DESCRIPTION OF THE EXPEDITION, Ze 
connection would certainly have been expected, especially if they are both considered 
to be remnants of a large continental mass which once connected 8. India to Madagascar. 
At the same time it must be remembered that, as far as the then existing soundings are 
concerned, it was still possible that a ridge or broad bank at a depth of 1500 or 1000 
fathoms might connect the 8. Maldives to the line of shallower banks between the 
Seychelles and Mauritius. 
We lay-to from the afternoon of May 17 for 24 hours to try our plankton nets and gear, 
which by that time we had got into order. We were in a belt of calm which apparently 
extended from lat. 3° 8. to lat. 5° S., so as to separate the northern monsoon from the 
southern trade-wind, the former with westerly winds and the latter with easterly. This 
weather, being particularly favourable, the ship was swung for the compass-error due to 
the iron gear on board and for the natural variation of the compass from true N. and 8. 
in position. The latter alters from time to time in all parts of the world, and it is 
essential to the navigator that he should accurately know the variation along his course, 
that he may be the better enabled to set a compass-course equivalent to the true course 
which he desires to steer. The operation requires as clear a day as possible, and, as the 
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A Medusa with widely open mouth from a deep plankton haul (JMesonema). x 1}. 
sun must not be at a high altitude, in the tropics is performed generally in the morning 
or evening. On the Sealark it was always carried out by Comm. Somerville himself, 
assisted by Lieut. Lay, his next in command. It consists in laying the ship’s head on 
each point of the compass in rotation and, when she is steady on each, in reading off the 
then directions of the sun in relation to the same on the ship’s compass; the operation 
is finally repeated in a reverse direction. The variation found here in lat. 4° 16'S. 
was 3° 41' W., and as subsequently in lat. 10°58’ S. to the north of Madagascar, less 
than 7° of lat. further south, we found a variation of 8 38’ W., the value of such 
work to practical navigation will be at once apparent. 
One of the objects of the series of plankton observations taken on this occasion was 
to ascertain the pelagic organisms found at different depths by night and day. In 
connection with the same we desired to find the depth to which the zoo- and phyto- 
plankton organisms extend. On account of their remarkably minute size we used the 
smaller nets with silk bags of 180 meshes to the inch for the latter, while for the 
animals we employed the larger nets throughout, 60 meshes to the inch. We had nets 
