Imagens das páginas

tion fair, with boiling. * * Plato is a grand and striking sight. Tint of floor medium. More than half the floor in shadow. Terminator just including the W. rim. The rim of the crater on the N. exterior slope finely seen. In three parts the rim appeared broken down to level of floor-close to m, opposite to c, and nearly so at W. II E42 [the breaks at m and opposite c are in the line of the well-known fault crossing Plato from N.W. to S.E.]. .Ś was throwing a long spire of shaduw the full length of the floor at 11" 40m. That part of the floor contiguous to the W. and S.W. rim was deeply shaded, with streaks of shade running towards the centre of the floor. Between the break near c and the shadow of % a straight shading as of a narrow valley was well seen. [These shadings appear to be roughly coincident with the dark spaces on the floor as seen under high illumination, the straight shading being, as Mr. Pratt suggests, between the “sector" and the E. arm of the “trident.” Is there really a valley here running into the central depression between 1 and 4, seen by Mr. Elger in January, 1870, and observed much earlier by Schröter?] Between these shadings and the shadow of the E. rim were three roundish lighter regions, the higher parts of the floor giving the appearance of a strongly marked convexity.”

“A strong suspicion arises that the apparently higher portions of the floor are the light streaks usually seen, and the highest parts are spots 1, 17 and 5.” Mr. Pratt further suggests that the light streaks are coincident with formations analogous to “spurs” from the chief centres of the residual activity on the floor.

It is not a little remarkable that on the occasion of such a very favourable oblique illumination the craterlets 1 and 17 should not have been detected by Mr. Pratt; both have raised rims of the nature of true volcanic cones, and 1 has been seen, and I believe 17 also, with interior shadows and bright interiors facing the sun. Mr. Pratt does not appear to have seen even the remotest semblance of a shadow. The spots properly so called do not appear generally until the sun has attained an altitude of 20°. If craterlets are recorded as spots earlier, it is probably in consequence of bad definition confusing the crater-form appearance. Is it possible that on the two occasions mentioned by Mr. Pratt, Oct. 17 and Nov. 1, the craterlets 1, 17, 3, and 4 were by some means concealed? As regards Nov. 1, the observation of the crater-cones as the shadows gradually recede from E. to W, is very frequent; indeed the surface of Plato as it just emerges out of night appears to be in a very different state to what it is about mid-day; objects are much sharper, and it is difficult to conceive of any agency so affecting such visible objects as to render them invisible at a time when they are generally most conspicuous. So far as contemporaneous observations are capable of throwing light on this phenomenon, three spots only were recorded on the same even, ing;. No. 1 by Mr. Elger, who noticed it from gh to gh 5m, near the shadow of the summit of the middle peak of the W. wall, three hours later than Mr. Pratt's observation. Mr. Gledhill at 6", same as Mr. Pratt, says, “ Moon so low and air so thick that very little light from moon can reach us ;" he says also, “I see 3 as double elevated cones [i.e. 3 and 30). No other objects can be seen.” Mr. Neison, 5.10 to 8.15 (probably 8.10 to 8.15] succeeded in seeing 3 only, which he records as very faint. He does not give the state of the atmosphere as to definition ; but from his remarking that “ a deep cleft in west edge of wall was very distinctly seen,” I should suppose that it was pretty good. Taking the four sets of observations it would appear that at sunrise on Plato Nov. 1, 1870, some agency was in operation capable of concealing the craterlets; and combining these observations with those of Oct. 17, it would also appear that the same agency was in operation at the time of the previous sunset.

1870, Nov. 1, 6h to 6h 40m. “A grand view again. Definition fair at times. The margin of the eastern end of the floor very distinctly shaded, showing that end to be convex as well as the western. This shading did not conform to the general form of rim, but ran inwards (as shown in the sketch); and three places on the floor were much brighter than the rest, which was free from shading (their localities I have no doubt are those of spots 3, 4, and 17), while the next bright parts of the floor are suggestive of the light streaks; and the shading or lower part coinciding with the narrowing of the streak between 4 and 3 as seen under higher illumination in a measure supports the impression.”

