4 Mr Griffiths and Mr Clark, Note on the [Oct. 31, 



of the bridge, increased. It appears probable that some of the 

 discrepancies in the above table are due to this cause. 



A simple method of graduating platinum-thermometers is thus 

 suggested. Assuming, as we are fully entitled to do, that the 

 curve t—pt is a parabola, three points only are necessary for its 

 complete determination : the points hitherto adopted have been 

 0°, 100", and 444°"5. The necessity of guarding against loss by 

 radiation, gain by superheating, &c., when determining the re- 

 sistance in sulphur, renders it a somewhat troublesome operation 

 to those observers who are not provided with the necessary appa- 

 ratus. In cases where a high order of accuracy is not a sine 

 qua non, and where the platinum is known to be fairly pure, we 

 may assume that when t = — 273°*7 then R = : therefore, if 

 i^j and Bg are determined by direct observations in steam (760 

 millim.) and melting ice, the instrument may be considered as 

 graduated, since 



- 21^-7 -pt 



273-7 



where 



100 



+ 



273-7 



100 



pt = p ~ ° X 100. 

 it, — M^ 



As an example of the order of accuracy which could be thus 

 obtained, we append the following table, the first column giving 

 the true temperature, the others the error introduced in graduating, 

 hy this method, the various thermometers previously mentioned. 



Table II. 



Although the above discrepancies may in some cases appear 

 large, it must be remembered that we have here the whole of 

 the error in each case, and that such causes of error as non- 

 uniform bore, zero changes (both temporary and permanent), 

 unequal graduation, changes of condition, sticking, &c. are totally 



