84 A Retrospect of Oceanography 



of compass and log which are necessary. Besides, a complete 

 practical knowledge of the craft of the seaman is required 

 for the work connected with the boats and buoys and cables. 



It is always advisable to begin operations by anchoring 

 a buoy as a fixed point to which all the observations are referred. 

 Its absolute position is determined astronomically with the 

 greatest care and repeatedly throughout the whole series, 

 in order to be quite sure that it has not drifted. Anchoring 

 the ship is not advisable, because she is wanted to follow 

 the drift-buoys with current-drags at different depths, and 

 to make other observations which are impossible when anchored 

 in a stream-way. Large current-drags suspended by wire 

 from suitable buoys, whose direction and speed are followed 

 from the ship, are the simplest means of investigating under 

 currents. Several forms of current-meter have been con- 

 structed in late years, and those of Admiral Magnaghi of the 

 Italian navy, and of Lieut-Commander Pillsbury of the 

 U.S. navy, have been used with effect by their inventors. 

 Never having had practical experience of them, I am unable 

 to give any opinion on their working. They are delicate 

 and expensive pieces of apparatus, but the object to be gained 

 is worth spending money on. Ascertaining the true direction 

 of an under current is very important and very difficult, but 

 if the investigation of the equatorial currents is seriously 

 taken up, there can be no doubt that the best instrument 

 will develop itself in the work, and will persist. 



Although there is demonstrably a transfer of cold water 

 from high latitudes along the deeper layers as far as the equator, 

 it is usually assumed that the motion is very slow so slow, 

 indeed, as to give none of the appearances which would be 

 produced by a current. Yet in many parts of the open ocean 

 we meet with rocky bottom, and it must be kept clear of 

 sediment by some agency, the most natural being a current. 

 When the rocky bottom of the ocean comes up to moderate 

 depths, as in the oceanic shoals which I had the opportunity 

 of examining in the s.s. "Dacia" in 1883, these currents, and 

 the tidal element in them, are very evident. In archipelagoes 

 like the Canary islands, which are separated by channels 



