INSULATING WATER-BOTTLE 37 



may be used. It is not often, however, that a surface 

 reading can be trusted to less than -^ C. ; and the 

 uncertainty is generally far greater. The most useful 

 kind is that known as a " milk-glass scale " ther- 

 mometer ; the graduations are marked on a strip of 

 white glass, which is protected from the water by an 

 outer glass tube. These thermometers have no metal 

 frame round them, and as they can easily be cleansed 

 from salt it is allowable to use the same bucket of 

 water for reading the temperature and bottling a 

 sample. A very useful pattern is that used by the 

 British Meteorological Office ; the glass is protected 

 by a metal frame, and is not easily broken, but as the 

 bulb is surrounded by a small bucket, which may hold 

 dried salt, a fresh lot of water should be used for the 

 collection of a sample. 



A very large number of water-bottles have been 

 devised for the collection of samples of water below 

 the surface, and the description will therefore be 

 confined to that with which the authors are practically 

 acquainted. For moderate depths, by far the best 

 instrument is the Nansen-Pettersson insulated water- 

 bottle, which has been made in various sizes, and can 

 be closed either by a " messenger " or by a propeller. 



The Insulating Water-Bottle. 



The Nansen-Pettersson insulating water-bottle, in its 

 latest form, is an instrument by means of which it is 

 possible to lower an ordinary thermometer, enclosed 

 in an outer tube to protect it against pressure, to the 

 desired depth, and there to surround it by a number of 

 envelopes of the w r ater to be examined. It can then 

 be hauled up through layers of widely different tem- 

 perature without the thermometer showing any ap- 



