An inspection of this table shows that, although the pendulums A and J were separated by a distance of about 270 yards, and were differently installed, there has been a general agreement in the direction of their displacements, and that this is particularly marked when the movements of A were decided. When there has been disagreement in the direction of motion, the movements of A have usually been small, and it is not unlikely that such discrepancies would disappear if a time had been taken to represent the daily mean position farther removed from the hour at which the western movement of the diurnal wave commences. The changes indicated in the table are also shown diagrammatically in conjunction with a curve showing the fluctuations of the barometer (p. 129).

What is true for A and J is generally true for J and K. Although between October 13 and November 20 there are six decided barometrical fluctuations, and during the same interval there are six decided westerly movements of J, the crests and depressions of these diagrams do not retain the same relative position. For example, on October 17 the barometrical crest corresponds to the westerly extension of J, while on November 5 a similar movement of J corresponds to a barometrical depression. Although at times it appears as if there were a close relationship between barometrical fluctuation and the movements of the pendulums, the diagram (fig. 4) indicates that, although both phenomena are nearly identical in having a periodicity of between two and seven days, it does not show that they are absolutely synchronous. It seems that the instruments at J and K, which were within six feet of exposed ground, with or after rain moved westwards, but equally large westerly motions have occurred without rain.

g. On the Diurnal Wave.

In considering the results towards which the observations of the daily wave point, it is necessary to consider the observations made during the past year in conjunction with those described in the Report for 1893-94.

When instruments were installed upon the rock in caves as at Kamakura (C and D), at Kanagawa (I), and in Yokohama (G and H) the daily wave was not perceptible. This by no means precludes the possibility of

its existence in such places, and had instruments of greater sensibility been employed it is likely that it might have been detected. It was perceptible and often measurable, but by no means pronounced in the records from my house (A), where the instrument was well founded and in an east and a west direction, well protected from temperature effects upon the surrounding soil. It must not be forgotten that this instrument was the most sensitive, being capable of recording changes of 01. Had the sensibility of this instrument not exceeded that of other instruments less favourably installed, it is doubtful whether it would have shown any marked trace of the daily wave.

Two instruments in an underground chamber (E and F), where as in the caves the daily change in temperature did not exceed 1° C., often showed the daily wave in a marked manner, but it was not so great as it was at stations J and K upon the surface. The conclusion to which these observations lead is that the daily wave is not due to fluctuations in temperature immediately near to the instruments, but that it is a surface phenomenon which penetrates to a depth of at least 12 feet in the alluvium.

An instrument upon the surface (J) the ground round which was exposed to the sun upon all sides excepting the east, and another (K) which was exposed on all sides excepting the west, showed large diurnal waves, and notwithstanding the fact that between these two stations there was a pond and a grove of tall trees, the pendulums usually moved in the same direction at about the same time. The magnitude of the movements was different, but with this exception the only other difference was that J, the open ground round which was exposed to the afternoon sun for one or two hours longer than the open ground round K, continued its westerly motion for one or two hours longer than K (figs. 7 and 8).

An experiment made at J was to dig a trench 5 feet in depth round the south and west sides of the hut. This did not appear in any way to affect the amplitude of the daily wave, but it seemed to increase the suddenness with which the westerly displacement commenced, and at the same time the number of hours occupied in making a complete wave was reduced. With an instrument at O, a few yards from J, which recorded north and south motions, the wave was regular but of small amplitude, the northern movement coinciding with the western motion of J and K. The direction of maximum tilting may therefore have approximately been W.N.W. and E.S.E. On the western side of a plateau facing the eastern slope of the plateau on which A, J, K, and O were situated, an instrument N showed a daily wave, but the westerly excursion of the pendulum was completed only a few hours later than the easterly excursion was completed by those upon the opposite hill. It appeared as if the two bluffs, or at least the trees upon them, inclined towards each other, and then away from each other once in twenty-four hours. Between the two bluffs there is an open valley, in which there is a lake or pond nearly half a mile in breadth.

There

That the records at N were small may be attributed partly to the fact that the instrument never had given to it any great degree of sensitiveness, and partly to the fact that on all sides excepting the west the ground immediately round the instrument was well shaded by tall trees. is, however, a large open space about 100 yards to the east of this station. At another station P, on the eastern side of the plateau, on which N was situated, and at a distance from it of about 200 yards, the movements,

especially upon frosty nights, were extremely erratic, and they may probably have been produced by the freezing of the ground. When the cold was not great the movements were small, and here, again, the ground around the instrument was so well shaded by trees that after a snowstorm it would take from ten to fifteen days to melt away the snow, which at other places disappeared in one or two days. At Q, R, S, and O the movement during the day was towards the side on which the ground ear to the instruments was most exposed to the sun.

