1870.] 



Deep-sea Researches. 



209 



nistic forces acting on the boat. — The same observations apply to the 400 

 fathoms' stratum. In the First set of experiments, the boat moved with 

 the surface -current, but at little more than a quarter its rate ; and this 

 retardation, taken in connection with the distinct Physical and Chemical 

 indications that the 400 fathoms' stratum was Mediterranean and not 

 Atlantic water, might fairly be taken as evidence that the force acting on 

 the " current- drag " was antagonistic in its direction to the surface-forces 

 acting on the boat, though less powerful than in the 250 fathoms' stratum. 

 This inference also was justified by the results of the Second set of experi- 

 ments (§ 62), which showed us the boat carried westwards, though at a 

 less rate than when the " drag " hung in the 250 fathoms' stratum. — It was 

 not a little remarkable to find in both sets of observations, that the water 

 of this lower stratum is of less density than that which overlies it at 250 

 fathoms, though still unmistakably Mediterranean ; and it may hence be 

 pretty certainly inferred that the denser middle stratum is drawn by cur- 

 rent-action from some intermediate part of the Mediterranean basin at 

 which the maximum density prevails (§ 93), and that it is flowing with a 

 gradual upward inclination, so as at last to pass over the ridge at the op- 

 posite extremity of the Strait. On no other hypothesis does it seem pos- 

 sible to explain the persistence of this condition, — supposing it to be uni- 

 form, as the close conformity of observations made after an interval of six 

 weeks would indicate that it is. 



116. Although we should have been very glad to repeat our experiments 

 at some intermediate Section, yet, as our time did not allow of our carrying 

 them out in more than one other locality, we considered it desirable to 

 proceed at once to the western extremity of the Strait, where its breadth 

 greatly increases, whilst its depth is yet more than proportionally reduced. 

 As already stated (§ 66), the bottom is here characterized by great in- 

 equalities; channels of from 150 to 190 fathoms' depth existing in the 

 immediate neighbourhood of shallows of not less than 50 fathoms (see 

 Section ef). In accordance with the greater breadth of this part of the 

 Strait, the easterly surface-current flows at a much lower speed than in 

 its narrower channel ; its rate being reduced from nearly 3 miles to little 

 more than 1| mile per hour. The use of the "current-drag" at 100 

 fathoms from the surface, in a part of the channel of which the depth was 

 147 fathoms, did not indicate any reduction in this rate; but a decided 

 reduction was shown when the "drag" was lowered to 150 fathoms in a 

 part of the channel of which the depth approached 200 fathoms (§ 67). 

 As Capt. Calver deemed it inexpedient to lower the "current-drag" to a 

 greater depth, since it would have been certain to foul against the rocky 

 bottom, we were unable to ascertain by Mechanical means that the stratum 

 of water immediately overlying that bottom has an outward movement ; 

 but whilst the existence of such an outflow may be regarded as a necessary 

 inference from the existence of a powerful outward undercurrent at the 

 opposite extremity of the Strait, valid evidence of it was afforded by the 



YOL. XIX. S 



