plains its mode of action. I am sorry I had not time to make a better finished instrument, for it was only at the eleventh hour that we thought of sending one at all.

If a water-wheel, with float-boards in the plane of the axis, or in planes passing through the pivots, be held so as to dip into a running stream, and free to revolve, the velocity of the circumference will be exactly equal to the velocity of the current, multiplied by the sine of the angle of deviation (in a horizontal direction) of the axis from the direction of the current; or, in the time the current flows from A to B (see fig. 1) the circumference of the wheel will move a dis

C

R

Fig. 1.

tance equal to A C. This is so obvious, it needs no proof. Now, if a moving plane be substituted for the stream, and a plain roller for the wheel, it occurred to me that it must obey the same law (for there is no other that it could obey), and, in that case, by communicating the motion to the roller instead of the plane by means of a lever in the line of the axis, a very simple measure of irregular plane surfaces might be made. I immediately set to work and made a rough model to test the principle, and it answered; I may say, beyond my expectations.

If the edge of the wheel is examined with a lens, it will be seen that the lines produced by the friction of the paper are all in the line of the axis, which proves that whichever position it is held in when being moved along the guide-way, the paper slides over the edge by the shortest route, viz., the line of the axis.

The length of the lever (from the centre of the sole plate)

multiplied by the circumference of the wheel, is equal to the area circumscribed to produce one entire revolution (50 square inches in the accompanying instrument).

Fig. 2.

The instrument examined by Mr Balfour had a wheel about half a pound weight. I made the accompanying one light on purpose to ensure its reaching Scotland in safety.

Report of Committee.

Your Committee having met and carefully considered the description of Mr Beverley's platometer, and having examined the instrument itself, beg to report as follows:

"The description is meagre and incomplete, so much so that, without the presence of the instrument itself, there would have been some difficulty in making out the nature of the action. It is therefore principally from an examination of the actual platometer that an opinion of its merits has been formed.

"It is assumed as the fundamental principle of the action, that when the edge of a wheel is drawn over a surface such as that of paper, in a direction not perpendicular to the axis of the wheel, the motion is proportional to the distance traversed and to the sine of the angle of obliquity jointly. This is also the principle on which other platometers are constructed: your Committee are not in a position to say definitely whether the doctrine be sound or not. When the surface is entirely free from linear structure it is highly probable that the proportionality of the motion of the wheel

to the sine of the angle is perfect; but when the surface is striated or wire-marked, it is likely that the direction of the striæ may form an element in determining the motion, and a careful analysis of the complete action is needed in order to ascertain whether such lines would interfere with the result. In using the platometer the tracer is led round the boundary of the figure to be measured. Thus the edge of the wheel must pass forwards, and also backwards, over the same line, and thus the influence which the striæ may exert on the one motion may be compensated by that exerted on the other, and so the resulting indication may be uninfluenced. The opinion of the Committee is, that the surface of paper is in all likelihood one of the best that can be chosen ; but it would be satisfactory to have the matter examined into by careful experiment.

"The contrivance by which Mr Beverley takes advantage of this proportionality is exceedingly neat and ingenious, leaving nothing to be desired in point of simplicity; and of this, which is the essential matter before them, the Committee desire to record their highest commendation.

"As to the practical, and particularly as to the comparative, advantages of the contrivance, it is not easy to speak decidedly. The platometer submitted is a specimen constructed only for the purpose of explaining the action, and it would be unfair to contrast its results with those of other elaborate instruments; at the same time, on the other hand, it would be as unfair to compare the cost of such an instrument as the platometer invented by Mr John Sang with the new one in its present state, because it may be found that when pains have been taken to give as great precision, the friction wheels and other appliances may bring up the costs more nearly to equality.

"Your Committee, however, are of opinion that Mr Beverley's platometer admits of considerable variety in the degrees of its elaboration-that it may be made simple, as in the example before the Society, or that it may be fitted up with various refinements according to the precision required.

"On the whole they beg to recommend it to the Society's very favourable consideration."

Description of a Holophone or Sound Reflector for Fog Signals. By THOMAS STEVENSON, F.R.S.E., C.E.*

During heavy falls of rain or snow, or when there is fog, rays of light are more absorbed than those of sound. Were it not for the difficulty that is felt in judging correctly of the direction of the place whence they proceed, it might be warrantably concluded that sound signals were in some respects more important than those produced by light, because the former are more available at those times when it is most essential that warning should be given to the mariner of his approach to the land or to dangerous sunken reefs.

The proposal to condense and direct the waves of sound by means of reflectors, is not new. Professor Babbage proposed to use a whistle in the focus of a parabolic reflector; and Mr A. Gordon, of London, suggested the employment for the same purpose of the form of reflector which is generally known as that of Bordier Marcet. In that kind of reflector the rays are only acted on in the vertical plane, while all the others are allowed to diverge naturally. It is obvious, however, to those who are acquainted with these instruments, that, as in the case of rays of light, much of the sound is lost by natural divergence past the lips of the reflector. Daboll has recently produced excellent results by means of a horn, containing a metallic "reed" or "stop," through which condensed air is blown by means of a caloric engine.

What I have now to suggest as an improvement on sound reflectors, is the application, with some modifications of the holophotal arrangement which I proposed for lighthouses, so as in like manner to parallelise and guide all the waves of sound into the required directions. I have elsewheret

*Read before the Society and instrument exhibited on 12th February 1866. Awarded the special thanks of the Society.

+ Lighthouse Illumination, London, 1859, p. 80.

explained, that, from the greater length of the waves of sound, a much larger instrument would be necessary for this purpose than in the case of the reflection of light. I had a remarkable proof of this sometime ago in the case of a stamping mill on the banks of the river Esk, at Musselburgh, the sounds from which were reflected with wonderful sharpness by a distant wall on the opposite side of the river. And my attention was more recently directed to this subject, when at Rome in 1862, by the well-known phenomena presented by the echoing gallery in the dome of St Peter's.

The same kind of arrangement is, of course, alike applicable to light and sound. The design for lighting purposes, to which I have given the name of Holophote, should, in the case of sound, take the corresponding name of Holophone, and consist of reflectors so arranged as to intercept the diverging waves of sound, and to parallelise them.

Fig. 1 represents a horizontal cross section of the first form of holophone, which I had made in September 1864, and consists of an arrangement of conical reflectors, which the late Dr Robison of Edinburgh considered superior to paraboloids for reflecting light, and which I adopted for a first experiment, as being more easily constructed, while figs. 2 and 3 represent elevations of paraboloidal reflectors. Fig. 2 represents the arrangement for diffusing the sound over 180°, and fig. 3 that for condensing the sound into one beam. The action of these instruments will be easily understood on looking at the diagrams. Figs. 1 and 2 are paraboloidal reflectors, shown in elevation and section, which reflect the rays in the vertical plane only. BCD are paraboloidal strips for parallelising vertically the waves of sound produced by the anterior half of the whistle or other radiant placed in the focus. EFG are similar paroboloidal strips for parallelising the waves proceeding from the posterior half of the radiant. HH are hollow cones for reflecting forwards the rays parallelised by E FG.

Fig. 3 represents the holophone which reflects the sound

*The drawing of this instrument is not given, as its construction will be easily understood by referring to figs. 2 and 3, which resemble it generally, VOL. VII. 2 D