42S 
MR. E. H. GRIFFITHS OH THE VALUE OF 
The correction in millimeters is given with sufficient accuracy by the formula 
85 (^1 - 19-3) X -00016, 
and as the mean value of 1 millim. is 0°’025 C., the resulting correction is 
85 { 6 ^ — 19-3) X -00016 X 0°-025 C. (a). 
This amounts in the greatest case to 0°-002 C. 
The readings of P were taken by a microscope (re-levelled before each observation) 
which was fitted with a micrometer scale. The one hundredth of a degree Centigrade 
was thus read directly and no difficulty was experienced in estimating the one 
thousandth, the scale being an unusually clear one, made by the Cambridge 
Scientific Instrument Company. 
The thermometer was placed in a tube, open at the lower extremity, and an 
opening at the upper end was connected with a rubber tube leading to the waste 
pipe. This arrangement was used as a siphon and the thermometer stem was thus 
maintained at the bulb temperature, and, therefore, as the mercury column rose, any 
alteration in the stem immersion was rendered unnecessary. 
In order that the readings might be obtained under the exact conditions observed 
by Dr. Guillaume, I implicitly followed the instructions given in a printed note 
added to the certificate. After the tank had arrived at the required temperature 
at least half-an-hour was allowed to elapse before taking a reading, and immediately 
after the observation the thermometer P was transferred to ice, and a series of readings 
taken to obtain the greatest zero depression. The admirable methods adopted by the 
Bureau International are less known, or, if known, are less practised in this country 
than they deserve to be. I therefore give one example in full showing the various 
corrections. 
