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degree in a hundred feet. It was observed with some astonishment by the resident engineer, that for a long distance there appeared no change of temperature, insomuch that he almost ceased to take interest in the observations. During this time, however, the work was progressing under a plateau varying in height from two to three thousand feet above the tunnel and extending for a distance of 13,000 yards from the entrance without much change in elevat ion.* Shortly afterwards, when the plateau had been passed and when the mountain side rises rapidly, the obsersrations showed a gradual rise, but they have not been reduced and are not at present accessible. It will be important and interesting to know if they show a rise in direct proportion to the increase of height above the tunnel. On the French side the largest spring was tapped at a temperature of about 62° with a superincumbent mass of about 2,000 feet of rock.
A good series of temperature observations on the northern side of the tunnel would have had extreme interest and great value if they had been carried on during the construction with good instruments and due precautions. It is to be feared that this enquiry was to some extent neglected. The holes were bored, and still remain; but it is believed that the temperature of the rock may be to some degree affected by the altered temperature of the tunnel when the works are completed and ventilation is ensured from one end to the other.
There is a difference of level between the north and south ends of the tunnel, the total difference being 435 feet. The Italian end is nearly level, the slope being chiefly on the other side. The contour of the mountain will be understood by reference to the annexed section, but it would be difficult for anyone who had walked over the ground to admit that the grand abrupt mountain pass from Modana and Bardonneche could be so apparently tame and regular as is there represented. The reader will, however, see that it fully exemplifies the above remarks.
The perforation of the Alps under Mont Frejus has been throughout a remarkably simple and easy operation. There have been no drawbacks of the smallest importance in an engineering sense, and the work has been carried on steadily from the commencement. There has been but a small proportion of hard, tough rock, no loose, treacherous shales, no influx of water, no crushing in of the roof, and no rise of the floor. It is not easy to imagine a more complete instance of plain sailing, or a great work less interrupted by natural or unexpected difficulties. The mere magnitude and novelty of the under
» This will be at once seen by reference to the section iu Plate LXIY., which is drawn to the same scale for vertical and horizontal distances.
taking, and the difficulties anticipated in obtaining fresh air, rendered it to a certain extent a hazardous and speculative matter at one time; but the successful adaptation of the apparatus for boring, by machinery driven by compressed air and carried into the tunnel in such a manner as to be ultimately applied by elastic tubes, completely settled all doubts, and renders it as easy to bore for ten miles, retaining good ventilation, as it had previously been to drive a level for a hundred yards. The geological and physical questions involved were not, however, at first considered, and have been to some extent neglected; but specimens of the rocks cut through have been preserved from the first, and two or three collections of this kind are available. One of these has been taken to Paris by Professor Sismonda, and forms the subject of a description by M. Elie de Beaumont, published in the Comptes-rendAis of the Academie des Sciences for the 4th July last. Other but less complete collections, with many duplicates, exist in the offices of the resident engineers at Modana and Bardonneche respectively. Both are in the highest degree interesting, and an inspection of them has greatly assisted the author in preparing this article. He has also to express his acknowledgements to the engineers for their kindness in placing before him the results of such observations as they have made on the various subjects alluded to.
It should be known that the present is a very favourable moment for examining the works and their geological results; although the inside of the tunnel is not easily accessible, owing to the great activity with which the works there are being pushed. There is no cessation, night or day, from one year's end to another, the only holidays in the year being on the occasion of the great festivals of the Church. To ensure the greatest activity and energy a premium is given to all hands employed in proportion to the rate of progress, and sometimes it has been found necessary to check the extreme eagerness to get on lest accidents should occur. The accidents hitherto have been very few and slight, and the general conduct of the works reflects the greatest credit on all concerned. The study of the rocks in the neighbourhood, and their comparison with the fragments brought out from the end of the tunnel, will afford ample occupation to the geologist for many days, and will not fail to render him familiar with some of the most interesting results of metamorphic action. The total absence of plutonic rock in the district near the tunnel will not fail also to attract his attention.
The whole group of rocks alluded to in this article belong, as has been already pointed out, to the middle part of the secondary system, from the middle oolite to the Khsetic inclu
VOL. IX. NO. XXXVII. A A
sive. They are called by Italian geologists the Rocce Antracitifere delle Alpi, and under this name form a connected group of" enormous thickness, repeated in this part of the Alps two or three times by anticlinal and synclinal axes, and generally very highly inclined. These are shut in on both sides by granite and protogine, sienite and diorite, and occasionally penetrated by these latter rocks and by a variety of serpentine and euphotide. The magnesian character of these rocks will be at once recognised, and is seen also in the talcose and steatitic nature of the metamorphosed schist*. Although the tunnel does not cut through any of the intrusive rocks, and does not seem even to approach them, it shows very clearly the presence of magnesia as connected with the metamorphosis. This is another matter especially and locally interesting. The great abundance of gypsum, the same in character and appearance in the tunnel and on the surface, is another point to be observed. We leave these matters to the careful study of geologists.
