early morning hours of June 25. After the flash had passed, the plate was left exposed for a few minutes in the hope that a second flash might illuminate the same part of the sky. This happened, the lower part of the field of view being brightly lit up by a flash which was itself hidden in the clouds. Where the consequent glare crossed the undeveloped image of the flash reversal has occurred, while no reversal can be detected in the other portion.

It will be noticed that this flash, like many others, shows a distinct ribbon-like structure. The repeated occurrence of this phenomenon has already given rise to considerable discussion, and Mr. W. Marriott and Mr. Cowper Ranyard have attributed it to a movement of the camera during the existence of the flash. Certainly many such photographs have been taken in cameras held in the hand or on no very firm base. Moreover, Dr. Hoffert's photograph, No. 1 B, shows this structure well in the successive bright flashes. Nevertheless, it must be noted that in this last case the camera was in rapid motion, and yet the ribbon-like structure is hardly more pronounced than it is in other pictures where any accidental movement was presumably much less. Moreover, the photographs Nos. 2B and 3B show this structure very plainly, though the camera was standing on a steady support, and movement during the flash was quite out of the question.

Alternative hypotheses are that the appearance is due to reflection from the back of the plate or in the lens. If either view were true the brighter parts of the flash should show the ribbon form the best, whereas

the contrary seems often to be the case. Again, if the former hypothesis were true, the position occupied by the reflected light could be ascertained by considering the direction of the incident light. Fact here disagrees with theory.

The evidence at present obtainable therefore points to the conclusion that a bright lightning flash may often take the form of a long sinuous ribbon, whose sectional thickness is very different in two directions normal to each other. Some of the appearances noticed also indicate that the greater thickness throughout all the parts of a given flash lies in one and the same direction, and the variations in its apparent direction are merely an effect of perspective.

This structure must be carefully distinguished from another, in which several distinct flashes follow precisely similar paths side by side. Sometimes the bright flashes (which may or may not show the ribbon shape proper) are connected by a less brilliant luminosity, which converts the whole phenomenon into a very broad ribbon. Photographs of this class are exemplified by Nos. 4 B, 5 B. The flash represented in Dr. Hoffert's photograph is evidently one of the same order, and the curious smudges which cross the plate must doubtless be due to the above-mentioned fainter light. Clearly we have here to deal with intermittent discharges, a number of discharges following each other along the same or closely contiguous paths. In some cases photographs of this kind show reduplicated images of buildings corresponding fairly well with the images of the component parts of the discharge. In such a case there seems little room for doubt that the flashes followed the same path or paths only a very short distance apart.

The secretary to your committee, however, secured the photograph No. 4 B on June 25. In this case the camera was certainly not moved. The flash, like many others, appeared multiple to the naked eye, but as the motion of the eyeball might have produced that effect, although the flashes formed the same path, little weight can be laid on that argument. Indeed, the fact that the camera was standing still and quite untouched is sufficient to prove that flashes of such a nature do occur. is really a rapid and almost simultaneous volley of flashes connected partly by a less vivid discharge which obliquely links the brighter lines. There is also evident a sort of half-twist of one part of the flash around another part.

It

In order to elucidate the unexpected facts brought to light in the numerous photographs belonging to the Royal Meteorological Society a number of experiments have been made by your secretary upon electric sparks obtained from an induction machine. As these tend to throw some light upon the questions in hand, a brief account of them may not be out of place.

First remove the small Leyden jars from a Voss or Wimshurst machine. The discharge is then pink in colour, of slight brilliancy, and strongly resembles the brush discharge. If the knobs are brought near each other the discharge passes along several lines, which arrange themselves side by side in a plane at right angles to the direction of discharge.

If now the condensers are introduced in the ordinary position, the spark at once becomes more brilliant, and the pink tinge disappears. This spark obtained from the ordinary size of condenser appears to be precisely the same as the commoner varieties of lightning. If larger

condensers are substituted the spark becomes thicker and brighter, and its minor irregularities frequently disappear.

Next remove the condensers from the machine, and connect their inner coatings with the prime conductors, while the outer coatings are imperfectly insulated, as, for instance, by placing them on a wooden table. If the jars are near each other, as each spark passes between the discharging knobs another will pass between the outer coatings.

Gradually increase the distance between the jars. The spark between the outer coatings will become more irregular as it grows longer, and at a certain distance it will suddenly cease. At this moment the discharge between the knobs entirely alters its character. If the striking distance is short, the form assumed is that of a bright pink band, generally brighter at its margins than elsewhere, and showing a beautiful fluted structure. Its duration is short, but it is nevertheless easy to see that it is a really intermittent.

Again increase the striking distance step by step. The discharge is still intermittent, but thin, brilliant white sparks make their appearance. At first the pink discharge can be recognised passing obliquely between these bright sparks, but as the distance increases the pink light disappears, and the discharge becomes a rapid volley of bright sparks.

The photographs from No. 1 E to No. 9 E show these phenomena. Again, if the discharging knobs are placed some distance from the machine, so that the field due to their charge is but little affected by the movements of the machine or operator, it may often be noticed that with ordinary bright sparks their form is repeatedly the same. No. 10 E shows a series of sparks taken under such conditions at intervals of about one secord.

Now, it is probable that all these forms of discharge have their analogues in lightning. The bright sparks with small condensers are the counterpart of the commoner type of lightning. Those from the large Leyden jars and between the outer coatings correspond to more powerful flashes, the latter being the 'impulsive discharge' described by Professor O. Lodge. The volleys of bright sparks are also the type of many observed multiple flashes. There remain only the pink discharges, and surely these are the counterpart of the flashes which yield photographs like No. 4 B.

