32 Drs. Ramsay and Young on 



The advantages which this apparatus appears to possess 

 over the usual forms are obvious. It is simple, compact, 

 easily made, and not easily put out of order. There is no 

 lateral stress, and in consequence of the rubbing action the 

 contact is always good. The deflections of the needle of 

 a " dead-beat " galvanometer in circuit with it are under 

 perfect control, responding to the rotation of the cylinder 

 with smoothness and regularity, and there is complete free- 

 dom from jerks and retrograde movements. It is not indeed 

 more suitable for use as a measuring instrument than the older 

 forms ; but when it is desired to adjust a resistance to a 

 nicety, or to cause a continuous variation of a current, it is of 

 great utility. If the interposed resistance is required to 

 be known with accuracy, it should be measured by the bridge- 

 method in the ordinary way. 



III. Some Thermodynamical Relations. — Part IY. 

 By William Ramsay, Ph.D., and Sydney Young, D.Se* 



SINCE the first portions of this research were published 

 Professors Ayrton and Perry have criticised our work, 

 and while we would thank them for the appreciative way in 

 which they speak of our labours, we must differ entirely from 

 their conclusion that " there is nothing further to be said 

 about the four laws in question." This conclusion rests on a 

 complete misapprehension of the whole scope of our papers, 

 and for the obvious reason that, while we have endeavoured 

 by a method of what may be termed approximation to arrive 

 at a workable plan of deducing from the known vapour- 

 pressures of a substance like water the unknown vapour- 

 pressures of any other substance with as little expenditure of 

 experimental work as possible, and have in our research made 

 the statement, which is only a very rough approximation to 

 truth, that the ratios of the absolute temperatures of any two 

 liquids at any given pressure are equal to the ratios at any 

 other pressure, but have materially modified this statement, 

 and asserted next, what is a very close approximation to truth, 

 that when the statement given does not hold (as in by far the 



affected by moisture. The depth of the groove should be about half the 

 diameter of the wire. The wire should be annealed, and should be 

 wound on when it is warm. The notch at the end of the copper pin 

 may be very shallow, for if the position of the spring is properly adjusted 

 the pin has little or no tendency to slip off the wire. 



* Communicated by the Physical Society : read May 8, 1886. 



