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marks 60. So that only at the instant of each hour are both breaks together united, and only then can a galvanic current pass. Each hourly current acts upon two electro-magnets. One is used at 1 h. P.M. only, to discharge the Greenwich time ball. The other has a far more extended use: by relay action it completes two other galvanic circuits, each giving hourly signals on a separate line of wire. One of these lines is in communication with the Central Postal Telegraph Office in Telegraph Street, London; the other passes to the London Bridge Station of the South-Eastern Railway. Hourly timesignals pass from the Royal Observatory along these lines day and night, and with the efficient performance of this duty the special responsibility of the Observatory terminates, the further distribution of the signals thus transmitted being under the control of other parties. [In speaking of "currents" and "signals," proper distinction cannot always be preserved, although, strictly speaking, by "current" we understand the unseen something which conventionally is supposed to pass along a wire; by "signal" the effect which the unseen current produces upon a telegraph needle or other indicator.]

The use made of the hourly signals in each of these lines must be considered separately. Taking that first mentioned, we proceed now to explain that in the Central Telegraph Office there is fixed an admirable and elaborate apparatus, designed by and constructed under the superintendence of Mr. C. F. Varley, engineer to the then existing Electric and International Telegraph Company, for the purpose, as he himself explains, "of sending exact Greenwich time simultaneously and automatically to numerous local and provincial stations." The whole collection of apparatus is known as the "Chronopher." It acts as a gigantic switch and relay, and by its means the one Greenwich current is transmitted on many different lines. These lines are, for convenience, disposed in two groups-one consisting of wires passing to points in London, the other of wires extending to distant parts of the kingdom, including such places as Manchester, Birmingham, &c. For the service of these groups of wires there are two relays-one the "local" relay for the London group, the other the "provincial" relay for the country group. On these relays only does the Greenwich time current act. In the local relay it causes a current to pass away simultaneously on each branch of the London group of wires; similarly in the provincial relay it causes a

In its simple form the relay is an instrument which, on receipt of a galvanic current-which may be from a distance-completes the circuit of a battery of its own, either for transmission of a signal to another station, or for the performance of some mechanical work.

current to pass away simultaneously on each branch of the provincial group. There is, however, this difference: the distribution in London takes place at every hour, the wires. being used for time-signal purposes only; but for the country, as it would at present be too expensive to employ special wires, those of the ordinary telegraphic service are used, general distribution of time on these wires being made once each day only, at 10 h. A.M. Every care is taken that this distribution shall be effective and certain. The chronopher includes arrangements by which (by means of a clock) the various provincial wires are, immediately before 10 h., automatically disconnected, each from its particular speaking instrument, and placed in communication with the chronopher commutator, in readiness for the Greenwich current. When it has passed and distributed time currents throughout the provincial lines, the ordinary connections of the lines are again automatically restored. And amongst minor arrangements there is one for preventing, almost entirely, interruption from accidental galvanic current arising, in any way, in the Greenwich wire. These 10 h. currents, being distributed along the principal lines of railway, give time daily through a large extent of country; they are used, to a considerable extent, for the direct correction of railway clocks, and indirectly rule them all, these clocks acting again each as a standard for the clocks of the neighbouring districts.

In some instances currents distributed from the Central Telegraph Office have been used for giving time to the public by public signal. In the year 1863 the River Tyne Commissioners promoted the establishment of two such signals, which have proved to be of great value. A 12-pounder gun at Newcastle and a 24-pounder gun at North Shields have, since the year mentioned, been fired daily at 1 h. P.M. Greenwich time. At London the chronopher, by automatic action, places the Newcastle wire for a short time in proper state to be acted on by the Greenwich 1h. current; at Newcastle proper changes of the speaking circuits are there also automatically made. As soon as at 1h. the seconds hand of the Greenwich normal clock marks 60, the relays at Greenwich, London, and Newcastle will each successively act, and galvanic currents will pass to the guns and fire them before the sound of the clock beat at Greenwich has well died away. The fuse used for the guns is Abel's chemical fuse, which explodes on passage through it of a galvanic current. Two fuses are inserted daily at each gun, to avoid failure should one of them by accident miss fire.*

* In taking time by the sound of a gun it is of course necessary to allow 4 seconds for each mile that the gun is distant from the observer.