The dip of the floor towards the border, as mentioned by Mr. Pratt, is now well established by numerous observations, also the comparatively greater elevation in the neighbourhood of the fault crossing Plato from N.W. to S.E. These characteristics will probably afford some clue towards framing a theory of the formation of the plain and rampart. Starting with the now acknowledged principle that the moon manifests on a large scale the operation of volcanic forces, we may first inquire as to their modus operandi in the forms we observe. So far as we know, volcanos and earthquakes are closely connected, and there is great reason to believe that both are the results of expansion occasioned by the intumescence of material beneath the

Fig. 8.

crust or surface. It was, I believe, Scrope who first called attention to the effect of the expansion of an intumescent mass elevating the superincumbent material; and Hopkins, twenty-two years later, clearly showed that when the surface was elevated to the point at which the tension and cohesion just balanced each other, the slightest increase of tension ruptured the surface and produced fissures, which might be considerably augmented by earthquakewaves accompanied by the sudden subsidence of the tract between two principal lines of fissures. In applying this reasoning to the explanation of the formation of “ Plato," the remarks of Scrope are so much to the point that a transcription of them is essential to the due apprehension of the forces concerned.

In chapter x, of his · Considerations of Volcanos,' p. 205 (1825), Scrope, speaking of M. de Buch's opinion that the intumescence and rise of the basalt elevated the superincumbent strata, says: “I differ from him, inasmuch as I conceive the intumescence and rise of the basalt to be not the cause but the result of the elevation of the overlying strata.

“A general fact, noticed by M. de Buch himself, proves this most thoroughly, viz. that wherever the basalt appears, the strata are invariably found dipping towards it, which is wholly inexplicable under the idea that the basalt elevated them. ... If, however, we suppose the expansion of the subterranean bed of crystalline rock to have taken place at a great depth, elevating the overlying strata irregularly along the line of various fissures, as for example at A and B (fig. 8), it is clear such fissures will open outwardly; but in the interval of two such fissures, as at C, another must be found opening, on the contrary, downwards, that is, towards the confined and heated lava, which in consequence must intumesce and fill the space afforded to it, and perhaps force its way through some minor cleft upon the external surface of the elevated rocks."

Plato we know to be a large cavity in an elevated region, between the Mare Imbrium and the Mare Frigoris, connected with the mountain-studded region of the Alps on the west, and descending with a precipitous slope towards the east. The whole of the surface around Plato is exceedingly rugged, containing at least the remains of three craters of more ancient date. It is the floor of Plato only that presents any appearance of a recent character; and even this when viewed by very oblique light is far from being level. The sketch (fig. 8) to which reference has already been made is intended to convey some idea of the successive steps by which it is probable that Plato has arrived at its present form. It is roughly drawn to scale, which is somewhat too small, and, consequently, the height of the rim rather exaggerated; the extent being 316,800 English feet, the height, under 4000 feet (i. e. of the rim exclusive of the four pinnacles), will be nearly 5th part. The letters A and B are placed over the supposed foci of expansion, the arrows indicating the direction of the elevating movements, the dotted line showing the extreme height to which the surface could be raised without fracture. Over A and B, and above C, are placed the three main fissures resulting from the increased tension and the general breaking up of the elevated mass, and which might have been accompanied with an almost immediate subsidence, as suggested by Hopkins, Report Brit. Assoc. 1847, p. 64, in the following passage : “If the intumescence of the subjacent fluid, and consequently its supporting power, were immediately afterwards diminished by the escape of elastic vapours, there would be an immediate subsidence.” Such a subsidence, or rather a succession of subsidences, would fully account for the formation of the floors of most craters; and the upwelling of lava from numerous small orifices would tend to produce such a floor as we observe on Plato. The section presents all the characteristics of the walled plain under consideration, the dip towards the border being strongly indicative of the main line of fissure opening outwardly at the foot of the rampart. It may be well to mention that no new principle is introduced in this explanation, which is based upon the views of two leading geologists, after comparing them with phenomena that have been assiduously and repeatedly observed.

Second Provisional Report on the Thermal Conductivity of Metals.

By Prof. Tait. SINCE the date of the former Report the Committee have obtained a splendid set of Kew standard thermometers. With these, complete sets of observations, at very different temperatures, have been made on iron, two specimens of copper, lead, german silver, and gas-coke. As great difficulty was found in keeping the source of heat at a constant high temperature in the statical experiments, they were repeated from day to day till satisfactory results were obtained. But a simple and ingenious device of Dr. Crum Brown (consisting in making the descending counterpoise of a small gas-holder nip an india-rubber tube) supplied so very great an improvement in steadiness of temperature that it was considered advisable to repeat all the statical experiments with this modification. This has accordingly been done, during the present summer, but it has not yet been possible to perform the large amount of calculation necessary to obtain final results. It may be stated, however, that the results as a whole will not differ very considerably from those formerly obtained, so far, at least, as can be judged from a comparison of the graphic representations of the experiments.

