176 D r. Marcet on the specific gravity and temperature 
of Gibraltar, and unloads its waters of their excess of salt. 
But however plausible this theory may be, it must be con- 
fessed, that scarcely any other argument has hitherto been 
alledged in support of the probability of this under-current, 
than the easy explanation it would afford of the phenomena, 
and analogies derived from the familiar fact of opposite atmo- 
spheric currents formed in confined places, from the mere 
admission of air of a different temperature.* The following 
fact, however, for which I am indebted to Dr. Macmichael, 
who had it from very respectable authority (the British Consul 
at Valentia), seems to give considerable support to the above 
theory. Some years ago a vessel was lost at Ceuta, on the 
Coast of Africa, and its wreck afterwards thrown up at 
Tariffa, on the European shore, full two miles west of Ceuta. 
How can this be explained, except by the action of what may 
be called a counter-submarine current, which would carry a 
body, sunk to a considerable depth, out of the Straits ? 
It was a favourite scheme of the late Mr. Tennant, to 
examine specimens of sea water from the Straits of Gibraltar, 
taken both from the surface and from some great depth, in 
order to ascertain whether the latter would have a greater 
specific density than the former, a circumstance which, if it did 
not establish the truth of the theory in question, would at least 
render it very probable. It was with a view to decide this 
point, that Mr. Tennant constructed the machine which 
* Thus it is well known that if the door of a heated apartment be partially opened, 
and two lighted candles placed the one at the top and the other at the lower part of 
the aperture, the uppermost candle will have its flame propelled outwards, by the 
rushing out of the heated and therefore lighter current, while the other candle will 
have its flame blown inwards by the opposite effect. 
