Discharges may consist of a single flash, but they frequently consist of a series of flashes following one another with considerable rapidity along the same or related paths. The eye is often able to detect alternations of brilliancy during a discharge, and may resolve it, as a moving camera will, into a series of flashes accompanied by a persistent luminosity, which it has already been suggested is probably the flame of burning nitrogen.

Last year your Committee referred to a photograph taken by Mr. Glew at Brixton. This was taken in a camera the lens of which was attached to the hammer of an electric bell and kept in oscillation during exposure. The object was to deduce from the known rate of movement of the lens the duration of the discharge. Unfortunately, however, there is nothing to show in which direction the lens was travelling at the moment of each component flash.

There is one very simple method by which it is quite possible to make a rough measurement of the duration of a discharge. Let two observers, A and B, agree that A shall carefully notice the seconds hand of his watch while B looks at the sky to be sure that A does not confuse two separate discharges. If the night is otherwise dark, A will see the hand only when the face is illuminated by the lightning. The secretary to your Committee has, with the aid of Mrs. Clayden, made many such observations, and has found that a lightning discharge often lasts as much as two or three seconds, and may extend further, the longest time hitherto observed being no less than seven seconds. During these times, though the brightness of the light varied considerably, it was quite possible to watch the hand moving steadily, and not in a series of jerks, as must have been the case if the continuity of illumination had been an illusion due to persistence of vision. In a similar way it is quite possible to follow the movements of swaying tree-tops and other objects. It was noted with some surprise that the light, as far as the eye can see, is often perfectly steady for as much as a couple of seconds. Since beginning these observations not a single discharge has been noted of sufficient brevity to prevent any movement of the watch hand from being seen.

Now, although such observations are rough, their bearing upon lightning photography is important.

An argument commonly advanced to prove that all photographs of reduplicated flashes are due to movement of the camera is that the track to be followed by successive flashes in a given discharge is marked out by the first, which creates a path of minimum resistance in the form of a partial vacuum.

But it seems to be forgotten how far this tube of rarefied air must be moved, and how far the discharging point of the cloud (so to say) may be displaced by the movement of the air. We know that the wind is often quite strong during a thunderstorm.

Now, a movement of one mile an hour corresponds to 176 inches a second.

Suppose, therefore, we take the first seven valnes of the Beaufort scale and see how far such a tube of minimum resistance would be displaced during the existence of a discharge.

Hence it appears that if a discharge lasts as long as three seconds, the path of minimum resistance marked out by the first flash might be displaced as much as fifty yards by a strong breeze. Moreover, since the clouds would be moving at the same rate as the upper part of the vacuous

track, there would be no disturbance of its relation to the discharging point.

It is frequently observed in photographs of reduplicated flashes that the various components do not follow absolutely similar paths, and it is often seen that the departure from similarity is near the ground.

Surely this is exactly what would be expected if the path of least resistance were swept along as suggested. The movements of the wind are not uniform, and the tube would frequently get bent or broken, such an event being most probable to occur within reach of eddies from the ground. It may be pointed out that the reduplicated flash photographed by the secretary to your Committee in a stationary camera was taken at right angles to that in which the storm and wind were travelling.

Movement of the camera or lens or plate would necessarily exaggerate the reduplication where it might not otherwise have been detected, but there can be no doubt that a single discharge often lasts for several seconds, and therefore that any path of minimum resistance created by the first component flash must be moved to an extent quite sufficient to reveal the multiple structure to the eye and to the camera.

It seems, moreover, that the narrow ribbon structure may be attributed to much the same cause.

In conclusion, your Committee have to state that their scheme of an atlas of typical clouds cannot be carried out without considerable expenditure, and they suggest that they be reappointed with a grant of 501. As they did not draw the 151. voted last year, this is really an application for only 351. for that which they believe would be a valuable piece of work.

