422 
LORD RAYLPRGH ON THE LAW OF THE PRESSURE OF GASES 
From this ecjuation Vg has been eliminated and B is expressed ]jy means of P], Po, 
and the actually gauged volumes V^, Vg, V 5 — Vj', — V/. It is important to 
remark that only the differences (V 5 — V./), (V 4 , — V/) are involved. The first is 
measured on the lower part of the side-apparatus and the second on the upper part, 
while the capacity of the intervening bid!) does not appear. 
If the principal volumes and Vo are nearly in the right proportion, there is 
nothing to prevent both V 5 — V 5 ' and V^, — V/ from being very small. When the 
temperature changes are taken into account, Vg, V^., Vg are not fully eliminated, but 
they appear with coefficients which are very small if the temperature conditions 
are good. 
Thermometers. 
As so often happens, much of the practical difficulty of the experiment turned 
upon temperature. The principal bulbs were drowned in a water-bath which could 
Ije eftectively stirred, and so far there was no particular impediment to accuracy. 
But the other volumes could not so well be drowned, and it needed considerable 
precaution to ensure that the associated thermometers would give the temperatures 
concerned with sufficient accuracy. As regards the side-tube, a thermometer 
associated with its bulb and wrapped well round with cotton-wool was adequate. A 
third thermometer was devoted to the space occupied by the manometers and the 
tube leading from C to J. It was here that the difficulty was greatest on account 
of the proximity of the observer. Three large panes of glass with enclosed air spaces 
were introduced as screens, and although the temperature necessarily rose during the 
observations, it is believed that the rise was adequately represented in the thermo¬ 
meter readings. A single small gas flame, not allowed to shine directly upon the 
apparatus, supplied the necessary illumination, being suitably reflected from four 
small jjieces of looking-glass fixed to a wall liehind the glass points of the 
manometers. 
As regards the success of the arrangement for its purpose, it is to be rememhered 
that by far the larger part of any error that might arise is eliminated in the final 
residt, since it is only a question of a comparison of observations with and without 
the large bulbs. Any systematic error made in the first case as regards the temperature 
of the undrowned, capacities will be repeated in the second, and so lose its importance. 
A similar remark applies to any deficiency in the comparison of the three thermo¬ 
meters with one another. 
Comparison of Large Bidhs. 
This comparison needs to be carried out with something like the full precision 
aimed at in the final result, although it is to be noted that an error enters to.only 