It should be further mentioned, as respects the distribution of time in London, that hourly signals are received at the Post Office; and also at the Westminster clock, for facility of regulating it; the clock gives also return signals to Greenwich, by which its error is there known. As the clock is not controlled or in any way acted upon by galvanic current, it may deviate slightly from true time; but its deviations are small, and seldom amount to more than two seconds of time.

We have now to consider the use made of the hourly signals which pass from the Royal Observatory in the line terminating at the London Bridge Railway-station. These signals, with the exception of that at 1 h. P.M., are placed at the disposal of the South-Eastern Railway Company, who, in return, accord to the Royal Observatory the necessary wire communication at 1 h. for an important special purpose. Mr. Walker, to whom the establishment of these relations is due, distributes the signals received by him on the lines of the South-Eastern Railway, principally for regulation of the station clocks, &c. We have before us his Table of the Distribution of Time Signals, showing how extensively and regularly the daily work of distribution is done. Many signals are sent by hand, but some are accurate transmissions of the Greenwich current itself, and these are managed as follows:-At London Bridge a clock (one of those controlled by the Greenwich normal clock) acts the part of an automatic switch. At each hour it, as it were, turns some railway or other wire on to the Greenwich wire, so that at different hours the signal is transmitted in a different direction. The special service accorded to the Observatory at 1 h. consists in the giving the necessary wire communication for enabling the 1 h. current from Greenwich to pass directly to Deal, to discharge a time-signal ball (similar in principle to that at Greenwich) placed on the old semaphore tower. The Greenwich current, by relay action at Deal, drops the ball, which, whilst falling, makes such momentary changes of wire connections as causes a signal (the "return signal ") to be received at Greenwich, showing there that the ball has really been discharged. This ball was erected by authority and at the expense of the Admiralty, to give time to shipping in the Downs, and it has been in use since the year 1855.

The Deal ball signal is of special use as giving to masters of ships the means of obtaining the errors and approximate "sea rates" of their chronometers; the rates of chronometers often changing when placed on shipboard. Other such signals might be very usefully established at important points of our coasts. The Astronomer Royal has himself urged on the attention of the Government the desirability of establishing hourly timesignals at the Start Point, and the Ship-Owners' Association of

Liverpool have likewise made enquiries as to the facilities for exhibiting an hourly time-signal on the Tuskar Rock. These points, in the English and Irish Channels respectively, are both advantageous for such signals; but some special wire communications would in both cases be necessary, so that the first outlay might be rather large, otherwise there would be no practical difficulty.

We have now completed our account, which intentionally has been confined to the time-operations in connection with the Royal Observatory at Greenwich. In conclusion we may however remark, that the local observatories in Liverpool, Edinburgh, and Glasgow have also for many years done much to give authoritative Greenwich time in their respective surrounding districts. In Glasgow there is a most extensive system of controlled public clocks, and Edinburgh and Liverpool both possess time guns.

THE ECHINUS, OR SEA-URCHIN.

BY ST. GEORGE MIVART, F.R.S.
[PLATE LXV.]

THE

HE Englishman, unacquainted with natural history, who for the first time visits Marseilles will, when he wanders down to its busy port, most probably have his curiosity awakened by baskets full of dark, round, spiny bodies (disclosing deep yellow parts within), each about the size of an egg. These are the sea-urchins, sea-eggs, or echini, which are largely affected by the good folks of Marseilles, and constitute one of the many objects of their fish-market which interest, surprise, or disgust the northerner on his first arrival on the Mediterranean shore.

A large specimen of echinus well indeed merits its name of sea-urchin, for externally it presents an amazing resemblance to a rolled-up hedgehog, or urchin, being covered over with spines which in size and general appearance are very singularly like those of the last-named animal.

The resemblance, however, between these two animals is of the most superficial character only, and two creatures more really distinct could hardly be selected from the whole animal kingdom.

The sea-urchin presents us with a singular mixture of great simplicity of structure united with very great complexity. It is indeed an animal formed on a very low type, which, while strictly preserving that low type, yet preserves it in a wonderfully ornate condition with a quite prodigious number of complications and adornments.

The creature, when deprived of its spines, presents the appearance of a spheroidal, melon-shaped body (the so-called shell), furnished with two poles (each being provided with an aperture), and with lines, like meridians, running from the vicinity of one pole nearly to the opposite one. The shell when thus stripped is seen to be formed of a multitude of parts, to be covered with small rounded prominences, or tubercles, and to be perforated by a vast number of minute holes, or foramina.

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