that in which in the segment A B the N.E. wind blows. Clouds apparently moving from the S.W. are often observed in this part of the storm, where N.E. wind first begins to exhibit itself. No

B

E

Fig. 4.

general movement of the air, however, takes place, because it is here that it first commences and continues to be generated from the still atmosphere ahead, where the curve of outward propagation advances upon it. This continued generation also represents what takes place in the clouds. Their apparent movement, which is due solely to their additional formation, is the result of atmospheric changes which must here take place. Hence occasionally they do not represent the force and direction of the wind. Similarly seamen often observe a gale approaching from some point to leeward.

It has been supposed by some writers that the progress of storms is due to the area of low pressure being impelled forward by the high pressure usually found near their rear, aided by the diminished pressure in front, which is due to heat and vapour. If this were the case, it is evident that a derangement would take place as regards the circular symmetry of the storm and the direction of the spirally inblowing winds, owing to the enormous friction which these widespread storms would encounter in being pushed forward over the earth's surface. The more evident of these derangements would be the steepening of the barometic gradients in front of the storm, a result the reverse of what observation shows to take place. The Rev. Clement Ley has shown that the isobars are widest, and gradient consequently lowest in the front of the storm.* Further, if this were the cause of the progress of storms, areas of depression would move more rapidly over the ocean than over the land. Professor Loomis has shown, however, that while the average rate of

* See Journal of Scottish Meteorological Society," vol. iv. p. 149.

progress of depressions over the ocean is 19 miles an hour, over the land the rate is 26 miles an hour. It is also evident that if a depression moves forward accompanied by spirally inflowing winds, considered simply as inflowing air-currents, and not as representing an efficient motive force capable of generating forward movement,-a derangement as regards their force and direction. would take place, especially on those occasions when the onward march of the storm attains a speed of 70 miles an hour. But in no case has such a derangement been observed.

Proposed Theory as to the Progress of Storms.-When, however, the depression is not being impelled forward in the mode above described, but in virtue of an opening out in front, and when the air-currents, inflowing spirally upon the centre, are regarded as an effective motive force capable of causing progressive motion, no derangement can be supposed to take place as regards the force and direction of the air-currents; and no steepening of the gradients in front will take place, since the change in the position of the depression is effected with facility by the transference of the air from the front to the rear along the north segment of the depression.

A depression advances by means of these spirally inflowing currents, owing to the difference which exists in their mode of inflow, and consequently in their force and direction; and it is in this way that outward extension in one direction is carried out. This difference in the mode of inflow is mainly due to the introduction in a much higher degree of the element of friction into the currents of the front segment; in this way scarcity of supply is produced in this part of the depression, and the effect is intensified by the ascending tendency of the air in this segment of the depression. On the other hand, the currents which enter in at the rear are much less retarded by friction, because there is a descending tendency of the air in this segment, and, consequently, supply is copious. The low central pressure, by travelling onwards towards the region in front, where supply is scarce, and where opening out takes place, practically overcomes this scarcity of supply, and thus uniformity of inflow is practically restored by progressive movement. If there was no progress, and if the depression was accompanied by this want of 'Proceedings," vol. viii. p. 613-615.

uniformity in the mode of inflow, it would then tend to fill up. Progress is therefore the means by which a depression is maintained.

Since the difference in the mode of inflow is due to a difference in the amount of friction called into play, we have here, probably, the reason why storms advance more rapidly over the land than over the ocean. The deflection of the winds produced by the earth's rotation, and the irregularity in the amount of pressure at different points immediately outside the area of the depression itself, have doubtless important bearings on this whole question.

It is very generally supposed that the spiral inflow to the centre is supposed to be there carried off by means of a vast ascending current from the centre. This opinion is, however, open to serious objections, but which it would be out of place here to discuss.

The general result is, that progress is regarded as being due to an irregularity in the direction and force of the spirally inflowing currents, arising from the different degrees in which the element of friction is called into play in the different segments of the storm, accompanied or aided by high pressure on the right of the direction in which the depression moves.

