376 On the Compressibility of Solids 



atmospheres. At great depths a very slight correction has to 

 be made ; the nature of this will be apparent from the following 

 table, in which, for different depths in metres, D, the pressure, P, 

 in atmospheres is given: 



D 1400 2000 3000 4000 5000 6000 



P 139-96 200-14 300-82 401-98 503-62 605-75 



Owing to the preponderance of water of low temperature 

 and of very uniform salinity in a vertical column of water in any 

 part of the open ocean, the pressure exercised by it per thousand 

 metres does not differ appreciably from TOO atmospheres. 



Inspection of Table I shows at once in which column the 

 true value of the apparent compressibility is most likely to 

 be found. It is the one which includes values between 49 and 

 50. Outside of this column it is only the adjacent one con- 

 taining values between 48 and 49 which enters into competition. 

 The mean of all the values in these two columns is 49-16, and 

 this figure forms the basis of the measurement of pressure in 

 this investigation, and it is used in interpreting the pressure- 

 value of the readings of Piezometer C. No. I, when being com- 

 pared with those of the manometer used for the ordinary 

 measurements of the pressure in the apparatus. 



The change of apparent compressibility per million of water 

 with change of temperature for the small range of temperatures 

 with which we are concerned was found in 1880 to be at the 

 rate of 0-33 per degree (Celsius), and this figure is used in the 

 present research. 



Micrometers. The same microscopes and micrometers, 

 which served in 1880, were again used in this research. Their 

 value was determined by reference to a stage micrometer, 

 ruled into hundredths and thousandths of an inch. This was 

 then verified at the National Physical Laboratory. The changes 

 of length measured by the micrometers are therefore given in 

 terms of the standard inch ; and, it may be added, the values 

 attached to the readings of the micrometers in 1880 were 

 exactly the same as those now found by reference to the 

 standard of the National Physical Laboratory. 



In the microscope which was always placed on the left 