Moreover there seems to be no prima facie absurdity in supposing that a short series of flashes may occur during a brief time along parallel paths. Such a phenomenon is conceivably explicable

(a) by an identity of conditions over the whole area traversed by the flashes;

(b) by the movement of the charged cloud causing the conditions which held in one place at a given moment to hold a short distance away at another;

(c) by the movement of the air sweeping along the disturbance caused by the first spark, so that a path of least resistance resulting from that disturbance occupies different positions. Your committee would draw attention to the similarity between the appearance of the bright pink discharge and that through rarefied air. Some of the discharges, Nos. 3 E to 7 E, look as if the passage of the bright sparks caused a partial vacuum between them, and the pink sparks then struck through this lessened resistance along the paths of the bright sparks and across the low resisting interval between them, the slope of these transverse

sparks being possibly determined by the difference of potential required to break through what resistance there was.

Possibly it may be found that the ribbon structure is also due to some such phenomenon. The passage of the first flash will produce for a short time a highly rarefied column of air, through which a stream of less luminous sparks may pass until the displaced air surges back. Resistance will then be abnormally high exactly along the track of the first spark, and this column of extra dense air will be surrounded by a tube (so to say) of lower resistance. Indeed, the paths of subsequent discharges in a series may conceivably be determined either by the outward movement of the wave of rarefaction or by the alternate compression and rarefaction along the original path. In either case the movement of the air may easily suffice to carry the position of least resistance along with it. That subsequent discharges do sometimes follow what may be called the trough of the atmospheric wave is indicated by the tendency sometimes exhibited for one spark or flash to twist partly round another.

However, your committee do not wish it to be understood that they put forward these suggestions as definite hypotheses. They merely state them in order to indicate various lines along which further research is desirable. They hope, if they are permitted to continue their task for another year, to add considerably to the experimental and observational facts at present available, and possibly to reach more definite conclusions than existing material allows.

Before ending their report your committee feel that a passing reference is due to the important paper read before the Royal Society in which the Kew Committee described some of their results, and also to the work which has been carried on at Berlin and elsewhere in the photography of the so-called luminous night-clouds and of clouds invisible to the naked eye.

They wish to express their thanks to the Kew Committee, to the numerous persons who have volunteered their assistance, and especially to the Council of the Royal Meteorological Society.

In conclusion they ask to be reappointed, with a grant of 15l., in order that they may have an opportunity of following up the beginning that has been made.

Report of the Committee, consisting of Professor O. J. LODGE, Professor CAREY FOSTER, and Mr. A. P. CHATTOCK (Secretary), appointed to investigate the Discharge of Electricity from Points.

MEASUREMENTS have been made of the strength of field necessary to start discharge at points of radius of curvature varying from 0.7 × 10-3 to 58 x 10-3 cm. The results show that the field strength increases rapidly as the radius of curvature diminishes. They also point to the gas surrounding the point as the seat of resistance to discharge, rather than to the surface of the metal; and, upon the assumption that discharge means the breaking down of Grotthuss chains in the gas, extrapolation indicates an atomic charge of dimensions approximating to those of the ionic charge of electrolytic ions.

The variations of the field strength with pressure of the gas seem to agree with the Grotthuss chain hypothesis as far as the measurements go.

Upon the assumption that the passage of electricity from a point to a plate is a one-way flow, it is possible to obtain a value of the ratio of mass moving to electricity carried by it (i.e., the electro-chemical equivalent of the discharged matter) in terms of the slopes of potential and pressure brought about by the discharge, and the density of the current passing. Experiments are now in progress to determine this ratio, if possible. So far they point to a number far in excess of the electrolytic value. This may be due to error in the measurements, or, possibly, to the presence of metal dust in the discharge.

Measurements, also still in progress, have been made on the mechanical forces which act on a point during discharge. They point to interesting differences between + and electricity, and it is hoped that useful information may be obtained as to the manner in which the two electricities leave the point by further work in this direction.

Your Committee asks for reappointment with a grant of 501.

Report of the Committee, consisting of Lord MCLAREN (Chairman), Professor CRUM BROWN (Secretary), Mr. MILNE HOME, Dr. JOHN MURRAY, Dr. BUCHAN, and the Hon. RALPH ABERCROMBY, appointed for the purpose of co-operating with the Scottish Meteorological Society in making Meteorological Observations on Ben Nevis.

DURING 1890 the hourly observations by night and by day at the Ben Nevis Observatory have been carried on uninterruptedly by Mr. Omond and the assistants, and as heretofore the five daily observations at Fort William have been made with great regularity by Mr. Livingston. As intimated in last report, a vitally important advance was made in the system of observations on Ben Nevis by the opening of the low-level observatory in Fort William on July 14, 1890, for regular continuous observations. This observatory has been equipped by the Meteorological Council with a complete set of self-recording instruments, such as are in use at the first-class observatories of the Council. The directors have thus now at their disposal the best information available for extending the scientific and practical inquiries they have undertaken through the unique facilities offered by these well-equipped observatories. A beginning has also been made with an elaborate discussion of this double series of hourly observations of which some account will be given in this report.

The directors were again able to give relief to the various members of the observing staff by the courtesy of the following gentlemen, who have given their services as observers for periods varying from four to eight weeks:-Messrs. R. C. Mossman, James McDonald, M.A., and Alexander Drysdale, M.A., B.Sc.; and Messrs. P. Gillies and C. Stewart, from Professor Tait's Laboratory, are now (August, 1891) assisting in the work of observing.

For the year 1890 the following were the monthly mean pressures and temperatures, hours of sunshine, amounts of rainfall, and number of fair