MEASUREMENT OF TEMPERATURE. 
221 
The value m&=10'578 is assumed in its reduction. This is a little uncertain, 
because in Observation (2) the sun had been allowed to shine on half of the barometer 
in the room upstairs, causing the high reading 74°'7 F. of the attached thermometer, 
and rendering the temperature correction uncertain ; the same uncertainty, of course, 
applies to the reduction of Observations (3), (4), (5), into which the same value of mk 
enters. A change of less than 1 in 1000 in the value of mk or Y would suffice to 
account for the deviation of this group from the curve. Observations (4) and (5) indi¬ 
cate a difference of 1° C. between the values of the temperature by air thermometer 
at constant pressure and at constant density respectively at 540° C. This may be 
similarly due in part to errors in the assumed volume or expansion of the glass, or 
merely to changes in the temperature distribution between Observations (4) and (5). 
Between 200° and 300° C. the observations seem to deviate in a regular way, as 
indicated by the dotted curve, from the parabola. That this deviation is not merely 
due to errors of observation is, I think, indicated by the curves of comparison of different 
platinum wires, which seem to show that the increase of resistance does not strictly 
follow the parabolic law, but is affected by some minor and comparatively unimportant 
variations which, from their very smallness (being of the order of 1° C.), are difficult to 
take account of otherwise than in a graphic record. Moreover, the observations in 
question were taken on different days and under different circumstances, so that it 
would be difficult to account for their agreement on any other hypothesis than that of 
some constant error affecting either the platinum or the air thermometer. 
Observations (11) and (13) differ excessively from those in their immediate neigh¬ 
bourhood. On reference to the Table A.—v., it will be noticed that they were both 
taken with the temperature varying much too fast for purposes of accuracy ; on 
the day in question the gas pressure was very variable, and it was difficult to get 
good observations at steady temperatures. Observation (11) was further vitiated by 
the fact that some of the water had leaked from the glass case surrounding the bulb A, 
and its level had been allowed to get too low. Observations (37) and (34) were taken 
at constant density, and differ from (35), (36), in the same direction and for the same 
reasons that (5) differs from (4). This apparent difference is greater than that indi¬ 
cated by theory, and may be due to some error in the value of Y, the volume of the 
bulb; further and more careful experiments are required to clear up this point. 
Observations at steady temperatures from the previous series of experiments 
(Tables A.-il, in., iv.) are entered in the diagram by means of small circles; they 
are seen to agree with the curve as well as can be expected, considering the various 
errors pointed out in each case to which the earlier experiments were liable. Had 
the observations been more complete, the agreement would probably have been more 
perfect. In order to render them comparable with the results of Series A.-v., the 
same value of the coefficient of dilatation, namely, 6 0 = 2 7 2 - 90, has to be employed 
in then' reduction in place of the coefficient d 0 = 270'l, which is used in the 
Tables A.-il and in. in the earlier portions of the paper. This anomalous value of 6 0 
