THE PRECIPITATION OF CALCIUM CARBONATE IN THE SEA, 489 



stream of water spreads itself in a fan-shaped manner over the sides 

 of the bulb, but I do not consider that any form of exhausted glass 

 flask is suitable even for depths as small as 80 fathoms. 



In Jamaica no apparatus for obtaining deep samples was available, 

 so the primitive method of lowering a sterilized stoppered bottle with 

 a string tied to the stopper was employed. At the required depth 

 the stopper was pulled out until the bottle was nearly full and then 

 allowed to fall back in place. This method can only be used for very 

 shallow depths owing to the pressure of the water at greater depths 

 making it impossible to withdraw the stopper. 



Temperature records were obtained in the Bahamas by means of 

 deep-sea reversing thermometers, specially made by Messrs. Negretti 

 and Zambra, of London. They were tested up to a pressure of three tons 

 to the square inch at the National Physical Laboratory at Teddington, 

 and a table of temperature corrections was furnished for each instru- 

 ment by the same Institution. These reversing thermometers differ 

 from ordinary thermometers in having a constriction and S-shaped 

 dilatation immediately above the main bulb, and in having a some- 

 what large secondary bulb at the upper end of the stem. The 

 graduations are reversed, so that the lowest temperature is marked 

 near the top of the capillary portion. On turning the thermometer 

 upside down, the mercury thread breaks at the constriction, and fills 

 the small bulb at the end of the capillary and also part of the capillary 

 itself. The thermometer is read in the reversed position, and when 

 certain corrections have been applied, the reading records the 

 temperature at which the thermometer was reversed. The eflect of 

 the pressure of the water is avoided by having the thermometer sealed 

 in an outer glass case. The lower end of this case is partially filled 

 with mercury in which the bulb of the thermometer is immersed, thus 

 allowing for rapid conduction of heat between the mercury in 

 the thermometer bulb and the surrounding water. An auxiliary 

 thermometer was sealed up in the same outer case as the reverser, so 

 that the temperature at which the actual reading was taken could also 

 be recorded. In order to calculate the correction that must be applied 

 to the temperature registered by the reverser, three factors must 

 be known : — 



(a) The temperature of the thermometer at the moment of reading. 



{h) The kind of glass of which it is made. 



(c) The volume, expressed in degrees of the stem, of the secondary 

 bulb and the portion of the stem below the 0° graduation. 



Of these (a) is given by the auxiliary thermometer, and (h) and (c) 

 were engraved on the back of the stem of each reversing thermometer. 



