Page images
PDF
EPUB

(a) The Instruments.

Although pendulums made of pieces of aluminium wire and held up with quartz fibres with their mirrors and lenses have given excellent results, the apparatus required good installation and careful manipulation. As these requirements were not obtainable, excepting in Tokio, before commencing observations in the country my first task was to design an instrument of a simple character, not easily put out of order, and which would give continuous records for at least one week. This was done, and as the six instruments which have been made have worked satisfactorily I give the following description of one of them.

[merged small][graphic][subsumed][merged small][subsumed]

The pendulum stand A, fig. 1, with its upright, which is 50 cm. high, is of one piece of cast iron.' The distance between the levelling screws working in brass sockets is 23 cm. The back screw tilts the upright and gives the required degree of stability to the pendulum; one of the lateral screws is used in adjusting and calibrating the pendulum. It carries a pointer moving over a graduated arc. By turning it, for example, one degree, which means raising this corner of the instrument 30 of a millimetre (the

The form of the bed-plate is that of a right-angled triangle with the right angle near A.

screws having a millimetre pitch), the corresponding deflection of the pendulum may be noted. The turning is done by a lever projecting from the head of the screw.

The boom of the pendulum is an aluminium tube 4 feet (120 cm.) in length, carrying a sliding weight, W, and a movable point to which the supporting tie can be attached. This tie, which is of thin brass wire, at its upper end terminates with about an inch of untwisted silk. On the inner end of the boom there is a quartz cup which bears on a steel needle projecting slightly upwards from the base of the cast-iron stand. The suggestion that the needle should project from the stand rather than from the boom is due to Dr. von Rebeur-Paschwitz. It gets over the difficulty of having anything which may be markedly magnetic in motion; and secondly, in case of violent disturbance, the relative verticality of the points of support is less liable to alteration.

The instrument is adjusted so that the needle bears normally on the centre of the quartz cup, or so that the centre of gravity of the system falls about G.

At the outer end of the boom a stiff wire rises vertically upwards. Clamped to this at the required height is a horizontal wire 15 cm. long, carrying a thin zinc plate p, measuring 6 cm. by 10 cm. In the centre of this, and parallel to the length of the boom, there is a slit about 0.5 mm. broad and 2 cm. long. As the boom moves to the right and left, this slit floats over a second slit about 5 cm. long in the lid of the box covering the drum which carries the recording paper. These two slits are at right angles to each other, so that the light from a lamp reflected downwards by a plane mirror only reaches the drum as a spot.

A well-defined spot, which means a clear, sharp line on the photographic film, can be obtained without fine adjustment. That is to say, the distance between the film and the slit, or between the stationary and moving slits, may be anything between 1 and 5 mm. Projecting an inch or so beyond the moving plate and attached to it is a pointer moving over a scale fixed on the cover of the box containing the clock of the recording drum. This can be inspected and the position of the boom at any time noted by looking through the glass plate at m.

The recording drum, on which the photograph-paper is fixed with a spring clamp, as in a recording barometer, is of thin sheet brass 5 cm. wide and 105 cm. in circumference (some are much less). It is turned at the rate of 15 cm. per 24 hours, and a film therefore lasts one week.

The clocks, which are an American type intended to run 8 days, have fitted to the slowest moving arbour four wheels, the last of which turns a disc with slots round its edges once a week. The recording drum, which can be dropped into its bearings, carries a large crank, When in position the clock is slid in a groove until one of the slots catches the outer end of the crank arm; after this the cover is put over the clock and drum, and the whole is pushed on grooves into the end of the case covering the pendulum.

Hollow wooden drums, which are easily driven by the clock-work, have a tendency to warp, and this may result in a want of uniformity in the motion.

Brass drums in the damp atmosphere of a cave in a month or so tend to rust, and this rust may act upon the photographic film to such an extent as to render it illegible.

Up to the present time ordinary kerosene lamps have been used, but

as they require attention at intervals of from 8 to 12 hours they are being replaced by lamps such as are used in magnetic observations burning benzine.

