33 
NATURE 
[ Fed. 23, 1871 

fathoms it was 55°°7; at 150 it was 55°°5 ; and from that depth 
to the bottom, at 517 fathoms, there was 20 further descent. 
Now, it will be shown hereafter that the thermal condition, 
which here so much surprised us by its contrast with that of the 
Atlantic waters, is that universally met with in the Mediter- 
ranean, the temperature of which, whatever may be its surface- 
elevation, falls to within 1° Fahr. above or below 56° at a 
depth of 50 fathoms, to a degree lower at 100 fathoms, and then 
remains uniform down to the greatest depth (1,743 fathoms) at 
which we examined it. And it thus appears that whilst the 
surface-water in this part of the Strait is certainly derived from 
the Atlantic, the deeper water, partaking of the thermal condition 
which so remarkably characterises that of the Mediterranean 
basin, may be fairly regarded as belonging to the latter. 
This inference is in harmony with another fact ascertained 
on the same occasion, viz., the great excess in salinity shown 
by water brought up from the depth of 250 fathoms over 
the water of the surface. Whilst the specific gravity of the 
latter was found to be 1'0271, that of the former was 1°0293 ; 
and whilst the proportion of chlorine in the latter was 20°034 
per 1,000, it was 21°775 in the former. Now in these par- 
ticulars the surface-water agreed well with what had been found 
to be the condition of the water of the Atlantic, whilst the 
water at 250 fathoms agreed equally well with what proved to 
be the condition of the water at the like depth in the adjacent 
part of the Mediterranean. We were not a little surprised, how- 
ever, to find that the water taken from the dottom (517 fathoms) 
was of much /ess density, as indicated both by specific gravity 
and by chlorine percentage, than that of the intermediate 
stratum ; its specific gravity being 1’0281, and its proportion 
of chlorine 21°465. This apparent anomaly (the existence of 
which was confirmed by observations made on our return 
voyage) pointed to the existence of an out-current in the zyer- 
mediate stratum as the probable explanation of the over- 
laying of the lighter by the heavier water. The specific gravity 
ofthe éoffom-stratum closely corresponded, as we subsequently 
found, with that of the bottom-water over the deepest part of the 
area of the western basin of the Mediterranean. 
These data having been obtained by the examination of the 
several parts of the vertical column at one and the same point, 
and this point being in the centre nearly of the narrowest part 
of the Strait, and at the deepest part of the channel, we pro- 
ceeded to test the actual movement of water on the surface and at 
different depths beneath it. 
The rate of szrface-movement was easily determined. The 
precise position of the ship having been ascertained in the manner 
already stated, a small flat basket, presenting no such elevation 
above the water as would cause it to be influenced in any consi- 
derable degree by a moderate wind,” was sent adrift, so as to be 
freely carried along by the current ; it was allowed to float for a 
determinate time, throughout which it was followed by the ship, 
and when it was taken up at the expiration of that time, the place 
of the ship was again ascertained as before. The space between 
the two points being then determined trigonometrically, the rate 
of the flow per hour, and its precise direction, could be readily 
calculated. Thus on the morning of August 15th the float was 
followed by the ship for fifteen minutes, during which it was 
found to have moved 4,377 feet in the direction E. by S. $ S.,or 
at the rate of 2°88 miles per hour. 
For the determination of the movement of the water at 
different depths below the surface, a current-drag had been con- 
structed by Capt. Calver on a plan suggested by his previous ex- 
perience ; which had led him to the conclusion that a submerged 
basket lined with sail-cloth, which of course fills itself with 
water, presented a better resisting surface than any vessel of wood 
or metal. Such a basket being made the basis (so to speak) of 
the apparatus, its resisting surface was augmented by fixing two 
pairs of arms at right angles to one another across its upper end, 
and stretching a piece of sail-cloth between each arm and the 
side of the basket, which device caused a uniform resisting sur- 
face to be presented to the current whatever the manner in which 
the sails might meet it. To the lower part of this ‘‘drag” a 
couple of sinkers, of 112Ilb, each, were attached; and the 
whole apparatus was supported by cords meeting in a ring above 
it, to which the suspending line was secured. 
* It is obvious that the movement of the ship itself would be liable to be 
considerably affected by even a slight breeze, on acc unt of the large surface 
of resistance presented by its transverse section (e-pecially by its paddle- 
boxes) above the water. This would cause its drift to be ore rapid than 
the curren:, if the direction of the wind should be zh that of the current, 
and Zess rapid if the wind should be opposed to it. 

This ‘‘current-drag” having been transferred to a_ boat, 
was lowered down by a couple of men placed in her, to the de- 
sired depth; and the boat was then left entirely free to move, 
being lightened by the return of the men into the ship. The 
motion of the boat would be the composite result of (1) the 
action of wind (if any) upon the tranverse section of the part of 
the boat above the water ; (2) the action of the surface-current 
upon the transverse section of the immersed part of the boat ; 
(3) the action of the upper current upon the suspending line ; 
and (4) the action of the current in which the ‘*drag”’ is sus- 
pended, upon the drag itself. Putting aside the jist of these 
agencies, which will be of very little account if (as in the ex- 
periment now narrated) the boat be small and the breeze be 
light, it is obvious that the relative influence of the second and 
third to that of the fourth will depend upon the proportion 
between the surfaces presented by the boat, the line, and the 
‘*drag ” respectively, and the strength of the current acting upon 
each. The surface given to the ‘‘drag” being larger than that 
of the boat and line taken together, the force acting on the 
“‘drag” will dominate, if it hang in an opposing current 
superior, equal, or even somewhat inferior in rate to that which 
acts on the boat and line, so that the boat would be carried along 
by the drag against the surface stream at a rate proportioned to 
the excess. If, again, the rate of the under-current should be 
greatly inferior to that of the surface, its action upon the *‘ drag” 
might still be sufficient to neutralise that of the surface-current 
upon the boat and line, and the boat would then remain staéionary, 
or nearly so, A still further reduction in the rate of the op- 

Fic. 1—CU. RENT DRAG 
posing under-current would make its action upon the ‘drag 
dess powerful than that of the surface-current upon the boat and 
suspending-line, and the boat would then move wt the surface- 
current but ata rate of which the great retardation would indicate 
an antagonistic force beneath. Supposing again, the water of 
the stratum in which the ‘‘ drag” is suspended to be stationary, 
the action of the surface-current upon the boat and line would be 
opposed by the resistance offered by the deeper water to the 
movement of the drag ; and the retardation of the movement of 
the boat would be less, though still considerable.—If, again, the 
stratum in which the ‘‘drag” is suspended should itself be moving 
in the direction of the surface-current, but at a reduced rate, there 
will still be a resistance to the movement of the ‘‘drag” at the 
more rapid rate of the surface-current ; and this resistance will 
produce a proportional retardation in the motion of the boat. 
Finally, if the stratum in which the ‘‘drag” is suspended, with 
the intermediate stratum through which the suspending line 
passes, move at the same rate with the surface-current, the motion 
of the boat with the whole suspended apparatus will have the 
same rate as that of the simple float. : 
Putting these respective cases conversely, it may be 
affirmed (1) that if the boat, having the ‘‘current-drag” sus- 
pended from it, should move zh the surface-currént and a¢ the 
same rate, the stratum in which the ‘‘drag” hangs may be pre- 
sumed to have a motion nearly corresponding with that of the 
surface-current ; (2) that if the rate of movement of the boat 
with the surface-current should be veéarded, a diminution of the 
