478 Scientific Proceedings, Royal Dublin Society. 



under high pressure. We are trying an arrangement, shown in the figure 

 at B, which so far has proved satisfactory. It consists of a rod of f inch 

 iron drilled for 1^ inch from one end to take the end of the manometer tube 

 which is cemented in with shellac. The other end is turned down in the 

 lathe for two inches to fit the tube of the rubber bag. It bears grooves to 

 hold the wire and has a narrow central passage opening into tlie larger 

 boring and thus communicating with the manometer. 



Corrections. 



In these preliminary experiments we did not consider it necessary to make 

 any correction for change in barometric pressure. If the original volume of 

 the air in the manometer tube is measured at an atmospheric pressure of say 

 750, all that would be necessary is to calculate the volume under a standard 

 pressure of 760 mm., and use this volume as a basis for calculations. Any 

 subsequent change of atmospheric pressure would be too small to affect 

 sensibly the volume reading of the compressed air. 



In the case of the lower pressures, readings can, of course, be obtained 

 accurately, but with pressures of 50 atmospheres, a rise "of about 0'4 of a 

 division is equivalent to an increase of an atmosphere in pressure. In 

 practice it is not easy to read any closer than to 0'2 of a division (i.e. O'l 

 of a millimeter), or about 0"5 of an atmosphere at 50 atmospheres pressure. 

 A sliding lens, with cross-lines, is used to facilitate readings. 



Heat. 



The correction for changes in temperature during the course of experiment 

 has to be considered under two heads : — 



(fir) Increase in volume of air column due to rise in temperature. 



{b) Expansion of mercury in iron due to rise in temperature. 



1. Correction for change in volume of air column. 



Tlie source of error is not very serious, as will be seen from the following 

 figures, which are representative of the readings actually obtained. 



Length of air column at start, . . = 60. 



Temperature, . . . . . = 10 C. 



Length of column later (high pressure), . = 2. 



Temperature at this time, . . . = 15° C. 



If not corrected for temperature, . P = 60/2 = 30 atmospheres. 



-n ^.A 5 X 60 

 If corrected for temperature, . . r = bO x — :yj^'~ 



2 

 = 30"5 atmospheres. 



