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The Hydrometer is a bottle like the Pyknometer, 

 only the volume of liquid displaced, or excluded, up to 

 a certain mark is weighed, instead of that included up 

 to a similar mark ....... 66 



Comparison of observations made with the Hydro- 

 meter in the Prince of Monaco's yacht in bad weather 

 and those made on shore on the same samples . . 69 



Summary of observations made with the Hydro- 

 meter during the voyage of the "Challenger" . . 72 



Importance of logging the time in all experimental 

 work, whether at sea or on land ..... 74 



Motion of the waters of the Ocean. In the ordinary 

 ship's log the positions by observation and by dead 

 reckoning are always entered, and the difference 

 between them is usually taken as the integral effect of 

 the non-tidal current over the twenty-four hours. Such 

 observations made in sailing ships by competent 

 navigators are of great value; but the modern steamer 

 passes too rapidly through any current for it to give a 

 reliable measure of its movement . . 79 



The "Challenger" made all her passages under sail; 

 she was, therefore, a sailing ship and much information 

 about currents was obtained from her daily reckoning. 

 But many observations were made from boats anchored 

 in mid-ocean, which gave data of a kind not before 

 obtained ......... 81 



The use of bottles or floats such as those used by 

 the Prince of Monaco, give very valuable results . . 82 



Currents measured by the author on board the 

 "Buccaneer" separated the tidal effect from that of 

 simple translation ..... 83 



Direct observation of currents of considerable 

 strength in the deep waters of the ocean obtained by 

 the author in the "Dacia" in 1883 ... 84 



The surface and undercurrents in the equatorial 

 region of the Gulf of Guinea were determined by 

 Captain Thomson of the "Buccaneer". ... 85 



No. 3. ON SIMILARITIES IN THE PHYSICAL GEOGRAPHY OF THE 

 GREAT OCEANS. [From the Proc. of the Royal 



Geographical Society, 1886, Vol. vm. pp. 753-769] 87 



A lecture, delivered before the Royal Geographical 

 Society in the year 1886. Its purpose was to direct 



