OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 39 



reservoir the capillary is constricted and branched, having a small 

 closed arm, and above this the thermometer tube is bent in a loop, from 

 which it continues straight and terminates in the smaller bulb. The 

 thermometer is so constructed that in the set position the mercury fills 

 the reservoir, the capillary, and part of the bulb. The amount of 

 mercury above the constriction depends upon the temperature, and, when 

 the thermometer is reversed, by turning through 180 degrees, the mercury 

 column breaks at the point of constriction and runs down, filling the 

 bulb and part of the graduated capillary, and thus indicating the temper- 

 ature at reversal. The loop in the capillary, which is generally of 

 enlarged diameter, is designed to trap any mercury that is forced past the 

 constriction if the temperature is raised after the thermometer has been 

 reversed. In order to correct the reading for the changes that result 

 from differences between the temperature at reversal and the surrounding 

 temperature at the time of reading, a small standard thermometer 

 known as the auxiliary thermometer is mounted alongside the reversing 

 thermometer. The reversing thermometer and the auxiliary thermome- 

 ter are enclosed in a heavy glass tube that is evacuated except for the 

 portion surrounding the reservoir of the reversing thermometer, which is 

 filled with mercury to serve as a thermal conductor between the sur- 

 roundings and the reservoir. Besides protecting the thermometer from 

 damage the tube is an essential part of the device because it eliminates 

 the effect of hydrostatic pressure. 



Readings obtained by reversing thermometers must be corrected for 

 calibration errors and for the changes due to differences between the 

 temperature at reversal and the temperature at which they are read. An 

 equation developed by Schumacher is commonly used for this purpose: 



^^ ^ Rr - t)(r + 7o) "[ L _^ (r + t)(r + Vo) '\ 



+ /. 



Here AT" is the correction to be added algebraically to the uncorrected 

 reading of the reversing thermometer, T'. The temperature at which 

 the instrument is read is t, Vo is the volume of the small bulb and of the 

 capillary up to the 0°C graduation in terms of degree units, and X is a 

 constant depending upon the relative thermal expansion of mercury and 

 the type of glass used in the thermometer. For most reversing ther- 

 mometers, K equals 6100. The term I is the calibration correction, 

 which varies with the value of T'. Where there are large numbers of 

 observations to be corrected, it is convenient to prepare graphs or tables 

 for each thermometer from which the value of AT can be obtained for 

 any values of T' and t, and where the calibration correction is included. 

 Reversing thermometers are generally used in pairs. The frames that 

 hold them are brass tubes which have been cut away to make the scale 

 visible and which are perforated around the reservoir. To hold the 



