was usually near to this point that fracturing occurred.

4. Experiments on the Vibration of Chimneys and Buildings.-Shortly after my fire Professors Tanakadate, Mano, and other Members of the Earthquake Committee to which I am attached took diagrams of the natural vibrations of the brick chimney stack which was left standing after my fire. The chimney is 18 feet in height, and has a rectangular section of 3 feet 8 inches by 3 feet 1 inch and two flues. With a rope and a windlass, a deflection of the top of the stack of one inch and a half was obtained, when the rope was suddenly released The result was that the chimney vibrated for about 20 seconds, and a record of these vibrations was obtained upon a band of paper. One of the diagrams is reproduced, fig. 19. The period of motion has apparently varied with the range of swing; for example

Range of Motion 1.25 in Period 1.7 second.

FIG. 19.

7 Seconds

·10,

When we remember that the greatest portion of the destruction occasioned by earthquakes is due to the fact that various portions of a building, in consequence of not synchronising in their movements, are mutually destructive, while solitary structures may be destroyed in consequence of the agreement between their natural period and that of an earthquake, it seems likely that observations like the one now described may lead to important rules being formulated for builders.

The next experiments will be on the vibration of wooden buildings.

5. The Earth Waves of Earthquakes.-On several occasions the apparatus described in the Report for 1893 has given diagrams showing the amount of tilting which accompanies certain earthquakes. The period of these angular displacements closely coincides with the periodicity of hori

zontal displacement, and its amount has varied between one and three minutes of arc. The original instrument has unfortunately been destroyed by fire.

6. Work at the Central Observatory under the Directorship of K. Kobayashi, Esq.--Athough records from the 968 outside observing stations have accumulated at a quicker rate than that at which they can be tabulated for analysis, many important improvements have been made in the working of the central station. In one room all instruments intended for country stations are tested. These instruments are simplified types of the Gray-Milne seismograph. One test is for the time-recording clocks, another for the clock driving the recording surface, and a third for the multiplication of the horizontal and vertical seismographs. Although several types of spring clocks have been tested, their rates are far from being satisfactory. Cheap pendulum clocks give good rates, but they are generally disturbed at the time of an earthquake. In another room an exceedingly large seismograph, without multiplication, is arranged to record severe motions. In a third room, which is alive with the ticking of clocks and chronometers which are at stated intervals compared with time signals from the Astronomical Observatory, there are four seismographs and various types of contact makers. A glass disc recording surface, on which the pointers for vertical and horizontal motion rest, is continuously in motion. The recording surfaces of the other instruments are set in motion by the contact makers, after which time intervals are marked upon them from a break circuit chronometer. Mr. N. Outska,. who is in charge of this department, finds that for horizontal and vertical contact makers having equal multiplication the former almost invariably closes its circuit before the latter.

APPENDIX.

On Causes producing Movements which may be mistaken for Earth

Tremors.

The following note refers to observations and experiments made at a small observing station which has recently been established at Shide, in the Isle of Wight. I reached Shide on July 30, and on the following day a pit was excavated in a dry stable, about 3 ft. 6 in. in depth, down to the upper surface of the disintegrated chalk.

On August 6 and 7 a brick pier, 6 feet in height and 1 ft. 6 in. square, was built on a concrete bed to rise freely in the pit. The necessary wooden covering for this was completed at noon on the 16th, and that evening an extremely light horizontal pendulum like R was installed and set to work. This instrument, which I call T, gave a beautifully defined two-line diagram until the 21st, when the clock ceased to drive the film, which had become damp and sticky. This was clearly due to moisture from the drying column being confined in the casing which covered its upper part and the instrument. To overcome the difficulty I placed inside the case two trays, each about 6 in. by 3 in., of calcium chloride. Immediately after this the pendulum commenced to swing, its range