Report on the Rainfall of the British Isles, by a Committee, consisting

of C. BROOKE, F.R.S. (Chairman), J. GLAISHER, F.R.S., Prof. PHILLIPS, F.R.S., J. F. BATEMAN, C.E., F.R.S., R. W. MYLNE, C.E., F.R.S., T. HAWKSLEY, C.E., Prof. J. C. ADAMS, F.R.S., C. TOMLINSON, F.R.S., Prof. SYLVESTER, F.R.S., Dr. POLE, F.R.S.,

Rogers FIELD, C.E., and G. J. Symons, Secretary. Your Committee have much pleasure in reporting that the organization under their supervision is believed to be in a generally efficient state. With a staff of observers, numbering nearly two thousand, spread over the whole extent of the British Isles, there can, however, be no question that, to ensure perfect efficiency and uniformity of observation, a systematic inspection of stations is absolutely necessary. In a paper read before the Society of Arts in 1858, Mr. Bailey Denton appears to have considered that there should be one inspector to about each 200 stations; at that rate we ought to have ten. The Meteorological Committee of the Royal Society have made it a rule to have all their stations inspected each year. On the most moderate computation it is indisputable that at least one inspector of stations is required for our large body of observers, the whole of whose time should be devoted to travelling.

Ever since their appointment your Committee have felt and acted upon this conviction; but want of funds has prevented them from employing a regular inspector, and obliged them to rely solely upon the unpaid services of their Secretary. Even under these adverse conditions considerable progress has been made with the work, and upwards of 400 gauges had been visited and examined previous to the Liverpool Meeting. At that Meeting, however, the Association only granted half the sum for which we asked, and we have consequently (most reluctantly) been obliged to stop this important and useful work.

As an interim measure, and with a view to ascertaining in what districts inspection is most requisite, it has been suggested that a schedule of questions as to the positions of their rain-gauges should be sent to every observer. The Committee unanimously approved of the suggestion, and annex a copy of the Circular and Schedule they are about to issue.

British Association Rainfall Committee,

62 Camden Square, London, N.W. SIR,—The above Committee feel that it is most important that precise information as to the position of all the rain-gauges in the British Isles should be promptly obtained. They are aware that under present circumstances it is impossible that each gauge should be personally inspected, and have therefore instructed me to ask you to fill up the accompanying form, which I shall be obliged by your returning as soon as possible.

As an indication of the kind of information which the Committee desire to collect, I have filled up one form for my own gauge ; but there are of course many subjects not touched upon in the specimen which will be acON THE RAIN FALL OF THE BRITISH ISLES.

Sersity o


ceptable in others, such as distance from the sea and from lofty hile well as their direction, &c.

The Committee will also be glad of any suggestions as to the conduct on rainfall work, and of information respecting any stations or old observations not included in the list published by them in 1866, and of which I shall be happy to send you a copy if you have not already received one.

Yours very truly,

G. J. Symons, Secretary.
[Illustration of mode of filling up return.]
At (Camden Square, London,]
In the County of (Middlesex.]

92 ft.



[ocr errors]




Year in which observations were first made (1858.]
Hour of observation [9 a.m.] If entered against the day of observation, or

the one preceding (Preceding).
Position [In garden, 120 ft. by 24 ft.]
Surrounding objects, their distances and heights :-

Distance. Height.
N. Wall

17 ft.

5 ft.]
N.E. House
E. Wall

5 ft.]
S.E. Wall

12 ft. .. 5 ft.] S. ¡Wall

5 ft.1 S.W. Summer House

24 ft. . 7 ft. W. (Raspberry-bushes

.. 6 ft. .. N.W.Wali

.. 12 ft. .. 5 ft.] Inclination of ground Quite level, but in N.E. rises 30 ft. in 4 mile.] Height of Ground above sea-level [111] ft. as determined by [Levelling from

Ordnance Bench-mark].
Height of top of gauge above ground [0] ft. [6] in.
Pattern of gauge. (If similar to any on plate, quote the number; if not,

give sketch.) [Similar to No. X., but the bent tube is made straight,
and a jar inserted for the purpose of ensuring more accurate mea-

surement.] Have the same gauge and measuring-glass been used throughout ? [No.] Has the gauge always been in the same position ? [No.]

| the previous position [300 yards further west.) If not, state briefly the reason for the alteration (Growth of trees.]

| the supposed effect [None perceptible.]

REMARKS. [Measuring-glass broken in 1861, and a new tested one obtained, the

rainfall of each day until its arrival being bottled separately, and measured by the new glass.]

Signed, [G. J. SYMONS.] Another branch of investigation which has been arrested by the same cause is the relative amount of rain falling in different months, or, as we have usually termed it, the “ monthly percentage of mean annual rainfall.” Several articles upon the subject have appeared in our previous Reports; and last year we pointed out that the observations for the decade 1860-69 offered data of completeness unparalleled, either in this or any other country, the

« AnteriorContinuar »