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should be directed. Of the remainder, three have been observed once only by Mr. Gledhill, viz. Nos. 26, 28, and 35; two have been observed twice, viz. Nos. 27 and 33; two thrice, both old spots, viz. 8 (Gruithuisen) and 15 (Dawes); and one, No.34, six times between January 15 and March 13, 1870*.

In his letter dated 1870, May 19, Mr. Pratt says that “ spot No. 8 could not be recovered even with the most minute attention.” Of spot No. 1 he says, " it was brighter than I have seen it before, quite rovnd and dense, much like the image of a star on a good night surrounded by the very least trace of a ring of light. [Neither] internal nor external shadows could be seen, although I constantly expected a slight glimpse."

Spot No. 22. In reference to this spot Mr. Pratt writes, under date 1870 August 26, as follows:

“ Spot No. 22, according to my observations, has manifested a remarkable increase of brightness, and those parts of the shaded portions of the floor of Plato which are nearest to the rim have come out more conspicuously darker than the rest than I remember to have previously noted. The tint of the floor, toe, has progressively paled. These three phenomena (the increased brightness of spot 22, the intensification of the darker parts of the floor near the rim, and the progressive paling of the floor] may possibly be connected by a common cause; for certainly in this lunation there is somewhat of a coincidence amongst them; for instance, spot 22 is intensely bright at the time the marginal portions of the shaded parts are most conspicuously dark, and these two, again, coincide with the time when the general tint of the floor is at its darkest. Again, after August 12 and 13, spot 22 decreased in relative intensity, although I am not ready to hazard the assertion that it had on August 16 positively declined to its usual intensity, as it was not seen. [It was on this evening that Mr. Pratt observed three spots only.] Two similar instances, I believe, I have noted before, when 22 manifested a singular brightness at sunrise. But the connexion between the visibility of the deeper-tinted margin and the general deepening of colour is perhaps more close still, as both certainly paled after August 13. The perplexity seems to be that the variation in intensity of the margin is relative in respect of the general colour ; and if differences of angles of illumination and vision do affect the general tint, it might be supposed that they would in the same manner affect the margin and so produce no relative variation of intensity.” · In connexion with the relative intensity of which Mr. Pratt speaks, the state of the border is somewhat important. August 12 and 13, when the marginal portions of the floor were intensified in colour, Mr. Pratt recorded of the border :-“ Definition fair at times, with much tremor, wind N.E.” This was on the 12th. On the 13th the record is: “ Border, definition bad,

* The history of spot No. 34 is curious ; the following are the only records which exist of it. The observations were all made by Mr. Gledhill with the Halifax 93-inch equatorial in the Observatory of Edward Crossley, Esq.

1870, January 15, 10 to 13 hours. "I am continually thinking I see an object close to No. 1 and to the west of it.”

February 11, 6.45. “No. 1 often comes out double ; last year I often saw it thus. I am now almost quite sure I see a minute object close to the west of it.”

February 12, 6.0. “Saw 9, 11, 30, and object close west of No. 1.”
March 12, 6 to 8 hours. No. 34 mentioned as having been seen.

March 13, 6 to 12 hours. “ Unless I am very much mistaken indeed 34 is an easy ohject, i.e. No. 1 comes out easily double.”

There are no records after this date. Instruments less than 9-inches aperture are not likely to redetect it.

as “

much boiling, wind N.E.” On the 12th, definition fair, the floor was recorded

very dark.” On the 13th it was dark, but not so much so as on the 12th. On the 16th, as well as on the 15th, the definition of the border was “ bad.” These records clearly throw a doubt upon the supposition of the “paling” having resulted from some lunar action, inasmuch as when the deeper tint was observed the definition was “good,” the “ tremor” and “ boiling" having a tendency to confuse the portions of the floor. On the other hand, spots have been much more numerous with bad definition than 3 as observed by Mr. Pratt on the 16th; and this would lead to the supposition that the apparent extinction of the spots with a pale floor was in some way differently connected than by a deteriorated state of the earth's atmosphere. I have often observed that the passage of a thin cloud over the moon has greatly contributed to intensify the tints of the darker portions of the surface; but in this case the intensification has been general and not partial, as it would be if dependent upon local lunar action.