sometimes reaching inch. This continued until the 25th, when, suspecting that the cause might be due to air currents resulting from rapid desiccation, I removed the calcium chloride, and the movements ceased. I have repeated the experiment several times, with the result that when the calcium chloride is introduced, movements are produced, which in the developed film have the appearance of a violent tremor storm, and when it is taken away the diagram is a clear straight line. We have here a cause of air currents which has not yet received serious attention. Another cause of movement, which is easily verified by experiments on a very light pendulum beneath a glass covering, may be due to the unequal heating of the surrounding walls. If portions of the tremors which have been recorded are due to causes, such as these which act within the casings, then it is understood why extremely small and light pendulums have shown more movement than those which are comparatively large and heavy. After my last twenty installations, and those which preceded them, the horizontal pendulums might be arranged according to their sensibilities as follows. The most sensitive are those with booms from one quarter to 3 or 4 inches in length, the next are those like R, and the one at Shide T, where the boom is of reed or straw about 2 ft. 6 in. in length, following which are booms about 5 feet in length of bamboo like A, and lastly, as the least sensitive, are the somewhat shorter and comparatively heavier booms of brass or aluminium used at the remaining stations. The only exceptions in the last group were the instruments E and F in the underground chamber, which recorded tremors almost equally as large as A. In this chamber, although there was but little appreciable daily change in temperature, the ventilation. was good, and therefore there may have been considerable changes in the hygrometric state of the atmosphere entering the covering cases, which were of wood resting upon a floor of asphalt. A difference in the rate at which moisture was absorbed or evaporated from the walls of this casing might possibly give rise to air currents. The five instruments in badly ventilated caves may have failed to show tremors, partly on account of their inertia, and partly perhaps because there was neither any sensible change in temperature nor in the dryness or wetness of the atmosphere. That tremors were practically absent from all the instruments on the surface, can only be attributed to their inertia, or to the fact that they were so well ventilated that no difference in temperature within their coverings was possible; but as the huts and casings were similar to that of R, the most probable explanation is the former.

The most difficult things which require explanation respecting earth tremors, assuming them to result from air currents due to differences in temperature or desiccation within the walls that inclose the instruments, are the facts that tremors have in all cases but one been most pronounced between 6 A.M. and 9 A.M., and during the night, and that they accompany certain meteorological conditions already formulated.

Before attempting these explanations it would be advisable to compare the movements of two light pendulums standing on the same column, one having walls varying in character, and the other, if possible, in

vacuum.

Earth Tremors.-Fifth Report of the Committee, consisting of Mr G. J. SYMONS, Mr. C. DAVISON (Secretary), Sir F. J. BRAMWELL, Professor G. H. DARWIN, Professor J. A. EWING, Dr. ISAAC ROBERTS, Mr. THOMAS GRAY, Sir JOHN EVANS, Professors J. PRESTWICH, E. HULL, G. A. LEBOUR, R. MELDOLA, and J. W. JUDD, Mr. M. WALTON BROWN, Mr. J. GLAISHER, Professor C. G. KNOTT, Professor J. H. POYNTING, Mr. HORACE DARWIN, and Dr. R. COPELAND, appointed for the Investigation of Earth Tremors in this Country. (Drawn up by the Secretary.)

APPENDIX.-Note on the History of the Horizontal and Bifilar Pendulums.

SINCE their last Report was presented, the Committee have purchased two bifilar pendulums from the Cambridge Scientific Instrument Company. These instruments are similar in most respects to the pendulum with which experiments were made in 1893 (Report, 1893, pp. 291–303), but several improvements have been introduced in order to correct one or two defects which those experiments brought to light. The changes made are described in the Report of 1894 (pp. 145-146, 158-160), and a detailed account of the new instrument is given in 'Nature,' vol. 1., 1894, pp. 246-249. Each pendulum is provided with a photographic recording apparatus. One of them has been erected on the old foundation in the cellar of the Secretary's house at Birmingham, but at too recent a date to allow any results to be included in this Report. It was intended that the second instrument should be placed in a building about threequarters of a mile to the east of the first, but it was afterwards found that the construction of the foundation might endanger the stability of the walls. Arrangements are accordingly being made for the erection of this instrument on another site in the neighbourhood of the former, and it is hoped that a comparison of the records of both may be completed before the next meeting of the Association. The second pendulum will then be available for use elsewhere.

