74 A Retrospect of Oceanography 



When the water is in the cylinder, its temperature is carefully 

 taken with a trustworthy thermometer, which must be divided 

 into tenths of a Centigrade degree. The thermometer is then 

 removed, and the hydrometer immersed and loaded with 

 small weights, until the water-level rises to one of the lower 

 divisions of the scale. It is unnecessary to point out that 

 the water in the cylinder must be ab rest, that the stem of the 

 hydrometer must be sheltered from wind, that everything 

 must be clean, and that the ordinary precautions usually 

 observed in every physical or chemical laboratory are to be 

 observed. Having obtained the first reading, further small 

 weights are added by steps of o-i gramme until at least nine 

 observations have been obtained. Sometimes it is convenient 

 to use the o-o5-gramme weight near the top or bottom of the 

 stem. Care is taken that the stem of the hydrometer is wetted 

 for a distance of one or two millimetres, but not more, above 

 the division where the hydrometer is going to float. This 

 is an essential precaution for ensuring precision. When the 

 last observation, which must be the one nearest the upper 

 extremity of the stem, has been made, the small weights are 

 removed, and the hydrometer lifted out and put in safety, 

 and the temperature again taken with the thermometer. It 

 should not differ from the temperature found at the beginning 

 by more than 0-3 C., and in work making any pretensions 

 to accuracy it should not exceed 0-1 C. If a difference of 

 temperature amounting to 0-3 C. is observed, and the tempera- 

 ture itself is above 20 C., then the mean temperature must not 

 be taken and used for the nine observations, but the 0-3 C. 

 must be distributed over them, and the temperature which 

 the water had at the time of each observation used. The 

 difference appears at once in the fifth place of decimals. 



Whether at sea or on land, I always log the time in my 

 laboratory work. A series of observations with the hydro- 

 meter as above described takes on an average about twelve 

 minutes; but in that time at least nine quite independent 

 observations have been made of the density of the water. 

 If a sufficient number of observations have already been 

 made with the hydrometer in distilled water of the same 