Every day from 12 noon to 1 P.M. the lamps are removed and a reading is taken, so that time intervals are marked on the photographs and scale values are obtained.

It does not seem necessary that the boom should be made of aluminium, as I obtain what appear to be equally satisfactory records with booms of brass or even wood. The most delicate pendulum I have has a boom made of varnished bamboo with brass fittings. It is about 5 feet in length, and when last rated had a period of 55 seconds. I say last rated because I find that this pendulum, like all others I work with, changes its period, and therefore its sensitiveness, from week to week. I notice that this source of error when computing results is also found in the infinitely better constructed and better installed apparatus used by Dr. von Rebeur-Paschwitz. When the pendulum has its 55-second period one millimetre deflection on the photographic plate is equivalent to a tilt of 0.08 second of arc. With this degree of sensitiveness a 14 lb. weight placed on the column, which is old and massive, at a distance of 2 feet from the instrument causes a deflection of 0.5 mm. My weight on the floor at the outer end of the boom produces no visible effect.

In this condition the pendulum is, however, often too sensitive, as it will, from time to time, wander an inch or so to the right or left of its mean position, and the spot of light fall outside the film. A sensitiveness of 1 mm. motion per 0-5 arc is usually quite sufficient, and I do not think that apparatus like those of Wolf, d'Abbadie, Darwin, or von RebeurPaschwitz capable of recording tilting of from tog of a second could be used on the alluvium of Tokio even when installed on a concrete bed underground.

Such apparatus might, however, be used on the solid rock which crops out round the Tokio plain.

An attempt to test the accuracy of one of the horizontal pendulums was made by placing it on an iron plate resting on a plank 18 in. broad, 1in. thick, which in turn rested on supports near its end 6 feet apart. It was then adjusted, so that trials with the test screw indicated that turns of 10° gave an average deflection of 11.5 mm.

Side by side with the pendulum a transit instrument having a good telescope was placed, and this read on a scale fixed on a brick wall at a distance of 720 feet. The supporting plank was then loaded at its middle until the telescope showed a deflection of 14 in. on the scale and the pointer of the pendulum moved 93 mm. From this it seems that the pendulum indicated a tilt of 1 in 562, while the angular tilt of the telescope was 1 in 616.

These are the means of a series of experiments, and assuming that the readings through the telescope were correct, then the pendulum indications are about 10 per cent. below their true values. On the other hand, assuming that the readings through the telescope were one inch too small, and it was difficult to read within that quantity, then the pendulum indications are 2-3 per cent. short of their true value. A great source of error no doubt resides in the test screw of the pendulum.

The instrument described will be recognised by those engaged in similar investigations as coarse in construction, roughly approximate in its records, and because it is large as being in all probability subject to

convection currents, unequal heating in its parts, and other interferences. In spite of these objections, I find it satisfactory. It is cheap, easily put up to read from 1" to 0"-5, easily worked, while the light is near the film, and therefore in the best position to use with ordinary bromide paper.

With more delicate apparatus, on the Tokio plain at least, no matter what the size of the foundations might be, the results of my experiments show that such instruments continually require readjustment in order to keep the light on a film of manageable breadth, while if installed on the rocks in the mountains I fancy that, owing to earthquakes or the gradual yielding of the column, there would be a constant change in the meaning of the deflections. In two of my pendulums I notice that sometimes they gradually become more sensitive and sometimes less sensitive.

The columns I am using underground are of brick, 2 ft. high and 2 ft. square, put together with pure cement. On the top of these, at first, I placed a slab of marble; but because I noticed that in a damp atinosphere there was a marked chemical action taking place between the brass screws and the stone on which they rested, the marble has been replaced by slate.

(b) Observations at Kamakura.