Summary. When a depression is formed with a narrow diameter, it fills up rapidly if it has a copious supply from the surrounding atmosphere, and the filling up is of course attended by a rapid rise of the low central barometer. But if the depression has a very large diameter, the element of friction must enter largely into the problem. The retarding influence due to this cause will necessarily be greatest in the denser currents near the earth's surface; while the light, rarefied, and mobile upper currents will move with great facility. Hence the main source of supply will be thrown on these freely flowing upper currents. The rapidity and facility with which they inflow give rise to horizontal extension or shallowing out aloft, and this extension would take place equally all round if the inflow were uniform in all the segments of the depression. If, however, this uniformity of inflow does not exist, but is more copious in one segment and scarce in another, then extension and shallowing out will take place towards that particular direction where supply is scarce. Hence progressive movement takes place in this direction, and is attended by filling up in the rear. When supply is equally copious in all directions,

the low central barometer rises rapidly; but when the supply is not copious and uniform all round, the barometer does not rise, because, though the depression fills up in the rear, there is a corresponding opening out in the front, and the central barometer remains low -thus producing the central spiral inflow which generates the motive force to which the progressive movement is due.

2. On the Thermo-electric Properties of Charcoal and certain Alloys, with a supplementary Thermo-electric Diagram. By C. G. Knott, B.Sc., and J. G. MacGregor, D.Sc. Communicated by Professor Tait.

3. On the Auriferous Quartz of Wanlockhead. By Dr Lauder Lindsay.

4. On the Discharge of Electricity through Turpentine. By A. Macfarlane, B.Sc., and Mr R. J. S. Simpson. Communicated by Professor Tait.

5. Remarks on the Phonograph. By Professor Fleeming Jenkin and Mr J. A. Ewing.

Professor Fleeming Jenkin and Mr J. A. Ewing exhibited a phonograph which they had constructed, and also some curves drawn on paper representing the indentions produced in the tinfoil of the instrument by vowel sounds.

The phonograph exhibited had been constructed from a description of the instrument invented by Mr Edison, and consisted of a barrel about four inches diameter and four inches long, mounted on a spindle on which a square-threaded screw had been cut. One bearing of the spindle was cylindrical, and the other was a nut in which the screw worked. A fly-wheel handle turned the spindle and barrel, which advanced during each turn by a distance equal to the pitch of the screw. A helical groove, about in. in breadth, was cut on the surface of the barrel, having the same axial pitch as the screw on the spindle.

A smooth strip of tinfoil was gummed round the barrel, and in

dentations produced in this strip by a point attached to a vibrating disc stretched across a short brass cylinder 2 in. diameter.

The same vibrating disc was used to produce the indentations and to reproduce the sounds.

The usual ferrotype plate had not been tried as a vibrator, for hard metal discs were found to give a disagreeable resonance to the voice and its reproduction. A slack tinfoil disc had been used with good results; the marker was attached firmly to a small disc of stiff paper in. diameter, gummed to the tinfoil; a short piece of watch spring was also attached to the marker, so as to give the disc a rapid period of vibration.

A sentence reproduced by this disc was loud enough to be heard by many people standing round, and sentences had been heard by several persons who understood them without any previous idea of what they were. This result could not, however, always be secured. When the sentence was known to the hearers, it appeared to be given back with startling accuracy. The vowel sounds were more distinct than the consonants, but the consonants also were distinctly to be heard. The tinfoil vibrator gave more articulate sounds when slack and irregular than when it was neatly strained over the end of the tube.

An oil-silk disc had also been tried with no spring, and a simple marker attached to a disc of mica, gummed to the oil-silk. This disc gave purer sounds than the tinfoil, but they were not nearly so loud. The indentations on the tinfoil were excellent to the eye, and quite as large as with the tinfoil.

The oil-silk answered best when irregularly stretched.

An india-rubber film, with a similar mica disc and rigidly attached marker, had also been tried, and gave beautiful records on the tinfoil strip, but this disc failed to reproduce the sounds accurately, having a note of its own.

This latter disc was used to produce records which were subsequently enlarged and shown on paper in the form of curves. The india-rubber was preferred for this purpose, because the gentle and uniform pressure which it gave did not tend to obliterate the records. The curves exhibited showed the form of the indentations magnified about five hundred times. This magnification was effected by two compound levers, of which the second was a glass siphon like that