BKTWKKN ONK ATMOSPHERE AN'!> HALF AN ATMOSPHERE OF PRESSURE. 357 



li l)eing a numericiil quantity which would be unity according to BOYLE'S law. In 

 the second pair of ojKjrutions with the tnmw nature but with a different ifiutittity of 

 gas, and with the .trtme /wvxw/vx, the mercury stands at GG throughout, and we 



have 



P,( 



whence by subtraction 



From this equation V., has been eliminated, and B is expressed by means of P,/P 2 

 and the actually gauged volumes V s V-, V, V' v It is important to remark that 

 only the different V 5 V 5 , V 4 V 4 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 bulb r/o^.v /,<>/ <t/ijH'nr. 



If BOYLE'S law be closely followed, there is nothing to prevent both V 5 V' B and 

 V 4 V 4 from being very small. Except the preliminary comparison of the 

 manometers, the whole of the data required for the verification are then contained in 

 the olervations of each set. 



When the temperature-changes are taken into account, V 3 , V 4 , V 6 are not fully 

 eliminated, but they appear with coefficients which are very small if the temperature 

 conditions are good. 



Thermometers. 



Of these four were employed. The first gave the temperature (T) of the 

 manometric columns ; the second gave the temperature (T) of V 3 ; the third that of 

 the bulb of the side apparatus (t). The temperature of the water-bath, in which are 

 contained the principal bulbs, is of course the most important. The water was 

 stirred continuously by a stream of air, and the temperature was taken by a 

 thermometer that could be read to T ^j of a degree C. No observations were begun 

 until it had been ascertained that the temperature of the water was slowly rising. 

 It is important to understand what really are the demands made upon this 

 thermometer. It was arranged that the mean temperature 6\ when the double 

 volume was in use should be almost the same as for the single volumes B z . The 

 difference was usually less than yJ D of a degree and rarely exceeded ifa or -j^. 

 Under these circumstances the use of the thermometer was practically only to 

 identify the same temperature on different days, and the actual error of its readings 

 and even of its scale of temperature were of but secondary importance. Comparisons 

 with other thermometers showed that there were no errors which could possibly 

 become sensible. The precautions necessary, in order that the other thermometers 

 should do their work satisfactorily, were indicated in the former paper. In the 

 present work the number of intervening screens was increased. 



It is desirable to emphasise that most of the errors that could arise from imperfect 



