78 A Retrospect of Oceanography 



the more valuable does it become. The repeated observations 

 in distilled water increase the knowledge of its displacement, 

 and when many observations have been made in distilled 

 water or in sea-water, and the single observations of each 

 series are compared with their mean, a calibration of the stem 

 is obtained which no other process can give. This is exemplified 

 by the table on p. 77, where the observations made on different 

 expeditions have been classified according to the stem readings 

 and their difference from the means in units of the fifth decimal 

 place. 



In the case of hydrometer No. 14, it will be seen that the 

 densities observed on the lower half of the stem are too high, 

 and those on the upper half are too low. This points to the 

 probability that the stem is slightly tapered, being thicker 

 in the lower half than in the upper. It also furnishes a correc- 

 tion which could be applied to future observations. The 

 other instruments also show probable very slight inequalities 

 in the calibre of their stems. 



For observing on board ship, I find that the method used 

 in the "Challenger," of placing the cylinder on a swinging table, 

 gives better results than any other. In the "Princesse Alice," 

 a ship of not more than 600 tons, the motion at sea is always 

 considerable, but in ordinary circumstances the maximum 

 amplitude of the motion of the floating hydrometer was not 

 over 3 millimetres. The vibration period of the hydrometer 

 always interferes with that of the ship and swinging table, 

 producing moments of rest, and my experience is that a moderate 

 rate of motion is an advantage. The individual readings of 

 a series made under favourable circumstances at sea generally 

 agree more closely with the mean than is the case with a series 

 made under similar circumstances on land. The limit to the 

 amount of motion with which trustworthy observations can 

 be made does not depend on the hydrometer, but on the 

 observer. When the motion goes beyond a certain amount 

 of violence, the observer's attention is entirely taken up in 

 looking after his own stability, and in preventing his coming 

 into collision with the swinging table. 



In the "Princesse Alice," I frequently compelled myself 



