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object of the latter installation is not simply to make a register of world-shaking earthquakes, but to obtain a continuous record of changes in level. The water of Lake Victoria fluctuates in its height, and it is suspected that this may possibly be due to rock folding. If this is the case, then a seismograph, which can be adjusted to record small changes in level, may possibly yield information connected with the water supply of Egypt. Between Victoria and Cape Town there is a stretch of some 2,000 miles, in which, if two or more stations were established, records would be obtained of immense value to the seismic survey of the world which is now in progress.
Not only would they be of value as a means of extending our knowledge respecting the nature of the interior of the planet on which we live, but from time to time seismograms would yield information of immediate practical value to South African communities. Certain colonies have established seismographs because they furnish information as to the cause of a certain class of cable interruption. Cables may cease to work in consequence of the operations of an enemy, in consequence of sub-oceanic seismic disturbances, and for other
A community that can be assured of the reason why its communications with distant places have suddenly ceased, should certainly be less liable to anxiety and alarm than one without similar information.
For the East Coast of Africa in 12 years I find that out of 19 cable interruptions in of these have corresponded with unfelt earthquakes which were recorded at many stations in different parts of the world.
It cannot be said definitely that these earthquakes were the cause of the interruptions, but the fact that both occurred on the same days, and that it has so frequently happened that cables have been parted by sub-oceanic convulsions, makes it appear likely that in certain instances, at least, we may be dealing with causes and their effects.
On Jan. 31st of this year an earthquake off the Coast of Columbia, the effects of which found responses in the W. Indies, was responsible for the parting of 8 or 9 cables.
When one or two more earthquake observing stations have been established in Africa, the origins of these sub-oceanic catastrophes will be localized, and their relationship to the cable interruptions will be better understood.
Seismographs have been established at Potsdam and at several magnetic observatories in the United States, mainly because the records they yield throw light upon perturbations noted in certain magnetograms. At other observatories records of unfelt movements of the ground have explained accelerations and retardations in timekeepers, sudden displacements on barograms, and unusual movements of electrometers, the assayers' balance, and other instruments. In addition to changes of level, which are only appreciable after long intervals of time, a horizontal pendulum readily records changes which take place with comparative rapidity. Diurnal changes in level, which are chiefly noticeable in fine weather, have been recorded at a depth of 19 feet in sandstone. On the opposite sides of two valleys where instruments have been established, it is found that these daily movements take place simultaneously, but in opposite directions. During the day we may picture a valley opening, and at night as closing.
Bearing in mind that these diurnal movements are only marked on bright, sunny days, and are practically absent in dull, wet weather, we may seek for their explanation either as a general expansion of the ground under the influence of sun heat, or to a diurnal loading and unloading of a valley bottom relatively to that which takes place upon its sides.
The fact that these deflections of the pendulum may be observed in cellars and chambers excavated in rock where temperature is practically constant, precludes the idea that they are due to any local heating of the foundation on which the instrument may be installed. It, however, does not preclude the idea that there may be a general superficial warping of a district as an effect of solar radiation. It is, however, remarkable that this action should extend to the depths at which it has been observed.
That valley beds convey more load at night than they do during the day is suggested by various observations. Engineers have shewn that under normal conditions certain streams carry the most water at night time. This is also true of certain drains and wells. The causes leading to these conditions may be various. A nocturnal increase in the flow of sub-surface water may be attributed to the expansion of air in soil by the slowly-descending heat of the previous day, which forces interstriaal water into channels of easiest flow.
Another explanation rests on the fact that during the day evaporation and vegetable transpiration are at a maximum, whereby the flow of sub-surface water is diminished.
At night, with the cessation of these activities, the flow is relatively increased, and valleys receive their greatest load, with the result that their sides close inwards. To support the idea that water load plays an important part in the fluctuation of level we have the repeated observations that during wet weather, when we see water accumulating in the beds of valleys, the heeling over of the booms of horizontal pendulums is towards the loaded district.
