300 



Mr. J. Y. Buchanan. 



[Feb. 11. 



column it is only the adjacent one containing 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. 1, when being 

 compared with those of the manometer used for the ordinary measure- 

 ments of the pressure in the apparatus. 



The change of apparent compressibility 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 micro- 

 meters 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 hand, one 

 division was equivalent, on the stage, to 0*0004219 inch. In the 

 one on the right hand one division was equivalent to 0'0004167 inch 

 on the stage. 



As the contractions or expansions are given directly in terms of 

 the inch, the total length of the rod is given in inches also. In order 

 to bring the ends into a suitable position for observation with the 

 microscopes the length of the rod or wire had to be not less than 75 

 or greater than 75*5 inches. The actual lengths were measured 

 exactly in each case. The average was 75*32 inches (1*913 metres). 



To facilitate the observation of the ends through the thick glass 

 tube a piece of microscopic covering glass was moistened with a drop 

 of water and laid horizontally on the tube, producing the same effect 

 as if a flat surface had been ground and polished on it. 



The effect observed and measured is the lengthening of the rod 

 when the pressure is relieved. As the compressibilit}^ of solids is 

 very small, the highest pressures have been used which were found to 

 be compatible with the reasonable persistence of the glass terminals ; 

 the usual pressure was in the neighbourhood of 200 atmospheres. 

 Very few of the glass terminals stood over 300 atmospheres. The 

 pressures actually chosen were as nearly as possible those at which the 

 manometer had been compared with the " Challenger " piezometer. 



The body under observation is in the form either of a rod or a 

 wire. If it is in the form of a rod then it is fitted with wire ends of 

 sufficiently small calibre to enable them to enter the glass terminals. 