EXPLANATION OF PLATE LXIV.
Fio. 1. Section from N. 14° W. to S. 14° E. on the line of the great tunnel from the mountains on the northern side of the Arc valley, in Savoy, to the mountains between the lateral valley of Rochemotte, to the valley of the Dora, in Pie'mont. This section is carefully drawn to the same scale of vertical and horizontal distances.
FlQ. 2. Plan of the country immediately adjacent to the tunnel, showing
the position of the principal valleys on each side the central axis.
N.B.—The following are the rocks as identiSed by Professor
Sismonda, and mot with in the tunnel commencing with the
north or French entrance.
1. Upper member, consisting of schists,
clay series 4,500 „ 7,707 „
2. Middle member. Gypsums, crystal-
corresponding to lower oolites . . 1,700 „ 2,815 „
3. Lower member. Calcareous schist
lihsetic series 17,500 „ 29,151 „
GREENWICH TIME AND ITS TELEGRAPHIC
By WILLIAM ELLIS, F.R.A.S., Superintendent Op The Time
THE object of our present paper is to describe that system by which Greenwich time, as found by astronomical observation at the Royal Observatory at Greenwich, is daily transmitted, by telegraphic aid, to distant parts of the kingdom; a subsidiary use of the telegraph which, although not directly contemplated in its establishment, yields practical advantages of no small value.
But before directly proceeding to consider its utilitarian applications we must give some account of the manner of reckoning and determining time. The astronomical considerations involved are, however, so fully treated in works on astronomy that we need only concern ourselves here with the more practical aspect of the subject.
When the sun reaches its greatest daily altitude in the heavens we call the time noon, and the interval which elapses between one noon and the next we call a solar day. But the natural solar day thus measured is (owing to the varying motion of the earth in its elliptic orbit, and the inclination of its axis of revolution to the same orbit) to a slight extent variable. Its length oscillates between certain small limits, which renders the ordinary use of such a day for many reasons inconvenient. The inequality is fortunately small as compared with the length of the day, so that its use in practice is avoided by assuming the existence of an artificial solar day—one of uniform length, and consequently better adapted to the wants of mankind. It is known as the mean solar day. Natural solar time (that shown by a sun-dial and variously called "true" or " apparent" solar time) is sometimes rather before and sometimes rather after mean solar time (that shown by a clock). Four times in each year they are together. The difference usually existing between them, which amounts to as much as 16 minutes in the month of November, is the "equation of time" of our almanacs. Its amount for each day at noon to the nearest second of time is contained in common almanacs usually under the heading either of " clock before sun" or "clock after sun :" for greater accuracy reference must be made to the Nautical Almanac.
Having shown the relation existing between apparent or sun-dial time and mean solar or clock time, we see how it is that, taking time from a sun-dial and allowing for the equation of time, ordinary clock time is obtained. But a sun-dial is useless for any accurate determination; and of other instruments and methods, giving something more of accuracy, space will not allow us to speak. We must hasten to describe that special instrument, the "transit instrument," which is always employed in fixed astronomical observatories. This instrument consists of a telescope fixed at its centre to a cross axis supported at the extremities on bearings firmly fixed in an east and west position, so that on turning the telescope on its axis it points successively to all parts of the meridian (that imaginary great circle in the heavens which corresponds to the brazen meridian of a celestial globe, and at which the heavenly bodies attain, between rising and setting, their greatest altitude). In order that it may do this precisely, the line of sight of the telescope must be at right angles exactly to the cross axis, and the axis itself must be truly level and also precisely east and west; but no instrument, if placed in exact position, will long remain so. It is therefore usual to register its small deviations, and apply corrections as necessary to the observations. The instrument made use of at Greenwich is the one meridian instrument of the Observatory, the noble transit-circle (designed by the present Astronomer Royal). Such an instrument is used for many purposes besides the determination of time, but it is with this use of it only that we have to do here. On looking into its telescope we see a number of delicate vertical threads across which objects must pass in their transit through the field. The centre thread represents the meridian, the others being uniformly distributed, an equal number on each side. The time at which, by the sidereal clock (always that employed in an observatory), the object to be observed is upon each thread being noted, the mean of the observed times gives a more accurate value of the meridian transit. Usually an observer counts the beats (seconds) of his clock, and estimates the time at which the object is on each thread; but at Greenwich this method is no longer pursued, for by means of the chronograph (brought into use in the year 1854) all transits are registered by galvanism. Of this instrument we cannot here attempt description further than to say, that by its means