formed at the bottom of running waters. 127 
the prominent bodies fixed to the bottom, present points of at- 
tachment to the forming ice, much more advantageous than a 
constantly agitated surface ; and it is well known what influence 
these points or nuclei have in general upon crystallization. The 
water sufficiently cooled begins therefore to be converted into ice 
at the bottom, particularly in the places where shelter is afforded 
by prominences from the impetuosity of the current. The conti- 
nual motion which takes place in the interior of the water, presents 
the same obstacle to the formation of large masses of ice, which 
is presented to the crystallization of common salt, by agitation of 
the liquid which holds it in solution ; and there are only formed 
in consequence simple agglomerations of small plates of ice im- 
perfectly crystallized. When these agglomerations have accu- 
mulated so as to present larger masses, they are detached from 
the bottom, either by virtue of their greater lightness, or from 
the impulse of the current, and rise to the surface, frequently 
carrying with them fragments of the soil itself. In fact, it is not 
uncommon to find sand, gravel, mud, or other substances, attach- 
ed to the ground-ice, and floating with it upon the surface. 
He then mentions his having searched, in different authors, for 
observations which might serve to confirm or refute the above 
theory, but with little success ; for modern writers seem to have 
overlooked the subject ; and it is almost exclusively in the older 
works that any particular notice is taken of it. Plot, in his his- 
tory of Oxfordshire, observes, that all the watermen with whom 
he has had an opportunity of speaking on the subject, agree in 
thinking, that the rivers of the country always begin to freeze at 
th£ bottom. He describes the manner in which the small pieces 
of ice, called ice-meers , rise from the bottom to the surface, and 
mentions their frequently containing gravel or stones, which they 
have carried along with them. Hales confirms these observa- 
tions ; and says, that the watermen of the Thames assert, that, 
some days before that river is frozen at the surface, they feel the 
ice at the bottom with their poles, and that they see it rising to 
the surface. He gives, also, several observations of his own, 
clearly establishing the fact These observations, M. Merian 
* On the 30th January 1793, at 7 in the morning, the temperature being about 
— 9° 6', he went to the banks of the Thames near Teddington, and found the surface 
frozen to the thickness of the fifth of an inch, in a pool where the current was very 
little felt. Under this ice he observed another layer upon the ground ; and having 
made an opening in the upper ice, he took up a piece of that which was formed 
