The following Communications were read :

1. On the Occultation of the Star 103 Tauri. (B. A. C. 1572.) By Edward Sang.

An occultation of a star, though not appealing to ordinary observation with the same force, is intrinsically an event as striking as an eclipse of the sun. It establishes the fact of the moon's proximity. Were it not that the moon's brightness overpowers the light of the small stars, occultations would be commonplace phenomena. As things are, we can watch, with the eye unaided, the eclipses of the planets and larger stars, not down, perhaps, to below the third magnitude; and the rarity of such conspicuous objects makes the occultations correspondingly rare.

By help of a telescope of two or three feet in focal length, we are able to examine stars even so small as of the sixth magnitude, and thus greatly to increase the number of observations, so much so that as many as 150 occultations may be visible from one place in the course of the year.

The particular case to which I would draw attention is thus one of many it derives its interest from the proximity of the planet Mars, whose occultations will have been carefully observed from many places.

The three objects-the moon, Mars, and the star-were all within

STAR

VERTICAL

MARS

MERIDIAN

the field of the telescope; their relative positions at the instant of the star's disappearance being as shown in the accompanying figure. The observation was made with a telescope having an aperture of 1.9 in., a focal length of 23.5 in., and a magnifying power of 26. The moon's dark edge was distinctly visible, the atmospheric tremor was slight, so that, notwithstanding

the moon's proximity to the horizon, the disappearance was watched under very favourable circumstances. The time was noted by an excellent chronometer, which was compared, twelve hours thereafter,

with the mean time clock of the Royal Observatory, and again rated after nine days. The comparison of the time may be held as true to within a quarter of a second. The observation may recorded thus:

North Latitude,

55° 55′ 42′′

be

West Longitude,

Oh. 12m. 42s.

Greenwich mean solar time of disappearance

1880, March, 17d. 12h. 23m. 55.88.,

as by the mean time clock of the Royal Observatory of Edinburgh. In order to compare this with the predicted motions, the moon's right ascension and declination were interpolated strictly from the hourly table in the "Nautical Almanac," and the star's place was taken from the "Elements of Occultations," while the parallax was computed on the supposition that the earth's equatorial is to the polar axis as 300 to 299. In this way the expected time was com puted to be 17d. 12h. 23m. 46.38., or 9.5s. earlier than the observed time.

There is no astronomical phenomenon more definite as to time than the occultation of a star, nor any perhaps more easily observed when the disappearance is against the dark edge of the moon. Provided that the telescope be sufficiently powerful to show the star, it is of little or no moment whether the definition be good, or even whether the instrument have been well adjusted to focus. In all cases the disappearance is instantaneous.

But, although the observation be thus satisfactory, there are various difficulties in the way of the calculations. In the first place, there is the error to which our lunar tables are liable; these tables, all founded on previous observation, are brought forward by an estimate of the laws and rate of change, and thus are unavoidably subject to a gradually increasing uncertainty. Wonderfully exact as these tables are, it would always be necessary, before drawing any exceedingly minute conclusion, to study the tabular error as obtainable from nearly contemporaneous observations on the

moon.

Next we have the possible error in the tabulated place of the star; an error, in the case of small stars, which is not to be

despised. The number of such stars is great, the number of observers small.

Next in order comes the size and shape of the earth. A relatively small difference in our position on the earth's surface makes a notable difference on the apparent position of the moon, and, in consequence, on the time of the occultation; even the height of the observer above the level of the sea has its influence. This may be well studied in the present instance; viewed from our latitude, the moon was seen to pass a little to the south of the planet Mars, whereas in the southern counties of England an occultation was seen. The oblateness of the earth has also to be taken into account. In the present instance calculations made as if the earth were spherical, would give the disappearance some eighteen seconds earlier than the above. Hence observations made on these phenomena from places in different latitudes afford a means for determining the earth's oblateness.

But, lastly, these observations are all deranged by the extreme jaggedness of the moon's edge. This jaggedness is well seen during an eclipse of the sun; it is also conspicuous against the disc of a planet. I recollect of witnessing an occultation of Saturn, some half a century ago, during which the corner of a lunar mountain was projected against the planet in such a way as to cut out a sector of about one-third of the surface. Irregularities of such magnitude cause serious variations in the times of disappearance and reappearance; and, for the purpose of estimating their possible extent, it might be useful to make concerted observations at places a few miles apart, so that the appulse may happen, here on the top of a lunar mountain, there in the hollow.

