portions of the approximately identical figures due to either fork by itself, took up, while both were sounding, a swaying movement about their mean position; one complete oscillation of figure synchronizing exactly with each beat* heard. The resonance-boxes had, in this experiment, been placed with their openings exactly opposite each other, and the film, which was rectangular in shape, midway between, but in a horizontal plane slightly above them. The swaying motion was, under these circumstances, on the whole rectilinear, as though each fork alternately gave the entire figure a pull in its own direction. The behaviour of the vortices was still more remarkable. With vigorous and equal bowing they rotated several times in one direction during the first half of each beat, and the same number of times in the opposite direction during the second half of it. If, instead of occupying the relative positions above described, in which the forks when sounding singly gave rise to antagonistic vortices, they were both placed on one side of the film, the result was rotation during one halfbeat and inaction during the next, followed again by similar alternations, but the direction of rotation remaining constant throughout. In this case the vortices moved most rapidly during the maximum and rested during the minimum of intensity. But in others it was not so, and I even observed instances where in one part of the figure the rotation coincided with the maximum and the quiescence with the minimum, while at another part of it the exactly contrary state of things prevailed.

In every experiment hitherto described the film was either acted on by the resonance of a spherical air-cavity, which practically reinforces only the fundamental tone of a compound sound in unison with it; or else the vibrations employed were exclusively those of a mounted tuning-fork which follow the pendulum law. Hence no other kind of movement was transmitted save that which gives rise to what Helmholtz calls a "simple tone." In order to examine the effects produced by composite sounds, it was desirable to let their vibrations act on a film unconnected with any resonant cavity. For this purpose one end of a caoutchouc tube of large bore was fitted into a metal ring fixed in a horizontal plane on which the film-bearing discs could be placed. Notes of the human voice, of tuning forks, organ pipes, &c., being sounded into the tube, either directly or through a funnel in the shape of an ordinary ear-trumpet, their effects on films of various forms and sizes could be conveniently observed. A very wide field for research was thus opened up, which I do not propose to enter upon here beyond simply mentioning one result obtained in this manner which possesses an independent interest. When two notes, identical in pitch and

The absence of an English equivalent for the German Schwebung, which denotes the whole phenomena from one maximum of intensity (Schlag) to the next, is most inconvenient, and makes itself very perceptible here.

loudness, but differing markedly in what is called quality or timbre, were alternately sounded, two perfectly distinct figures were obtained, each presenting itself again and again for many alternations the instant its own note was sounded, and remaining constant until that note ceased. By this apparatus, therefore, permanent pictures of the relative quality of musical sounds may be secured.

I content myself here with a description of the phenomena I have observed, and make no attempt at determining the mechanical conditions under which they occur. It may, however, be worth while to remark that the most striking feature of the figures above described, the vortices, can be exactly reproduced with the caoutchouc tube apparatus by gently sucking a little air through it, taking care that the whole opening of the tube is not in simultaneous contact with the lips, and that its other end is not completely closed by the disc and film. Before concluding I wish to draw attention to some allied phenomena described by Mr. E. B. Tylor in "Nature," for May, 1877, P. 12. Distinct patterns were obtained by him, but it would appear from the directions he gives for producing "a film more free from interference-colours, so as to display the vibration-figures on an almost clear ground" that no permanent colour-patterns are in question in his letter. He speaks indeed of "the gorgeous scenic effect of the masses of prismatic colour whirled hither and thither by the musical vibrations," but of nothing more fixed and regular. The mode of experimenting adopted by him on that occasion suggested the third form of apparatus described in the present paper.

February 7, 1878.

Sir JOSEPH HOOKER, K.C.S.I., President, in the Chair.

The Presents received were laid on the table and thanks ordered for them.

The following Papers were read

I. "On the Comparison of the Standard Barometers of the Royal Observatory, Greenwich, and the Kew Observatory." By G. M. WHIPPLE, B.Sc., Superintendent of the Kew Observatory. Communicated by order of the Kew Committee, WARREN DE LA RUE, F.R.S., Vice-Chairman. Received

November 26, 1877.

The Standard Barometers of these two important establishments, up to within a recent date, had never been compared directly, although

from time to time indirect comparisons have been made by persons not connected officially with either.

Attention has recently been drawn to the subject by the publication of a memoir by Dr. H. Wild, Director of the Central Physical Observatory, St. Petersburgh, entitled "Ueber normal-Barometer und ihrer Vergleichung," in which the author states (p. 100) that the standard barometer of the Royal Observatory reads 0·465 m.m., or 0·018 inch lower than that of the Kew Observatory, arriving at that conclusion by means of somewhat circuitous comparisons. Professor Mohn, of Christiania, in 1876, writes also that he finds the difference Kew Greenwich to be + 0.49 m.m., or +0.019 inch.

In 1869 the late Colonel Strange made a comparison between the two standards by means of a barometer which he had conveyed from the one observatory to the other, the results of which are stated by him to be as follows:

By comparison No. 1 Greenwich lower than Kew 0.009 inch.

In consequence of these statements the Kew Committee made application to the Astronomer Royal for permission to make a direct comparison between the two instruments, and received from him a courteous consent to their proposition.

Accordingly, having selected four barometers-Adie 655, Adie 657, Negretti and Zambra and Negretti and Zambra all standards

C

429'

C 431'

on the Fortin principle-as suitable instruments for the purpose, I made arrangements to convey them to Greenwich and back safely in a carriage.

On April 23rd, 1877, Mr. Baker and I made a set of observations of the two large Kew standards, constructed by the late Mr. Welsh ; Newman 34 [the working standard of the Observatory], Adie 655 and Adie 657, and subsequently of Newman 34, and the two Negretti and Zambra standards.

The latter we also compared again on the morning of the 24th, before starting for Greenwich.

Having carefully packed the instruments, accompanied by an assistant, I went to the Royal Observatory, and there suspended the barometers in the large room of the Magnetic Observatory, facing and about eight feet distant from the standard barometer (Newman 64).

After leaving the instruments about an hour, in order that they might assume the correct temperature of the room, which did not differ greatly from that of the external air at the time, we made a set of twelve readings of all the four instruments and of the Greenwich standard, myself and the assistant reading them alternately, the tem