1871.] 



Theory of the Ocean. 



533 



and the maximum, or Mediterranean density of 29°, was found to be 

 constant from that depth downwards, whilst the surface showed the same 

 density as that of the Sea of Marmora, viz. 20°, the Bosphorus being 

 about 14°, and Black Sea surface 13^°, and about 15° below 100 fathoms. 

 Therefore in this part of the Dardanelles, between the surface and the 

 depth of 20 fathoms, there was an increasing density from 20°-29° as the 

 result of the intermixture in the deeps of the narrows and from the en- 

 circling eddy or return-current by the south shore in that part, as in- 

 variably occurs ; and in proportion as the density of the water increased 

 to that depth, so did the rate of the current decrease, as shown in the 

 following Table :— 



Depths. 



Density. 



Temperature. 



Current. 



Eemarks. 





o 



20 

 22 

 25 

 27 

 28 

 29 



o 



50 

 50 

 53 

 55 

 55 

 54 



Eate of 2\ knots. 

 Eate of li knot. 

 Eate of % knot. 

 Eate of \ knot. 

 Eate very slight. 

 No current. 



At twenty miles 

 westward of the 

 Dardanelles,inthe 

 .ZEgean, the sur- 

 face-density was 

 the same as the 

 Mediterranean. 





10 fathoms 





20 fathoms 







The plan I adopted for ascertaining whether any such undercurrents 

 existed, as well as of the rate of the surface-current in descending depths, 

 was as follows : — 



A suspended sinker, or current-anchor, was made in several forms to 

 test the most simple and effective form. Sometimes a vertical cross was 

 used, which was formed of boards, so as to offer a resisting surface every 

 way when hanging vertically with the weight or lead attached to its base. 

 More generally a large weighted disk or enlarged log-ship was used, and 

 when weighted with a weight consistent with size of the disk, and also of 

 the line alid float, was slung like a kite, the weight being in the place of the 

 tail. When thus slung it would of course hang nearly vertical in all 

 depths, and offer a sufficient resistance to prevent its being moved by the 

 friction of the surface-current upon the float used to suspend it. It will 

 be thus seen that the operation of testing slow currents in the deeps is one 

 of great delicacy, and therefore requires great nicety and care in the mode 

 and apparatus for doing it, for scientific dependence and aims. 



The float that, after many experiments, I found to answer best was 

 one made of thin copper or block-tin, in the form of an elongated air- 

 tight cylinder, 4 or 5 feet in length, and pointed at one end to offer least 

 resistance to the surface-current passing it. The other end was truncated 

 or flat, where two loops were fixed in which a small rod or staff with a flag- 

 could he placed, to render its position conspicuous, without adding to its 

 resistance or weight. The suspended kite, or current-anchor, was then 

 weighted to about one-third of what the float would bear in perfectly still 

 water, without being wholly immersed. 



