238 
MR. J. J. MANLEY ON THE APPARENT 
two pans, together with their contents, were enclosed by means of the copper cylinders. 
Taking all the usual precautions numerous determinations of the E-.P. were now made ; 
in any one series the observed rate of decay in the amplitude of the vibrations was, 
with very few exceptions, almost perfectly uniform, and even in the exceptional cases, 
the deviations from uniformity were so small as to become negligible. The conclusion 
was therefore drawn that this device amply fulfils the functions for which it was 
designed. 
The success which attended the introduction of the enveloping cylinders is primarily 
attributed to the fact that the fluctuations in the temperature of the air within an 
ordinary balance case are so extremely small; so minute are they that all ordinary 
attempts to discover their existence must necessarily end in failure. We desire to 
emphasise the importance of this fact: for we believe that in the presence of 
abnormally large fluctuations, measures even more stringent than those adopted here 
might be required for maintaining perfect uniformity in the temperature of the air 
within the cylinders. It is only by preventing all fluctuations in the temperature of 
the air immediately surrounding the pans and stirrups that we can hope to completely 
suppress disturbing air streams. We do not, of course, assert that the disturbing 
effects of air streams were completely neutralised, but we are convinced that their 
magnitude was so far reduced that, for all present practical purposes, the assumption 
of a zero value was permissible. 
(6) /3i. Of certain Errors imoducible hy Differences in the Areas of two Reaction 
Vessels .—In the immediately preceding section is given a method whereby small air 
streams flowing in the vicinity of the balance pans may Ije suppressed ; and in the 
same section it is also stated that under the new conditions a very concordant set of 
values for the reduced E.P. may generally be obtained from any one series of pointer 
readings. It was, however, found that the mean R.P., deduced from one series of 
observations, frequently deviated very slightly from the mean R.P. value calculated 
from another series ; slightly differing values were obtained not only from day to day, 
but sometimes also from hour to hour during the same day. After many tentative 
experiments, the conviction was borne in upon us that the deviations were, in all 
probability, due to corresponding variations in the amount of moisture condensed 
upon the surfaces of the reaction vessels. 
Lanuolt and others have laid great stress upon the fact that the volumes of the 
two vessels used in any given experiment were adjusted by trial until they were not 
apprecialdy different; any difference that might exist between the two was removed 
by adding to the one possessing the smaller volume a suitable length of sealed glass 
tuhing. Now, if nothing more than a cursor}^ thought is bestowed upon the matter, 
it may easily be unconsciously assumed that as the two vessels are almost, though not 
quite, equal in volume and very similar in form, their superficial areas are, for all 
practical purposes, identical. An analysis of the actual facts may lead, as will be seen, 
to a very different conclusion. We would draw attention to several veiy important 