Kamakura, one of the ancient capitals of Japan, lies on the western side of the Miura Peninsula, facing the Pacific Ocean. On account of its ancient temples and its enormous bronze Buddha it is visited by almost all travellers to Japan. The geologist has an interest in visiting this place, as it has been the site of a series of earthquakes, which, with their accompanying sea waves on more than one occasion, are said to have laid waste a city of a million people. The place is prettily situated amongst Tertiary hills which rise to heights of from 100 to 600 feet around the plain on which the ancient capital stood. The cliff-like faces of these hills show a series of conformable beds dipping about 30° N.E. These beds, which are soft grey coloured clay stones or beds of consolidated ashes, are from a few inches to a few feet in thickness, and are traversed by numerous small faults the throw of which, so far as I have observed, does not exceed six feet. Near to the temples, caves have been excavated, which are used as shrines; while similar but smaller caves have been made by farmers, and are used as storehouses.

The general relationship of the strata at Kamakura to the alluvium and diluvium overlying tuff at Yokohama and Tokio is shown in the accompanying section.

[blocks in formation]

At Kamakura the tuffs are crushed and faulted, but before reaching Yokohama they pass into gentle folds, and then become horizontal and are capped with some fifty feet of reddish earth and gravel. This condition, with the exception that the overburden is perhaps 100 feet in thickness, continues up to Tokio.

At Yokohama the tuffs, which almost entirely consist of a light grey

coloured clay rock, are visible as cliffs from 50 to 80 feet in height. In Tokio, however, they only crop out at one or two places at the bottom of deep cuttings. The depressions in the section represent the flood plains of rivers which are filled with soft alluvium.

Rather than working on strata which had been so far crushed and crumpled that further yielding is hardly to be expected, I should have preferred a site on the strata which are gently folded, and where a measurable amount of yielding may yet be in operation.

Although I had the choice of several caves as Kamakura, all of them were situated at some distance from the railway. To go and return from the one I selected took six hours, and it was therefore seldom that it was visited more than once a week. Very fortunately I received assistance from Mr. P. E. Heerman, a gentleman who happened to be staying in the neighbourhood, while one of the officials from the railway station kept the lamps burning, and three times a day took readings of the instruments. That the latter was attended to regularly was shown by a slight change in the intensity of the photographic trace at the times when the lamps were adjusted, a gap when they were removed to be refilled and a slight notch in the diagram from a self-recording thermometer at the times when the doors of the cave were opened, and the times at which these various marks were made coincided with the times at which the readings were noted as having been made.

The cave seems to have been excavated on the line of a fault which, curiously enough, is with difficulty recognisable on the face of the cliff itself, but which is quite apparent in a photograph. The entrance to the cave, which faces S.E., was, with the exception of the door, blocked up with a wooden wall faced on the outside with a bank of earth and rubble work 4 feet in thickness. The dimensions of the cave were 20 feet by 20 feet, with a height of from 7 to 10 feet. One corner of this was partitioned off with wooden walls to form a room 10 feet square, and from this the débris was cleared out to reach the solid rock on which two brick platforms were built.

These were one brick thick and laid with pure cement. On the end of each of these platforms, which were at right angles to each other—one running N.W. and the other N.E.- a small pillar three bricks high and one brick square was built and capped with a slab of marble. These were finished on January 7, and the cave was left open for one week to facilitate the drying. At the end of that time, on January 14, as the cement appeared to have set, the instruments were placed on the slabs and the records commenced. From that date, with but few interruptions continuous photographic traces were obtained until March 18. These machines I have called C and D. Machine C recorded tilting parallel to the strike, while D recorded movements parallel to the dip. By a lifting on the S.E. side the readings of the index attached to C increased in value, while the readings of D increased with a lifting on the S.W. side.

At the end of each week, when a photographic film was renewed, the sensitiveness of the instrument was determined, after which it was readjusted. These determinations are given in the following table. The ratio of unity to the numbers in the first two columns is the tangent of the angle through which the instrument would have to be tilted to produce a deflection on the photographic trace of one millimetre ; the corresponding angles in seconds of arc are given in the third and fourth columns. At the commencement it will be observed that to produce a deflection of one

« PreviousContinue »