308 



Mr. J. Y. Buchanan. 



[Feb. 11, 



as well as great patience. The necessity to have, as part of the 

 apparatus, two glass tubes which are exposed to the high pressure on 

 the inside only, introduces an element of chance into the work 

 which is sometimes annoying and sometimes exciting. It is im- 

 possible to say beforehand whether a particular glass terminal will 

 stand or not. It is necessary to be provided with a large reserve of 

 them before beginning work, and when one fails another is put in its 

 place without loss of time. Hitherto I have taken no particular care 

 of my glass terminals, because I can always depend on finding plenty 

 of them which will stand from 200 — 300 atmospheres, and there is 

 abundance of work to be done at these pressures. When, however, it 

 is desired to use higher pressures, it will be prudent to take some 

 measures for preventing the points of the wires scratching the internal 

 surfaces of the terminals. When some precaution of this kind has 

 been taken, casualties will be less frequent, and the attainment of higher 

 pressures will be merely a question of how many glass ends the observer 

 is prepared to sacrifice in the service. 



In the w T ork connected with this paper, which extended over the 

 greater part of 4 weeks, fifteen glass terminals gave way ; and oddly 

 enough, the failures were as nearly as possible equally distributed 

 between the two ends ; eight of them fell to the left arm and seven of 

 them to the right arm. The bursting of a terminal causes no incon- 

 venience beyond the trouble of replacing it, because the construction of 

 the instrument enables air to be completely excluded from it, and the 

 quantity of water in it to be kept within such limits that its resilience 

 is of no account. When a tube bursts it usually splits longitudinally 

 up the middle into two slabs. One of these almost always remains 

 entire, the other is sometimes broken into fragments, but there is never 

 any projection of material unless the instrument has been carelessly 

 put together and air admitted. 



Microseismic Effects. — In a research like the present where the primary 

 object is the numerical determination of a physical constant, the 

 secondary phenomena which reveal themselves are often of equal and 

 sometimes of greater interest, because they generally affect pre- 

 ferentially the natural history side of physics. To this class 

 belong the phenomena observed in connection with the behaviour 

 of ice under the relief of high pressure in my earlier investi- 

 gation.* In the present case the frequent bursting of the glass 

 terminals afforded the opportunity of observing another and very 

 interesting phenomenon. It is illustrated in fig. 2. It was first noticed 

 when copper wire was being experimented with. The pressure had 

 been raised to 300 atmospheres, and had begun to fall when the tube 

 gave way. On proceeding to replace the broken tube with another I 

 was astonished to find the copper wire twisted into a regular spiral 

 * 'Eoy. Soc. Edin. Trans.,' vol. 29, p. 598. 