The main point, however, to which I desire to draw attention is not so much the explanation of curious phenomena, but simply to the fact that Horizontal Pendulums may, under certain conditions, be influenced by hidden water loads. The investigations that this suggests should be of particular interest in certain parts of South Africa. For example, do the fluctuations of the subterranean water tapped in the Karoo affect the surface level? If they do, then the revelations of a pendulum may play an important part in the opening up and the settlement of a district.
Whether this type of seismograph will be used as an assistance in the prediction of weather is a matter worthy of some consideration. The Barometer gives the atmospheric pressure where it is installed. The Horizontal Pendulum, under certain conditions, swings to the
side, where pressure is at a maximum, or the side from which bad weather might be expected.
The most general and popular use of this instrument is that it yields records of all the great seismic disturbances in the world. From the character of a seismogram you can judge of the magnitude of the earthquake it represents. You can frequently say where it occurred, and when it occurred. This information is usually obtainable long before the arrival of cablegrams which, if they emanate from a devastated district, suffer not only delay, but may convey exaggerated and alarming impressions. Seismograms written by our earth have frequently extended, confirmed, or disproved telegrams written by man. To pressmen and the inquiring public seismograms have an increasing importance, and Africa, although it is the poorest earthquake-producing continent in the world, can with advantage to itself report upon disasters off and beyond its shores.
Large earthquakes are announcements of accelerations in hypogenic activity. When this ceases, and that which is epigenic becomes paramount, it would seem that the ultimate effects of surface denudation in general would be to reduce continents to sea level and to wipe out surfaces which are habitable. The panacea for such a fear rests in the idea that as mountains are washed down to load our sea boards, these may sink, whilst the high lands, which have been lightened, would be buoyed up. By such a process isostacy would save the features of our world.
HUMIDITY CHARTS OF THE UNITED STATES.
By KENNETH S. JOHNSON, HARVARD UNIVERSITY.
(Received through Professor R. de C. Ward, Harvard University,
and communicated by J. R. Sutton).
[ABSTRACT.] Loomis in 1880 * published a chart with relative humidity lines for a few stations east of the Rocky Mountains. There were but four such lines, and these were based on only one month's recordJanuary, 1875—but the purpose which Loomis had in mind was accomplished, namely, to show that on the east side of the Rocky Mountains there is a narrow belt of territory where the mean relative humidity is less than one-half.”
In 1902 the U.S. Weather Bureau published † three charts of relative humidity, one for the year, one for January, and one for July. The charts are incomplete in that they do not cover all the months. Moreover, they are based on records of varying length, ranging from 4 to 14 years. The present charts for all the months and for the year are based on data for the uniform period of 14 years, from 1888-1901 I In a few parts of the country, where the stations are scattered, records of slightly shorter periods have been used in determining the position of some of the lines, but in no case have the short period records been given equal weight with those of the 14-year period.
The curves are drawn for differences of 10 per cent. only; for the sake of clearness in presenting the main facts. Furthermore, the length of the records is not sufficient to warrant greater detail.
Among the most striking features shown on the charts is the uniform high relative humidity along the coasts, in contrast with which is the extremely low relative humidity in the S.W. interior, especially in Arizona, Nevada, S.E.California, and the adjacent districts. While the high relative humidity on the coast remains fairly constant throughout the year, the low in the interior basin (or, as it used to be called, the Great American Desert) becomes more marked as summer approaches, thereby increasing the already strong relative humidity-gradient between the Pacific coast and the interior.
Another interesting fact is the annual movement of the lines in the northern part of the interior basin. These travel north in summer, reaching the northern limit in June or July, returning south or perhaps entirely disappearing in winter.
Among other features worthy of note is the distorting effect of the Great Lakes, where the relative humidity tends to remain at or near 80%. an effect quite similar to that produced by the oceans. Also—a thing which rarely occurs—there is a prevailing north and south trend of the lines over the Great Plains in the vicinity of the 100th meridian, showing that the relative humidity there does not vary with the latitude.
* American Journal of Science, Third Series, Vol. XX., p. 22. + Report of Chief of Weather Bureau, 1901-2, p. 320. 1 Report of Chief of Weather Bureau, 1901-1902, p. 318.