DECEirSER 1, 1896.] 



KNOWLEDGE. 



279 



the blocks, to which it is my principal object in this paper 

 to draw attention. 



A little more than forty years ago, part of the lead on 

 the roof of Bristol Cathedral was found to have crept 

 slowly downwards, the total descent being about a foot and 

 a half in two years. Soon afterwards the now well-known 

 explanation of the movement was given by Canon 

 Moseley. When the lead is heated it expands, but it is 

 easier to move down the slope than up it ; so that, while 

 some of the lead at the top of the sheet moves upwards, a 

 far larger part moves down. When the lead is cooled it 

 contracts ; part near the lower edge moves up the slope, 

 but a far larger upper portion moves downwards. Thus, on 

 the whole, with every change of temperature to and fro, the 

 whole sheet of lead makes a short creep down the slope. 



In this case, since the sheet lead is thin, the movement 

 is principally due to the fact that the lead and wooden 

 surface on which it rests have unequal coefficients of 

 expansion. If they had been of the same material the 

 creep would have been almost imperceptible. When, 

 however, one stone rests upon another of the same kind, 

 the two are unequally heated on account of the small 

 conductivity of rock, and the ultimate result is the same. 



A few years ago I made some experiments on the creep- 

 ing of stones arising from changes of temperature.* In 

 the first, one brick was placed on another inclined to the 

 horizon at an angle of twenty degrees. On the upper 

 edge of the upper brick, and on another independent brick 

 support, a level was placed with the bubble in the centre 

 of the tube. The level was read frequently, and it was 

 found that while the upper end rose slightly during the day, 

 it descended by a greater amount during the night, so that 

 at the same time each day it stood at a lower level. This 

 experiment showed that the stone did creep in the manner 

 supposed by Moseley, though it gave no idea of the extent 

 of the movement, as the angular value of the scale- 

 divisions of the level was undetermined. 



The object of a second experiment was to ascertain how 

 far a stone would creep in a year under given conditions. 

 I had two slabs of York stone cut, each three feet long, 

 five inches wide, and two inches thick. One face and one 

 side of each stone were smoothed, and the stones were 

 placed with their smoothed surfaces in contact, and 

 inclined at an angle of seventeen degrees to the horizon. 

 The smoothed sides of the two stones were continuous and 

 three fine scratches were made on them, in the middle 

 and at each end, so as to be at starting in the same 

 straight lines. At the end of a week the displacement of 

 the upper stone, though exceedingly slight, was quite 

 perceptible, and it continued so throughout the year, the 

 total creep in this interval being thirteen and one-sixth 

 millimetres, or a little over half an inch. 



So small a movement may be considered hardly worth 

 taking into account. But it must be remembered that it 

 affects every stone free to move and resting on an inclined 

 surface. Moreover, the creep is proportional to the length 

 of the stone and to the range of temperature to which it is 

 subjected, and the length of the lunar compared with that 

 of the terrestrial night, is probably more than counter- 

 balanced by the magnitude of this range. 



If changes in the manner hero described do take place 

 upon the moon, it is evident that they must be exceedingly 

 gradual. Years must elapse before they become per- 

 ceptible to us ; but as the changes would take place always 

 in one direction, it seems not impossible that a careful 

 scrutiny of a few selected districts would in time reveal 

 their existence. 



• " Note oil tlic Movement of Sci-eo Material," Quart. Joiirii. Qeol. 

 Soc, 1888, pp. 232-237, 825, 82R. 



THE LUNAR METROPOLIS. 



By E. Walter Maunder, F.E.A.S. 



THE lunar photograph we give this month is from 

 the same negative as that which appeared in the 

 October Number of Knowledge, and forms a con- 

 tinuation of it ; indeed, it overlaps it to a small 

 extent. It likewise shghtly overlaps at the ter- 

 minator the photograph given in our April Number of 

 Cuvier and Licetus. The general characteristics of the 

 district are the same as around Clavius. If anything, the 

 complexity of detail and the intricacy of overlapping 

 formations is greater in the region now presented than in 

 the one shown in October. 



The interest of the district centres m Tycho, not on 

 account of its size, though this is very considerable, as it 

 is over fifty-four miles in diameter — almost as large as 

 Copernicus ; but whereas Copernicus is so placed as to 

 have attention drawn to it, Tycho is dwarfed by many of 

 the neighbouring objects, such as Clavius, Longomoutanus, 

 and Maginus, shown in our October photograph, and the 

 extraordinary complexity of the region surrounding it 

 takes oil' the due effect of its proportions. 



It is important, however, as being in the very centre of 

 the most disturbed region of the moon — a land of the 

 wildest and most rugged character, and sown so thickly 

 with crater pits and bowl-like depressions that it is hard 

 to refrain from adopting the hypothesis of some theorists, 

 that the moon, when in a plastic state, has been subjected 

 to a fierce bombardment by great meteorites, who have 

 left the tokens of their assault in these thickly clustered 

 scars upon its surface. 



The great claim, however, of Tycho to distinction — the 

 circumstance which has won for it th§ apt title of the 

 " Lunar Metropolis " — is the streak system which radiates 

 from it in all directions. This system— the most strikmg 

 feature of the moon at full — is, of course, not visible in our 

 present photograph, which shows the moon at her third 



( 





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quarter. On the photograph, therefore, the broad and 

 lofty rampart, deep floor, and bold central peak of Tycho 

 are the features that most ilistinguisb it. 



The other walled and ringed plains shown in our photo- 

 graph arc far too numerous to describe in detail, and, 

 indeed, the pliotograph itself will yield more information 

 than could be given by much writing. Longomontauus 



