12 Dr. C. Chree. ///// .s///////Mm* on 



prratiirr. />., about 7 per cent, of the fundamental interval of one of 

 the ordinal v thermometers. Thus a change of one-thousandth part in 

 tin- resistance of one of the leads would answer to an alteration of 

 aliout 0-014 in the quantity tabulated in Table III, or to an apparent 

 shift of 0'007 in the zero of an ordinary thermometer. 



There are various suggestive features in the table. Between 

 ()< tol>er 18 and November 11, 1898, the difference between two leads, 

 immaculate to the eye, altered steadily in one direction by an amount 

 equivalent to a change of 0'05 in the zero of a platinum thermometer. 

 Then, without apparent cause, there occurred a sudden alteration in 

 the opposite direction, equivalent to a change of 0"12 in the zero. 

 After this very erratic behaviour the leads seem to have remained 

 practically constant for ten days. 



Next we come to absolutely new leads of the best construction. No 

 one, I think, can question the reality of the differences between the 

 results of December, 1898, July, 1899, and September, 1899. But with 

 the occurrence of differences of the size shown during 1899, we must 

 anticipate errors of the order of at least 0"01 or 0'02 in the absolute 

 readings, unless adequate provision is made for eliminating this source 

 of uncertainty. 



Changes in the leads have unquestionably been the principal cause 

 of the apparent changes in the zeros of the platinum thermometers 

 during the investigation. These changes have added considerably to 

 the difficulty of working up the results. 



The difference between the proportional arms was recently found 

 by Dr. Marker to be less than 1/6000 of either resistance. Doubtless 

 the resistances were originally made as nearly equal as possible, so that 

 there is a strong presumption that if any differential change has 

 occurred it has been small. There is, however, unfortunately no direct 

 evidence bearing on this point. 



10. It is only proper to remark that from some points of view 

 slow, regular changes in the relative resistances either of the leads or of 

 the proportional arms are not of primary importance. Such changes, 

 at least when small, are equivalent, the one to the addition of a con- 

 stant quantity to the observed resistances, the other to the multiplica- 

 tion by a constant factor, so long as the measurements dealt with 

 cover only a short time during which no sensible variation occurs. 

 Supposing sufficiently numerous observations made of the resistances 

 R and EI in ice and steam, it is clear that the platinum temperature 

 Pt, given by 



pt = 100 (R - Ro)/(Ri - Bo), 



would remain practically unaffected. 



But, on the other hand, the necessity of frequent zero point obser- 

 vations is one of the drawbacks most frequently dwelt upon in the case 



