1886.] 



On. connecting and disconnecting a Receiver. 



249 



barometer, and the amount of rainfall supplied to the author by 

 Professor Brioschi of the Capodimonte Observatory. 



From the discussion of these tables, it is concluded by the author 

 that there is a striking relationship between the curves which mark 

 sudden changes in atmospheric pressure and those which indicate 

 distinct variations in the volcanic activity. As regards the relation 

 of changes in volcanic activity with the lunar positions, the author 

 speaks with greater doubt, the period over which the observations 

 have extended being insufficient to justify definite conclusions ; but 

 he believes that his observations point to distinct tidal influences as 

 affecting the liquid magma beneath the volcano. 



II. " On an Apparatus for connecting and disconnecting a 

 Receiver under Exhaustion by a Mercurial Pump." By 

 J. T. Bottomley, M.A., F.R.S.E. Communicated by Sir 

 William Thomson, F.R.S. Received March 1, 1886. 



In experimental work with vacua, and especially with the high 

 vacua given by the Sprengel pump, a connecting tap has often been 

 much wished for which would enable the experimenter to remove 

 a piece of apparatus from the pump for examination or preliminary 

 experiment, and afterwards to reapply it to the pump without dis- 

 charging the vacuum. So far as I am aware nothing satisfactory has 

 hitherto been suggested. The ground glass stopcocks now made by 

 some of the German and English glass workers are undoubtedly very 

 highly finished ; but sooner or later, even with the best of them, the 

 air begins to work its way round the grinding marks, in spite of lubri- 

 cants, and, worse than this, when the apparatus under exhaustion 

 has been removed from the pump and gauges, there is no way of 

 knowing whether or not the air is leaking in round the interstices of 

 the ground glass stopcock. 



To meet this difficulty, I have recently constructed a mercurial 

 vacuum tap, which is certainly impervious to air, and which will, I 

 think, be found to work easily and conveniently. In constructing it 

 I have taken advantage of a tap described by Mr. C. H. Gimingbam 

 ( rt Proc. Roy. Soc," No. 176, 1876), by means of which a piece of appa- 

 ratus may be disconnected from the pump without discharging the 

 vacuum of the pump ; and thus by means of the complete tap, which 

 I proceed to describe, the apparatus under experiment can be separated 

 from the pump and replaced without either the pump or the appa- 

 ratus being discharged. 



The tap consists of three parts. AB is a tube containing a glass 

 float, of which the upper end is conical, and ground very carefully at 



