92 W. J. M. Rankine, Esq., on the 
Scandinavia which is ascertained to be undergoing an eleva- 
tory movement, even at the present day. I was able to visit 
the celebrated stone at Lofsgrund, near Gefle, on the Gulf 
of Bothnia, which has been marked with the height of the 
water at various periods; lastly, by Sir Charles Lyell in 
1834. I found his mark 2 feet 7 inches below that of 1731; 
and the sea, on the day of my visit (2d September), was about 
6 inches below Sir Charles’s mark, or rather more than 3 feet 
below that made 118 years before. It occurred to me as un- 
fortunate to have selected for this kind of test a loose block 
lying near the shore; for we cannot exclude the suspicion, that 
the ice may carry it a little way up the beach every winter. 
Nevertheless, as there are other marks presenting similar 
results, where no such liability to fallacy exists, the proba- 
bility is, that no movement has actually taken place. Two 
days afterwards, I visited the mark made by Flumen upon 
the cliff of Grasée, near Oregrund, in 1820. The sea was 
so calm as not to wet more than an inch of the cliff above its 
ordinary level of the day; and the seamen informed me, that 
the water was ata very fair average forthe season. I found 
the surface of the water 11 inches below Flumen’s mark, 
which had been made only nine days later in the year. Thus, 
if the sea at the two periods was in similar circumstances, 
or at its mean level for the season, there appeared to have 
been arise of the land in this district to the extent of 11 
inches in twenty-eight years. 
A Conjecture as to the Forces which produce the Tails of 
Comets. By WILLIAM JOHN MACQUORN RANKINE, Esq., 
C.E. Communicated by the Author. 
The immense velocity with which the tails of comets, on 
the approach of those bodies to their perihelia, are projected 
in a direction opposite to that of the sun, has always been 
held to indicate that the particles of those nebulous enve- 
lopes are acted upon by some powerful force directed from 
the sun. Various hypotheses have been proposed as to the na- 
