1C8 PROCEEDINGS OF THE AMERICAN ACADEMY 



metres, supplemented by a calibration by the observer rather than an 

 approximation by the maker.* The best form of tube for almost all 

 work is one backed with white enamel, with an inverted pear-shaped 

 bulb at the upper end of the capillary (a very important feature), and 

 with a scale of equal arbitrary linear parts (0.7 mm. to 1 mm. is a 

 suitable length for estimation of tenths) or of approximate degrees, 

 for convenience, etched or engraved upon it. 



Without reviewing here the methods proposed by various writers, it 

 may be said that it has been the general plan to select beforehand 

 upon the scale two points between which to make the calibration, 

 this space being considered the " calibration unit " ; the errors of these 

 points being, of course, zero. This plan has led to unnecessary com- 

 plexity in the resulting methods. Such an assumption is no more 

 requisite in calibration after a scale has been put upon the tube, than 

 in calibrating by the dividing-engine or micrometer before making the 

 scale. It is obvious, therefore, that the selection of these points is 

 wholly arbitrary, and, if used at all, one or both of them may. if de- 

 sirable, be chosen after the observations with the calibrating thread 

 have been made. The choice should be made with the view of facili- 

 tating the work. Hence the use of the observed freezing and boiling- 

 points, upon which some methods are based, is most undesirable. 



In the method which will now be given, either one or both of these 

 points may be left to be selected, according to the combined conditions 

 of length of thread employed, shape of the tube, and numerical con- 

 venience, after the observations with the thread have been made. 



Let it be desired to find the calibration corrections for a given tube. 

 Determinations which will give the errors of every 3 cm, of length 

 will ordinarily be sufficient, but this must depend upon the result 

 sought. Separate a thread of mercury of about that length. The 

 actual length of the thread within two or three millimetres is of no 

 consequence whatever; and hence a suitable thread can be obtained in 

 a very short time. 



Set the thread with its lower end at or near the beginning of the 

 graduation: call the reading t of the lower end of the thread /j, 



* It should be noted tliat thermometers intended for measurements above 

 about 280°C. almost always contain sufficient air to render tlie separation of a 

 thread for calibration difficult, if not impossible. The object of the air is that 

 its pressure upon the top of the column may prevent the mercury from enter- 

 ing into ebullition. 



t Tenths of a division are supposed to be read by estimation. 



