308 MR. HE. H. GRIFFITHS ON THE LATENT 
if not perfect, immediately adjusted by Observer No. I. Owing to the alternative- 
circuit method previously described, only a very trifling adjustment was as a rule 
required. Observer I. had now to direct his attention to the maintenance of the 
thermal balance. This, as I have anticipated, was found at first to be matter of some 
difficulty, but practice rendered the task comparatively easy. 
The discrepancies observable in the earlier experiments (Table XI.) are, I expect, 
in some measure due to fluctuations in the value of 6, As a general rule, the 
evaporation at the start was too rapid, the pressure having been too much reduced 
by the last stroke of the pump—the galvanometer swing amounting to as much as 
— 500 or — 600 (nearly 0°11 C.). The tap T, was closed and (evaporation being 
thus prevented) the electric current was allowed to raise 6, until the + galvanometer 
swing announced by Observer I]. was about equal to the previous negative swing ; 
the time occupied by these two large oscillations being only a minute or two. T, 
was then partially opened until it was found that @, was slowly falling. When 6, 
became equal to @) the rate of cooling was further decreased and during the remainder 
of the experiment the galvanometer swing (which was called aloud by Observer II. 
every 15 or 20 seconds) rarely exceeded 50 or 60 (2.e., about 0°01 C.) and in many 
cases the difference between 6, and 6, did not exceed 0°:002 C. in the course of half- 
an-hour. 
As the whole of the aqueous vapour passing into the condenser was not at once 
absorbed by the H,SO,, the pressure slowly increased, and thus gave a further power 
of adjustment. The large bulb of the Geissler was kept vacuous and thus, by 
opening the tap T, the pressure in the condenser could at any time be decreased. 
By one rapid turn of the tap an almost imperceptible rise of the manometer M was 
caused, and a very slight difference in pressure produced a considerable alteration in 
the cooling rate; thus, if @, was rising, a single revolution of the tap T, would check 
the rise, another revolution would probably give the cooling a slight mastery. The 
operations by which the thermal balance was maintained may, as I have described 
them, appear both cumbersome and difficult. I can only say that (after the first 
three or four experiments) the control was nearly perfect and a single oscillation of 
as much as 0°01 C. would have been considered excessive. From beginning to end 
of each experiment great care was taken that each positive oscillation should be 
succeeded by a corresponding negative one. 
The electric balance was also maintained by Observer I.; but as the temperature 
of the coil did not appreciably alter throughout, this balance required but little 
attention. 
The taps T,, Ty, T;, and T,,.as well as the handles of the rheochord and all the 
electric keys, were so placed as to be within reach of Observer I. without change of 
position. The high resistance galvanometer screen was immediately in front of him, 
and also the manometer, M, whose readings were constantly observed. Near the 
commencement and end of an experiment the physical strain was great, but when both 