Mr. Pratt records a case of partial obscuration which was well seen on August 13. “It appeared,” says Mr. Pratt,“ on this wise. A general view of the floor showed it much speckled and streaked in other parts ; but over the area specified (Mr. Pratt has not mentioned the particular part of the floor; but from what follows I apprehend it must be in the neighbourhood of No. 3) there seemed an absence of markings ; close attention, however, enabled some to be seen, but not nearly so richly as the remainder of the floor, and we know well enough that that particular area is not wanting in markings. The evening's view has just occurred to memory when I first discovered that spot 3 was a triple one, and had a remarkable view of its neighbourhood (Qy. Was this on May 13 ?], therefore exactly the reverse being the case. August 13 seems as conclusive a proof as one observer is likely to obtain in a year's work.”

Of four observers on the same evening, two record No. 3, and the other two appear not to have seen it. Taking them in chronological order, Neison, 9.5 to 9.15, records it as distinct; Pratt, 10.30 to 12.30, did not observe it; Ormesher, 11.0 to 11.30, does not show it in his drawing ; Gledhill, 14h, records it as a bright disk : he also records 30. As these observations are not contemporaneous, with the exception of Ormesher's, having been made while Pratt was observing, it appears, from its absence in both their records, that from 10.30 to 12.30 it was really not visible ; and this tends to support Mr. Pratt's idea that for the time it was hidden by something like an obscuring medium. What this could have been it is difficult to surmise. The remark, however, of Neison that 30 was not to be seen between 9.5 and 9.15 is interesting in connexion with Gledhill recording both spots at a later epoch, 14”, and also detecting five not seen by Pratt, viz. 3, 30, 9, 11, 18. Neison suspected he saw 14, not recorded by Gledhill nor Pratt, but seen by Ormesher. Pratt saw 22, not seen by either of the others. The case of 14 is a little perplexing; it might, however, have been missed by Pratt on account of the bad definition. With regard to the greater number of spots seen by Gledhill, two circumstances may have contributed to this result, the larger aperture of Mr. Crossley's instrument and the epoch at which Mr. Gledhill observed. It may possibly be found that the greater number of spots recorded after the sun's meridian passage at Plato depend upon the steadiness and purity of the air mostly experienced after midnight.

Sunset and Sunrise on Plato. Extracts from Mr. Pratt's notebook, 1870, Oct. 17, 11h to 12h. Defini

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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.]. 5 was throwing a long spire of shadow 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 5 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 evening; No. 1 by Mr. Elger, who noticed it from gh to 9h 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 61, 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

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much boiling, wind N.E." On the 12th, definitie
as " very dark,” On the 13th it was dar)
12th. On the 16th, as well as on the

view again. Definition fair at “ bad." These records clearly the “paling" having resulted frodeeper tint was observer “boiling" having a ter other hand, spots hav 3 as observed by Mr that the apparer

or lower part coinciding with the way differently sphere. I b moon has gr the surfar partial,'

Mr. Augr

inst sacrement et the floor very distinctly shaded, war erremi as the western. This shading did not *********/A rim, but ran inwards (as shown in the

stirrups the foor were much brighter than the rest,

* fter throuw shiling their localities I have no doubt are those of murowing of the strak between 4 and 3 as seen under higher illumination

***ť 4 un 1% while the next bright parts of the floor are suggestive mor well established by numerous observations, also the comparatively

The dip of the Aoor towards the border, as mentioned by Mr. Pratt, is

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gmaker cloration in the neighbourhood of the fault crossing Plato from framing theory of the formation of the plain and rampart. Starting with the now acknowledged principle that the moon manifests on a large scale the

W. to S.E. These characteristics will probably afford some clue towards operation of rolcanic forces, we may first inquire as to their modus operandi results of expansion occasioned by the intumescence of material beneath the closely connected, and there is great reason to believe that both are the

Fig. 8.

observe. So far as we know, volcanos and earthquakes are

in the forms we

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个 erust 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,

for example at A and B (fig. 8), it is clear such fissures will open outwardly; in the interval of two such fissures, as at C, another must be found opening, je contrary, downwards, that is, towards the confined and heated lava,

in consequence must intumesce and fill the space afforded to it, and

3 force its way through some minor cleft upon the external surface of vievated 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 4th 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 elastio 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 expe

1871.

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