The Best Methods of Recording the Direct Intensity of Solar Radiation.-Ninth Report of the Committee, consisting of Sir G. G. STOKES (Chairman), Professor A. SCHUSTER, Mr. G. JOHNSTONE STONEY, Sir H. E. ROSCOE, Captain W. DE W. ABNEY, Professor H. McLEOD, and Mr. G. J. SYMONS. (Drawn up by Professor MCLEOD.)

DURING the last year Mr. Casella has constructed for the Committee a thermometer with a lenticular bulb similar to that described in previous Reports, but consisting of colourless instead of green glass. As stated in the last Report, there are great difficulties in constructing an instrument with a green-glass bulb, and it was believed that there would be little

difference in the readings obtained with a thermometer of ordinary white glass.

On May 22 three sets of observations were made, two with the greenglass and one with the white-glass thermometer: those with the green were made between X.17 and X.50 and between XI.35 and XI.53, that with the white-glass instrument between XI.0 and XI.25.

The observed excesses of temperature of the green-glass thermometer above the temperature of the case were 48°.3 F. and 490.3. The observed excess of the white glass was only 32°.8. The corresponding calculated excesses obtained by the method described in the last Report were respectively 50°.29, 49°.24, and 33°.30.

It is thus seen that the white-glass bulb rises to about two-thirds of the excess indicated by the green-glass bulb. This, however, is no disadvantage, for when the temperature of the insolation thermometer is much above that of the case the simple law which for smaller excesses connects the rate of cooling with the difference of temperatures is no longer a sufficiently near approximation, and the reduction of the observed results becomes more difficult.

As the simultaneous reading of the three thermometers is not an easy operation, an attempt has been made to replace them by two thermoelectric junctions. A copper disc, 20 mm. in diameter and about 75 mm. thick, was soldered at its centre to a piece of iron wire. The wire was so bent that when the centre of the disc opposite to the soldered joint is exactly behind the hole in the copper cube, the other end of the wire makes contact with the copper cube midway between the front and back. To the edge of the disc a thin copper wire is soldered, which passes through a glass tube in the central opening of the cube, and is thus insulated from it. The experiment being only preliminary, the iron wire has been fixed in a hole drilled in the copper plug which usually holds the insolation thermometer, the glass tube carrying the insulated wire being passed through the hole in the same plug. The other terminal from the copper cube is made by fixing a piece of copper wire in the plug which closes the hole of the case thermometer B in front of the cube. In a permanent instrument a binding screw should be attached to the cube in the plane of the disc. To increase the absorption of heat by the copper disc, it was blackened by being placed for a short time in sulphuretted hydrogen. The black surface thus obtained does not, however, completely absorb the radiation, for, on throwing a beam of sunlight on it, it is observed that some of the light is scattered. The surface thus obtained may, in addition, be not permanent.

The terminals of the thermo-couples were connected to a reflecting galvanometer of 97 ohm resistance, and the disc exposed to the rays of the sun, the lens of the instrument being used. The deflection of the galvanometer became steady after an exposure of from five to eight minutes, whereas twenty minutes were required when the green-glassbulb thermometer was used.

In order to determine the value of the deflections a double thermocouple was made by soldering to two stout copper wires a bent piece of thick iron wire. Close to the junctions delicate thermometers were tied, and the apparatus was so arranged that the thermo-junctions and thermometer bulbs could be plunged in test tubes containing paraffin oil: one of these test tubes could be heated, and the connections were so made that the current produced by the heated junction opposed that from the 1893.

L

actinometer. Whilst the disc in the actinometer was exposed to the solar radiations, one of the thermo-junctions was heated, and when the galvanometer indicated that no current was flowing the thermometers were read. In one case, in which a deflection of 172 divisions was obtained, the current was balanced by a difference of temperature of the two junctions of 8°.27 C.

If an instrument of this kind could be made photographically selfrecording it would constitute an excellent sunshine-recorder, giving not only the time of the shining of the sun, but also a measure of its intensity. An ordinary reflecting galvanometer would not be very suitable for this purpose, for variations of the earth's magnetism and the possible movement of magnetic bodies in its neighbourhood would vitiate the results. An instrument on the principle of the D'Arsonval galvanometer would be more appropriate, but a few experiments made with such an instrument have not given satisfactory results. Another source of error must be mentioned, namely, the variation of the resistance of the long conducting wires by changes of temperature. No doubt all these difficulties might be overcome in a properly appointed observatory.

