induced or exhibited in Iron , fyc. by Rotation. 125 
14. If, instead of a horse-shoe magnet, the contrary poles of 
two bar-magnets be used, the effect is the same as before ; but, 
if the poles of the same name, viz. both north or both south, be 
employed, then the effect is scarcely perceptible. This is an 
important result, as it shows, that the effect is not due to any 
kind of resisting medium, as was supposed in the first instance. 
Art. XII. — Remarhs upon Ground-Ice , or Ice formed at the 
bottom of running waters. By Professor Merian *. 
HE name of ground-ice is given to the detached and sepa- 
rated masses of ice, which running waters carry at their surface, 
during a frost of some duration. This ice differs from that con- 
tinuous kind which is formed along the edge of rivers, and par- 
ticularly in places where the water is quiet ; it never forms in 
lakes, pools, or other stagnant water, and motion appears to be 
a condition essential to its existence. At first sight it might be 
taken for an aggregation of snow penetrated by water, swimming 
at the surface, rather than for ice ; but a closer examination will 
not fail to discover its true characters. In fact, it is formed of 
an assemblage of a multitude of small, thin and rounded plates 
of ice, having a diameter of a few lines. They are individually 
transparent, but their aggregation presents at a distance the ap- 
pearance of a semi-transparent mass like wet snow. It is known 
that, before rivers begin to carry the ground-ice, the temperature 
of the air must have been for several days previous some de- 
grees below zero ; and it is observed in general, that a cold 
wind, blowing in a direction contrary to the current of the river, 
is singularly favourable to the formation of this sort of ice. 
We might at first be induced to suppose, that the ice in ques- 
tion must be formed at the surface, as is the case with that of 
smooth water, because the effect of the cooling of the atmosphere 
is more intensely felt at the surface, and water at a temperature 
below + 3° C., that is, below the point of its greatest density, 
becomes lighter, in proportion as the cold augments. It might 
* The above observations of Merian, are abridged from his Memoir, read be- 
fore the Natural History Society of Basle. 
