324 



NATURE 



\_yan. 31, i««9 



complete absence of haze, and it is well known that the 

 air is peculiarly clear in early morning, when we get 

 above the fog-level. 



According to this account of heat haze, it stands in 

 sharp contrast to fog, of which it is so often supposed to 

 be a relative in reduced circumstances. While the one 

 requires convection, the other usually occurs when the 

 air is in stable equilibrium, the lowest strata being the 

 coldest. In the fog, for example, which so frequently 

 heralds or accompanies the break-up of a frost, the lower 

 strata are still cold, while above the wind has changed, 

 and the air comes up warm and vapour-laden. The 

 vapour diffuses downwards into the lower, cold strata, 

 and is there condensed, and it is possible that the more 

 rapid diffusion of water vapour has something to do with 

 the continuance of the fog, for it would diffuse down- 

 wards more rapidly than the rest of the air with which 

 it has come. 



There are other cases of haze which may, perhaps, be 

 explicable by optical heterogeneity. In the dry east winds 

 of spring we frequently have a haze when the air is far from 

 saturation-point, and the clouds, if they exist, are at a 

 high level. It appears possible that this haze is due to 

 small convection currents of the cold air from above, the 

 temperature falling too rapidly from below upwards for 

 equilibrium. 



Sometimes the heterogeneity may be due to water 

 vapour. After rain, when the ground is still wet, the 

 drying of exposed surfaces often shows that the air is not 

 saturated, yet there is a haze or mist which is supposed 

 to be thin fog,z>. water-dust. Evaporation must be going 

 on, and the air must certainly be unequally charged with 

 vapour. With this inequality there must also be convec- 

 tion. I have never been able to detect, with certainty, 

 quivering of outline either in this case or in the previous 

 one of the east wind haze, though I have sometimes 

 suspected its existence. Possibly someone who has 

 used a high-power telescope for terrestrial objects might 

 be able to give information on this point. But it is to be 

 noted that the differences of density in both these cases 

 are much smaller than in the case of summer haze, and 

 the currents should therefore be on a smaller scale. 



It seems worthy of inquiry whether the haze observed 

 under cumulus clouds, referred to by the Rev. W. Clement 

 Ley, may not also be due to water-vapour heterogeneity. 

 The cumulus cloud indicates the existence of a large body 

 of vapour-laden air extending no doubt below, as well as 

 above, the condensation level. As this mass sweeps along, 

 the lower part of it is retarded by the earth and by the 

 lower strata, and more or less disturbance and mixture 

 with the surrounding air will occur. There will therefore 

 be heterogeneity. I do not know whether this would be 

 at all sufficient to account for the haze observed, but the 

 suggestion seems worth considering. 



J. H. POYNTING. 



THE EARTHQUAKE AT EDINBURGH. 



'T' HOUGH no earthquake of great destructive energy 

 ■^ has passed across the area of the British Islands 

 within historic times, the number of actual shocks which 

 have been experienced and recorded amounts now to 

 many hundreds. Scotland, being more generally moun- 

 tainous than the other divisions of the United Kingdom, 

 has hitherto enjoyed by far the largest share of these 

 manifestations of terrestrial disturbance. Hardly a year 

 passes without adding to the list. The latest addition is 

 that which startled the inhabitants of Edinburgh, on 

 Friday morning, the i8th instant, a few minutes before , 

 7 o'clock, when a large part of the population was still in 

 bed. It agreed in general character with the usual type 



of Scottish earthquakes, and might indeed be taken as 

 an illustrative example of them — not strong enough to 

 do damage to property or life, but yet quite sufficiently 

 marked to arrest attention and to exhibit some of the 

 more prominent features of seismic movements. 



The earthquakes that visit Scotland so frequently are 

 always singularly local in character. Now and then a 

 more vigorous shaking may be felt across the breadth 

 of the Highlands, but. as a rule the tremor is confined to 

 a comparatively circumscribed district. Again, not only 

 are they local, but there is usually some centre or median 

 belt of ground where the effects have been more distinctly 

 perceptible than elsewhere. Some districts are specially 

 liable to them. Thus, the tract around Comrie, in the 

 south of Perthshire, has long been noted for the frequency 

 of its earthquakes. Another area often similarly affected 

 embraces the West Highlands up to Inverness. It is 

 worthy of remark that this latter region was consider- 

 ably disturbed by a series of shocks almost exactly 

 a year before, viz. on February 2, i888. If we look 

 at the geological structure of these earthquake districts, 

 we observe that they are traversed by some of the most 

 important lines of fissure by which the crust of the earth 

 within the area of Britain has been disrupted. Comrie 

 stands on the line of a powerful fault which runs across 

 Scotland from sea to sea, and which, by bringing the 

 softer and less prominent sandstones of the low country 

 against the harder and more precipitous schists of the 

 hills, has been in some measure the origin of the line 

 between the Lowlands and Highlands, and thus of the 

 limitation of the Saxon and the Gael. Again, in the West 

 Highlands, a profound dislocation has defined the line of 

 the Great Glen from Inverness to the Linnhe Loch. The 

 relation of the chief earthquake centres to these ancient 

 lines of fracture in the terrestrial crust can hardly be 

 accidental. The actual shock that starts the seismic 

 wave is probably in these areas to be traced to some 

 slipping of the rocks along these lines of rupture. Though 

 the faults are of great geological antiquity^ they doubtless 

 remain lines of weakness, along which any changes due 

 to the secular cooling and contraction of our planet may 

 be expected to show themselves. 



The area shaken by the earthquake of Friday, January 

 1 8, is not one which has been much subject to experiences 

 of this kind, though a few shocks are spoken of as having 

 been felt there in the past. But its geological structure 

 shows how well fitted it is to become a theatre of dis- 

 turbance of the usual feeble Scottish type. It extends 

 from the edge of the Firth of Forth southwards into the 

 interior across the site of Edinburgh, and thence along 

 the chain of heights known as the Pentland Hills. These 

 hills consist mainly of volcanic rocks belonging to the 

 period of the Lower Old Red Sandstone. The beds of 

 lava and sheets of tuff increase in thickness towards the 

 north-east, until in the Braid Hills, which lie immediately 

 along the southern outskirts of Edinburgh, they are re- 

 placed by the materials that fill up what seems to have 

 been the chief vent from which they were discharged. 

 These volcanic eruptions had long ceased when the Car- 

 boniferous rocks of the district began to be laid down 

 upon their upturned andworn edges. During a prolonged 

 period of deposition and depression they gradually sank 

 and were buried under some thousands of feet of sedi- 

 mentary strata. In later times the area was disrupted by 

 a group of long parallel fractures running in a general 

 north-east and south-west line. Vast denudation likewise 

 took place. By degrees the deep cover of Carboniferous 

 strata was worn away from the tops of the volcanic ridges 

 which now once more appear at the surface as the chain 

 of the Pentland Hills. Thus the site of the earthquake 

 coincides with an ancient volcanic centre and with a 

 belt of country which has been dislocated by lines of 

 fault. 