One very important observation especially marked at stations where large diurnal waves were recorded was that on rainy or cloudy days these instruments were steady, and no diurnal wave was recorded.

The general conclusion to which these observations on the diurnal wave point is that on the alluvium on open ground, the daily wave is most pronounced on the surface, it is less in amplitude, but it may be decided at a depth of 12 feet; while on a massive foundation the ground round which is well protected by a building or trees the wave is slight. Deep underground on a rock foundation with instruments such as I have had at my disposal it is not perceptible, but it is not improbable that a residual effect of the surface motion might be detected with more delicate apparatus. The cause of the motion is not any immediate effect of temperature upon the instruments, nor if we except the case where actual freezing of moisture in the ground round and possibly inside one of the huts took place does it appear to be due to expansion or contraction in or near the foundations accompanying the acquisition or withdrawal of heat.

The most active cause producing the movement which takes place during the day may be the fact that the ground on different sides of an instrument is unequally exposed to effects producing evaporation. The retrograde motion during the night, which is smaller and more gentle than that which has happened during the day, may be due to the unequal condensation of moisture on two sides of a station.

1. Effects accompanying Evaporation (Daylight Effect). As the side of a station from which most moisture is withdrawn to be dissipated in the atmosphere has been relieved of a load, we should expect it to rise, and this effect ought, in alluvium, to be perceptible to some depth. The same area, because it is contracting like a drying sponge, may sink, but this would be a superficial action.

On open ground, under favourable circumstances, the load taken away from a surface of earth by evaporation may amount to 4 or 5 lb. per square yard, or from an area 20 yards square, about 1 ton. Experiment has shown that 2 tons of water taken out of a well and run off down a hill will cause a pendulum at a distance of 20 or 30 yards to behave as if the ground upon the well side had risen. If these premises are correct, then an instrument well surrounded by trees or buildings, because the evaporation is slight and is not likely to be much more marked upon one side than it is upon another, should show but little motion. A pendulum at a station freely and uniformly exposed upon all sides should also show but little change. During the morning a north-south pendulum would be expected to move slightly towards the west. For some hours after the sun's meridian passage there would be a pause in such displacement, after which a retrograde motion would set in. An instrument with open ground upon its eastern side would, during the morning, be expected to move westwards; while at the same time another instrument, with the western side as an evaporation area, would move eastwards. It

A.-Moves west from 3 A.M or 6 A.M. to 3 г.M., or from noon to 9 P.M., or midnight. 3 Noon 21 18

21 18

15

12

9 6

Nov 6th 1894

15 12

Ꮽ

6

3

0

is not likely that pronounced movements would be recorded upon rock, neither should there be an appreciable trace of diurnal waves on days when it was wet or cloudy.

2. Contraction due to Desiccation. -If the desiccation due to heating by the sun is followed by contraction, we should expect that during the day a pendulum would move towards the side losing the greatest amount of moisture; that is to say, its movement would be in a direction opposite to that accompanying the removal of a load due to evaporation. On ground covered by trees or buildings, or on ground uniformly open all round, but little motion should be expected. Whatever effect was observed it is not likely that it would penetrate many inches beneath the surface. The following is a comparison of these considerations, with the observed movements of the various pendulums and the character of the surrounding ground.

For 100 yards to the east and west of A the ground is equally open. The most open ground, however, lies to the east. It would therefore be expected that movement during the day due to unloading would be westwards. The observed movements, however, although generally westwards, showed too many irregularities, and were too feeble to justify a conclusion that they were due to such an influence (fig. 6).

For 100 yards round station J the ground is more open upon the western side than upon the eastern side, and the westerly motion might therefore be attributed to desiccation and contraction upon this side. Beyond this limit, however, the ground is most open upon the eastern side, which might therefore, by evaporation, rise. This would give a westerly motion (fig. 7).

FIG. 8.

K.-Moves west from 6 A.M. to 3 or 6 P.M.

21 18 15 12 9 6 3 Noon 21 18 15 12 9 6 3 0

For 100 yards round K the ground is most open upon the eastern side, and desiccation would result in an eastern displacement. The westerly motion recorded seems to find its only explanation in the fact that the eastern side of the instrument is more open than the western side, but the reason that these movements were greater than those at J is not clear (fig. 8).

Immediately to the west of R there are tall trees and a deep cutting. So long as the sun shines over these trees upon an area 50 or 100 yards long to the east of the instrument, the pendulum moves towards the area