The Committee recommend that they be reappointed, with the addition of Mr. G. F. Deacon.

APPENDIX.

Note on the History of the Horizontal and Bifilar Pendulums.
By C. DAVISON.

In previous Reports of this Committee, as well as in those of the Committee on the Lunar Disturbance of Gravity (1881-82), reference is made to the history of the horizontal and bifilar pendulums. A recent work by Mr. Claudius Kennedy contains an interesting chapter on this subject, and gives some additional facts which it seems desirable to embody in this note.

A Few Chapters in Astronomy (London, 1894), pp. 93–103.

In the following bibliography, the first date is that of the which, so far as known, the instrument was originally constructed.

6

year in

1. 1832. L. Hengeller: Phil. Mag.,' vol. xlvi., 1873, pp. 412-416. 2. 1851. A. Gerard: Edinburgh, New Phil. Journ.,' vol. lv., 1853, pp. 14-16; Kennedy, pp. 94-95.

3. 1862. Perrot: 'Paris, Acad. Sci. Compt. Rend.,' vol. liv., 1862, pp. 728-729, 851-852.

4. 1869. Rev. M. H. Close: Barrett and Brown's 'Practical Physics,' 1892, p. 241; Kennedy, p. 96.

5. 1869. F. Zöllner: Phil. Mag.,' vol. xliii., 1872, pp. 491–496. 6. 1871. C. Delaunay: 'Paris, Acad. Sci. Compt. Rend.,' vol. xcvii., 1883, p. 230.

7. 1879. Lord Kelvin : 1880-81, G. H. and H. Darwin: Brit. Assoc. Rep.,' 1881, pp. 93-112.

8. 1887-88. E. von Rebeur-Paschwitz: Nova Acta der ksl. Leop. Carol. Deutschen Akademie der Naturforscher,' Bd. lx., 1892, pp. 1-216;' Brit. Assoc. Rep.,' 1893, pp. 303-309.

9. 1892. J. Milne Brit. Assoc. Rep.,' 1892, pp. 107-109 ; 'Fed. Inst. Mining Eng. Trans.,' 1893, pp. 6-7; 'Seismol. Journ.,' vol. i. 1893, pp. 88-90.

·

10. 1893. H. Darwin: 'Brit. Assoc. Rep.,' 1893, pp. 291-299; 1894, pp. 145-146, 158-160; Nature,' vol. 1., 1894, pp. 246-249; Seismol. Journ.,' vol. iii., 1894, pp. 61-63.

In every case, I believe, except those numbered 8 and 10, the principle of the instrument was discovered independently. The horizontal pendulum has also been designed as a time-recorder for small disturbances by Professor J. Milne (Japan, Seismol. Soc. Trans.,' vol. iii., 1881, pp. 6162; 'Nature,' vol. xlii., 1890, p. 347); Professor T. C. Mendenhall (Amer. Journ. Sci.,' vol. xxxv., 1888, p. 105); and Professor G. Grablovitz ('Boll. della Soc. Seismol. Ital.,' vol. i., 1895, pp. 12-17).

All the different forms of horizontal and bifilar pendulums agree in one respect the vertical distance between their points of support is very great compared with the horizontal distance between them. In principle they merely differ in the method of suspension; and, according to this method, they may be grouped in the following three classes :

1. The pendulum in which the rod or mirror is suspended by two wires. These may be again subdivided: (a) The pendulums of Close and H. Darwin, and practically also of Delaunay, and Lord Kelvin and the Darwins, in which the centre of gravity of the rod or mirror lies between the two points of attachment of the suspending wires. (b) The pendulums of Hengeller, Perrot, and Zöllner, in which it lies outside them.

2. The pendulums of Gerard and Milne, on which the rod is supported by one wire and on one steel point.

3. The pendulum of von Rebeur-Paschwitz, which is supported on two steel points.'

In this class should be included Professor Ewing's horizontal pendulum seismograph, which, though designed for a different purpose, also records slow tilts of the ground (Encycl. Brit. vol. xxi. p. 628).