On the Present State of our Knowledge of Electrolysis and Electrochemistry. Report by W. N. SHAW and T. C. FITZPATRICK. Table of Electro-chemical Properties of Aqueous Solutions, compiled by T. C. FITZPATRICK.

The comparison of the numerical results of electrolytic observations is rendered difficult from the fact that the data are scattered in various periodicals and expressed by different observers in units that are not comparable without considerable labour. The following table has been compiled with the object of facilitating the comparison.

In the table are included all the observations, as far as they are known to the compiler, for the metallic salts and mineral acids; but amongst the solutions of organic substances are not given all those for which Ostwald has made observations, as it was thought that they would add unnecessarily to the size of the table. Observations for a number of additional substances will be found in Ostwald's papers in 'Journal für Chemie,' vols. xxxi., xxxii., and xxxiii., and in the Zeitschrift für physikalische Chemie,' vol. i. With this restriction it is hoped that no important observations have been omitted, and that in the reduction of results, expressed in such varied units, the table is sufficiently free from mistakes for it to be of service. The data included refer to the strength and specific gravity of solutions, with the corresponding conductivities, migration constants, and fluidities. The several columns are as follows:

I. Percentage composition, i.e. the number of parts by weight of the salt (as represented by the chemical formula) in 100 parts of the solution. II. The number of gramme molecules per litre, i.e. the number of grammes of the salt per litre divided by the chemical equivalent in grammes, as given for each salt.

III. The specific gravities of the solutions: in most cases the specific gravities of the solutions are not given by the observers, and the numbers

given have been deduced from Gerlach's tables in the Zeitschrift für analytische Chemie,' vol. viii. p. 243, &c.

IV. The temperatures at which the solutions have the specific gravities given in the previous column for the given strength of solution. V. The conductivity, as expressed by the observer. In the cases in which the observer has expressed his results for specific molecular conductivity no numbers are given in this column.

VI. The temperature at which the conductivities of the solutions have been determined.

i.e.

VII. The temperature coefficient referred to the conductivity at 18°, 1 (k18

VIII. The specific molecular conductivity of the solutions at 18° in terms of the conductivity of mercury at 0°; specific molecular conductivity is the ratio of the conductivity of a column of the liquid 1 centimetre long and 1 square centimetre in section to the number of gramme equivalents per litre.

In some few cases in which no temperature coefficients have been determined the results have been given for the temperature at which the observations were made.

The numbers given in the column are the values for the specific molecular conductivity × 109.

IX. This column contains the values for specific molecular conductivity at 18° in C. G.S. units: they are obtained from those in the previous column by being naultiplied by the value of the conductivity of mercury at 0° in C.G.S. units. This factor is 1.063 x 10-5.

X. The migration constant for the anion; for instance, in the case of copper sulphate (CuSO,) for (SO4).

XI. The temperatures at which the migration constants have been determined.

XII. The number of gramme molecules per litre, as defined for column II., for which the Auidity data are given in the following columns. XIII. The fluidity of the solutions of the strength given in the previous column.

Most of the results given for the fluidity of solutions are expressed in terms of the fluidity of water at the same temperature: to obtain the absolute values for the solutions they have been multiplied by the value for the fluidity of water at the given temperature. The values used for this purpose have been taken from Sprung's observations for the viscosity of water given in 'Poggendorf's Annalen,' vol. clix. p. 1.

To obtain the values for fluidity in C.G.S. units the numbers in this column must be multiplied by the factor 1019.

XIV. The temperature at which the solutions have the fluidity given in the previous column.

XV. The temperature coefficient of fluidity at 18°, that is,

:

1

fie (9/10).

papers

XVI. In the last column are given the references to the various from which the data are taken against each reference will be found a number, which appears also against the first of the data which have been taken from the paper in question.