MICROSCOPY. 59 
such a low temperature exists at great depths in tropical seas as 
can only be accounted for by the hypothesis of undercurrents 
from the Poles to the Equator. The temperature soundings taken 
on the last English expedition show that the bottom of the sea off 
Portugal, below one thousand fathoms, ranges from thirty-nine to 
thirty-five degrees, or about the freezing point. In the Mediter- 
ranean the temperature beneath the hot surface stratum of water 
is uniform to any depth—namely, about fifty-five degrees. It is 
naturally inferred that in the Atlantic and Pacific oceans an 
undercurrent of polar icy water is flowing southward under the 
warmer: tropical waters; and this is sustained by the discovery at 
great depths of polar animals in the seas of Florida and Cuba. 
, The English expedition under Professor Carpenter, the well known 
physiologist, has also detected the existence of an outward under- 
current in the Strait of Gibraltar, which carries back into the 
Atlantic the water of the Mediterranean that has undergone con- 
centration by the excess of evaporation in its basin. Professor 
Carpenter confirms the theory previously urged by Captain Maury, 
that the cause of thé superficial in-current and the deep out-cur- 
rent is to be found in the excess of evaporation, the Mediterra- 
nean water being from evaporation denser than the water of the ; 
Atlantic. Catpankst then compares the polar and equatorial areas, 
and shows that there is a tendency in the former to a lowering of 
level and increase of density, which places it in the same relation 
to the latter as the Mediterranean bears to the Atlantic. 
Coat Beps 1N Panama.—In a paper read before the Geolog- 
ical Section of the British Association, Dr. Hume stated that, 
during a recent residence upon this isthmus, he learned that a 
series of seams of coal had been found in a secluded and prim- 
itive portion of. the country, not far distant from the railway. 
He had procured and analyzed some specimens of the coal, and 
had found 75 per cent. of carbonaceous matter, the balance being 
water and ash, and a very small quantity of sulphur. The coal 
possessed a fair heating and large illuminating power. 
r MICROSCOPY. 
ANGULAR ÅPERTURE. — An anonymous querist in the “ Monthly 
Microscopical Journal,” incidentally to asking the aperture of a 
certain lens, urges the importance of angular aperture as an ele- 