2. On Currents produced by Friction between Conducting Substances, and on a new form of Telephone Receiver. By James Blyth, M.A.

In former papers laid before this Society, I showed that when any two metals are rubbed against each other, a current of electricity is produced; and that this current agrees in direction with the thermo-electric current for the same two metals, and is greater. approximately at least, in proportion as the metals rubbed are far

apart on the thermo-electric scale,-the greatest current, as far as I have yet observed, being got from antimony and bismuth.

It is very difficult to decide as to the cause or causes of such currents. They may be (1) purely thermo-electric; (2) the currents, which are the supposed cause of friction; (3) currents produced by contact force between adhering films of air, moisture, or other substances with which the surfaces rubbed are tarnished; or (4) they may arise from all these causes combined. The following experiments were made in hopes of getting some information on these points.

My first experiment was to obtain the exact difference, as far as the production of a momentary current is concerned, between rubbing two pieces of metal together, and knocking the one against the other. For this purpose I repeated, with greater care, an experiment which I formerly described. It consisted in attaching a wire firmly to an ordinary hammer, and leading it to one of the terminals of a telephone circuit, while the wire from the other terminal was rigidly attached to a stiff bar of copper held vertically in a small table vice. When the face of the hammer was rubbed against the end of the copper bar, a very distinct grating noise was always heard in the receiving telephone; but the sound was almost inaudible when the bar was knocked by the hammer, if proper care were taken not to combine rubbing with knocking. This is, however, so difficult practically, that it is just possible that the sounds which I heard are due to faint rubs accompanying the knocking.

Should this not be the case, however, this difference of effect seems to show that the currents are not wholly, although they may be mainly, thermo-electric, as it is hard to believe that the heat produced at the junction of the surfaces by a smart blow can be less than that produced by a faint rub. Granting that the knocking is actually heard, it seems not unlikely that this effect may be due to the currents associated with rapid changes of form in matter. As has been remarked by Professor Tait (Proc. Roy. Soc. Edin. vol. ix. p. 552), these currents are such as would be capable of detection by the telephone.

In order to detect what effect, if any, the presence of the air had upon these friction currents, I employed the apparatus commonly called the electric egg. Having unscrewed the interior balls, I

fastened in their places two metallic strips, one of copper and the other of iron, so arranged that they could be made to rub against each other by moving the upper rod up and down in its air-tight socket. Before being fixed on, the metal surfaces were both well cleaned by scraping. When this apparatus was included in the circuit either of a galvanometer or telephone, no difference could be detected either in the deflection or the sound produced, by exhausting the air, as far as could be done, with an ordinary good air-pump. It is possible, however, that there may be films of air adhering to the metals which cannot be removed by pumping. Indeed, in the whole of this inquiry, the great difficulty is to be sure of what are the surfaces that are in contact.

Having ascertained that the current produced by the friction of antimony and bismuth is of some strength, and fairly constant when the friction is constant, I proceeded to make a small dynamo machine for producing currents on this principle. It consists of a cylinder of antimony 3 inches long and 2 inches in diameter, mounted on an axis which runs in centres. By a fly-wheel and band this cylinder is driven rapidly round against a plate of bismuth pressed tight against it by a stiff spring. Wires are led from the plate of bismuth and from one of the pivots on which the cylinder revolves to two binding screws, which form the electrodes of the machine. When this machine is included in the circuit with a microphone transmitter and a telephone, the current from it can be used for the transmission of musical sounds and even loud speaking. There is, however, heard along with the transmitted sound the noise arising from the friction of the antimony and bismuth. I have succeeded in transmitting, in this way, very distinctly, tunes played on a violin to which a microphone was attached. It is very curious, in this experiment, to hear so distinctly the music, notwithstanding the friction noise which accompanies it. It is to be noticed that the sound heard in the telephone of the rubbing of two pieces of metal together in a distant room is an effect precisely identical to this. In this case the rubbing produces the current, and the more or less loose contact of the metals acts as the microphone whereby the sound is transmitted through means of that current to the telephone.

I have also tried, with varying success, several other forms of this friction current-producer. In one of the most effective of these the