Intelligence and Miscellaneous Articles, 479 



had been deferred last Session, would soon be completed, and that 

 on it would be represented the most sweeping change in the Geo- 

 logical Map of Scotland which had ever been proposed ; inasmuch 

 as a vast region, which has up to this day been considered to be 

 of a primary age, and anterior to palaeozoic life, will be referred to 

 the Lower Silurian epoch. 



Passing from the North Highlands, which he had more especially 

 examined of late years, the author expressed his opinion that the 

 mica-slates, clay-slates, and quartz-rock of the Southern Highlands 

 would probably all fall into the same Lower Silurian category as the 

 rocks of Sutherland and Ross. 



In conclusion, he begged to repeat what he had stated five years 

 ago at the Glasgow Meeting of the British Association, that, with 

 the exception of examples in the slate-rocks of Loch Lomond, Eas- 

 dale, Ballyhulish, &c., on the west coast, and very partial cases in 

 the clay-slate of Aberdeenshire, he had never seen any examples of 

 mechanical cleavage in the crystalline rocks of the Highlands, as had 

 been maintained by a distinguished deceased Fellow of the Society. 

 (See Mr. D. Sharpe's Memoir in the Phil. Trans, vol. cxlii. p. 445.) 



LXXIII. Intelligence and Miscellaneous Articles. 



SUGGESTIONS AS TO THE STRUCTURE OF THE TAILS OF COMETS. 

 BY THE REV. T. W. WEBB. 



nPHOUGH there seems to be no question that the dark space so 

 -*- frequently included in the tails of great comets is the result of a 

 hollow structure, an attentive consideration of the appearances ex- 

 hibited by the comet of Donati has led me to think it probable that 

 some other cause may concur in its production. The ordinary laws of 

 perspective certainly seem inadequate to its explanation, except in 

 cases in which the darkness bears a large proportion to the brighter 

 streams on each side of it ; for, unless the difference is but small 

 between the radius of the hollow interior and that of the whole tail, 

 the sine will not exceed the versed sine in a sufficient ratio to ac- 

 count for so great an increase of luminosity as is frequently witnessed. 

 On the contrary, at one period in the course of Donati's comet, at 

 the end of September and during the first few days of October, the 

 central darkness in the train, though very intense, occupied a com- 

 paratively small part of its whole breadth. My own estimate on 

 September 30 gave it but one-eighth of the entire width of the tail, 

 in which case a simple calculation will j)rove that the sine would 

 exceed the versed sine only by something less than one-eighth part, 

 and consequently the resulting difference in brightness would by no 

 means accord with observation. The supposition of a shadow pro- 

 jected from the nucleus might seem at first to assist us with a sup- 

 l)lementary amount of darkness; but it will be found unavailing when 

 we have compared the case of the comet of 1811, in which a trans- 

 parent sjjace surrounded the nucleus alike on every side. Hence it 

 may be thought proljable that there must be some other cause for 

 this appearance ; and I have been induced to conjecture that, admit- 

 ting the existence of a hollow interior, the difficulty might be met by 

 the additional supposition of a radiated structure, in consequence of 



