of Edinburgh, Session 1868-69. 
553 
and he was inclined to think the last process, however it was to be 
accounted for, seemed more in accordance with the facts, and, in 
particular, with the horizontality of these sea-margins, which, 
being over an extensive area, could scarcely have resulted from 
volcanic elevation of the land. 
10. The author adverted to an opinion by Mr G-eikie, and 
countenanced by Sir Charles Lyell, that the last change in the 
relative levels of sea and land took place in the Firth of Forth 
since the time of the Romans, which opinion he thought was dis- 
proved by the number of military works of the Romans in the 
district about Stirling, which could never have been made had the 
sea then stood 20 or 30 feet higher than at present. 
At the same time, he thought there was evidence to show that 
human beings occupied this part of Scotland when the sea had 
stood at that higher level, and that as there were also grounds for 
believing that the climate of Scotland had then an arctic character, 
the subterranean dwellings, called “ weems ” and “ Piets’ houses,” 
found in many parts of Scotland, might very probably be referred 
to the epoch which preceded the change of sea-level. 
2. On Comets, By Professor Tail. 
[Abstract.) 
The principal object of the paper is to investigate how far the 
singular phenomena exhibited by the tails of comets, and by the 
envelopes of their nuclei, the shrinking of their nuclei as they 
approach the sun, and vice versd, as well as the diminution of period 
presented by some of them, can he explained on the probable sup- 
position that a comet is a mere cloud of small masses, such as 
stones and fragments of meteoric iron, shining by reflected light 
alone, except where these masses impinge on one another, or on 
other matter circulating round the sun, and thus produce luminous 
gases, along with considerable modifications of their relative motion. 
Thus the gaseous spectrum of the nucleus is assigned to the same 
impacts which throw out from the ranks those masses which form 
the tail. 
Some of the most wonderful of the singular phenomena pre- 
sented by comets, such as the almost sudden development of tails 
