I >r. ( '. < 'lnvf. / 



boiling points; lut an undetected change of zero in the barometer, or 

 SIM alti-ration in the method of reduction, would simulate a change in 

 K| and K,, the same for all the thermometers. 



After the first few observations the Royal Society's old double tul>e 

 iiHMvury barometer repaired some years previously by Messrs. 

 ;ti and Zambra was set up in the platinum thermometer room, 

 and it has since then l>een used in all the steam and sulphur point 

 o]ervations. A recent comparison between it and the Observatory 

 standard barometers shows that no certain change of zero has occurred 

 since it was set up. The surface of the mercury in the cistern of the 

 barometer is about 6 inches lower than the surface of the water in 

 the hypsometer, and about 11 inches lower than the surface of the 

 boiling sulphur. No allowance has been made for these differences of 

 level, as they would introduce extremely small and practically constant 

 errors in the two boiling points. 



The same reduction tables have been in use throughout, and the con- 

 stant factor 1 '0006 has been applied in the reduction to gravity at 

 latitude 45. Until standard gravity is more exactly defined, and 

 relative values of gravity more carefully determined, the accuracy of 

 the reduction is open to question, hut for the purposes of the present 

 inquiry the uncertainty is quite immaterial. 



As to the accuracy of the individual barometer observations, I need 

 only say that readings by Mr. Hugo and myself seldom differed by 

 more than O001 inch. The instrument has a tube of 0'6 inch in 

 diameter and responds rapidly to alterations of pressure, so that no 

 serious error need be apprehended from lag. 



fif in tin 1 S"Jj>hnr t c. 



34. The sulphur made use of in the experiments has all been 

 obtained from 'Messrs. Baird & Tatlock; it has been produced by 

 Chance's process. Though several supplies have been obtained, at 

 different times, no discontinuity has l>een observed in the sulphur 

 points; thus, unless our experience has been unduly favourable, 

 sulphur has at least one great merit as a medium for supplying a fixed 

 high temperature. There are, however, certain disadvantages which 

 have introduced some variability into the conditions of experiment. 

 After an experiment the sulphur solidifies in the tube containing it, 

 and the remelting it on a subsequent occasion not infrequently ends in 

 cracking the tube. On one or two occasions a crack went right round 

 the tube, at a considerable height above the bulb, after the sulphur had 

 been melted and experiments were progressing, and on other occasions 

 the tube was found to have cracked a day or two after the sulphur had 

 cooled. Of one batch of tul>es, of thicker glass than usual, several Hew 

 into pieces after the sulphur had liquefied, and the observer was lucky 



