236 
MR. J. J. MANLEY ON THE APPARENT 
(y) A want of strict equality in the temperature of the contents of the two 
vessels.* 
We now proceed to a more detailed consideration of each of the above-named 
variables ; we shall also attempt to show how the errors incidental to each may be 
either overcome or eliminated. 
(6) a. Of certain Irregularities produced by Air Streams. —From the law of 
uniform decaj^, we know that the path of a vibrating pendulum will shorten by nearly 
equal steps during each successive vibration ; and therefore in a series of observations 
of the extreme positions of an oscillating balance pointer, we expect to find the same 
law operative ; in practice, however, we frequently get not uniform but well marked 
irregular decay. 
On a former occasion t I was able to show that the temperature of the air within an 
ordinary balance case is seldom or never either constant or uniform. Any differences 
in temperature, however small, tend to set up a more or less complicated and ever- 
changing system of convection currents; and these currents produce certain effects 
upon any object that is being weighed. If the volume of the object be considerable, 
then by using the most refined methods available for weighing, the existence of the 
air streams should be rendered evident by corresponding irregularities in the successive 
differences in a series of pointer readings. That such irregidarities are not only 
theoretical possibilities, but also insistent realities, may be seen by referring to the 
columns headed “pointer readings” in Table I., p. 232. In the first of those columns 
of “ pointer readings ” the successive differences have a common value, namely 2 ; it is 
suggested that the equality of the differences is due {a) to the absence of appreciable 
air streams, or (h) to the presence of two practically equal air streams acting in 
opposition, and therefore mutually compensating each other’s effects. In the third 
and fourth columns of “pointer readings” {Table I.) we observe that successive 
differences, instead of possessing a common value, range in value from 1 to 4. Any 
or all of these variations might well be brought about by— 
(1) A single air stream acting upon one pan only ; 
(2) Two or more air streams acting in unison upon both pans ; 
(3) Differential effects produced by opposition air streams. 
Probably, however, they were sometimes due to one cause and sometimes to another 
—for it is difficult to see how an ever-changing temperature, such as the one the 
existence of which I was able to prove upon a former occasion, could possibly give 
rise to perfectly steady and uni-directional air streams. 
* There is also a possibility that during sudden changes in the temperature of the room, the balance 
shelf may undergo a minute but perceptible warping and thus temporarily tilt the balance case and so 
introduce a fourth variable. As this point has been dealt with in the ‘Roy. Soc. Proc.,’ A, vol. 86, p. 598, 
we do not further allude to it here. 
t ‘ Phil. Trans.,’ A, vol. 210, p. 405. 
