Chemical and Physical Notes 93 



that which governs the behaviour of thermometers in air. 

 The thermometer loses equal fractions of its excessive heat 

 in equal intervals of time. These intervals are very much 

 shorter when the instrument is immersed in water than when 

 it is in air. When the difference of temperature is at all 

 considerable the thermometer falls very rapidly at first, and 

 more slowly as it approaches to the temperature of the water. 

 The divisions of the scale of a thermometer ought to be 

 about one millimetre apart. If in reading the thermometer 

 we estimate tenths of this amount, then it is important to 

 know how long the thermometer takes to assume the tempe- 

 rature of the water within one-tenth of one of its own divisions. 

 If we estimate only to one-half of a division, then it is 

 sufficient to know how long the thermometer takes to arrive 

 at that of the water within one-half of one of its own divisions. 

 It does not matter what the thermometric value of each 

 division is. The following imaginary case will illustrate this. 

 It is assumed that when immersed in still water the thermo- 

 meter loses half its excessive heat in twenty seconds. When 

 immersed in the water the excessive temperature of the 

 thermometer is represented by 4/8 of its own divisions. 

 Thus we should have: 



Excess temperature, divisions 4*8 2*4 i'2 o'6 0*3 0*15 0^075 

 Time elapsed, seconds ... o 20 40 60 80 100 120 



In this imaginary case the temperature of the thermometer 

 will have attained that of the water in 120 seconds, or two 

 minutes. 



When the water touching the thermometer is moving, 

 whether the water runs past the thermometer or the thermo- 

 meter runs through the water, then the time which the 

 thermometer requires to lose its excessive heat is very much 

 shorter. In actual sounding practice the thermometer arrives 

 at the depth at which it is to register the temperature, having 

 already very nearly the temperature of the water at that 

 depth. It is, therefore, generally speaking, quite safe to 

 despatch the messenger, which is to overturn the thermometer, 

 so that it may arrive at the required depth not later than one 



