370 THE POPULAR SCIENCE MONTHLY. 



researches that the whole scale be simply a millimetre one, and care 

 only be taken to have the millimetre graduation extremely accurate. 



The dividing of the tube so that an equal volume of mercury may 

 occupy the same number of degrees at the various parts of the tube is 

 called the calibration of a thermometer, and on the pei-fection of this 

 work, if it is attempted at all, largely depends the value of the ther- 

 mometer. As Fernet has remarked, the labor of determininsf the 

 errors of a thermometer is much increased by having to determine the 

 errors the maker has introduced in the imperfect calibration of its 

 scale. In observations not requiring an accuracy beyond 0-1 F., it 

 it might be quite safely left to the skill of a reputable maker to free 

 the instrument from errors of this kind. It is accomplished by de- 

 taching a small portion of the column and measuring its length at dif- 

 ferent, and usually consecutive, parts of the tube. Obviously fron-i 

 these results may be computed the value of 1 at successive parts 

 of the thermometer scale, in terms of the dividing engine used by 

 the makei'. 



The precision attained in the calibration of standards when the 

 greatest care is exercised is surprising ; thus, in the three Kew stand- 

 ards of the Yale Observatory, the maximum sum of the errors depend- 

 ing on imperfect calibration is very nearly 0.01 in each of them. 



Supposing that several thermometers, by different and equally skill- 

 ful makers, have been prepared with the greatest care, it is found in 

 comparing them that they differ sensibly among themselves, owing to 

 the difference in the glass used in their construction, their varying 

 sensitiveness to the slight changes caused by the cii'culation of the 

 water in which they are immersed, and a variety of less obvious causes. 

 It becomes necessary, therefore, that some definite construction of the 

 mercurial standard thermometer should be adopted, and the standard 

 chosen by the Yale Observatory is defined upon the certificates issued 

 with standards compared, as follows : 



"The theoretical mercurial standard thermometer to which this 

 instrument has been referred, is graduated by equal volumes upon a 

 glass stem of the same dimensions and chemical constitution as the 

 Kew standards 578 and 584. The permanent freezing-point is deter- 

 mined, by an exposure of not less than forty-eight hours to melting 

 ice, supposing the temperature of the standard has not been greater 

 than 25 Cent. = 77 Fahr. during the preceding six months. The 

 boiling-point is determined from the temperature of the steam of pure 

 water at a barometric pressure of 760 mm. = 29*922 inches (reduced to 

 Cent.) at the level of the sea and in the latitude of 45." 



This standard has its and 100 C. identical with the standard of 

 the International Commission of Weights and Measures, and the physi- 

 cists generally have agreed upon the pressure and latitude given as 

 the most advisable. It is practically coincident with a pressure of 

 29 "905 inches in the latitude of London, and at the sea-level the con- 



