[ 63 ] 



VII. Notice of Researches in Thermometry. 

 By Edmund J. Mills, D.Sc, F.R.S* 



IN the course of some researches, commenced some years 

 since, wMeh required a series of accurate measurements 

 with the mercurial thermometer, I had occasion to make a 

 somewhat minute inquiry into the properties of that instrument. 

 The publication of the completely reduced results has been de- 

 layed by ill-health and pressure of other work ; my present 

 wish is to indicate them, as they may be of interest to those 

 who are engaged in observations of temperature. 



I. If an old thermometer be immersed in boiling water, its 

 zero descends. In the course of two or three years, at the 

 ordinary heat of the air, the zero may attain its original posi- 

 tion, subject to some slight oscillations according to the season 

 of the year. If x represent the time in months, y the remain- 

 ing depression, and (A + B) the total depression, the equation 

 to the ascent is 



A« depending on the diameter, B/3 on the length of the bulb. 

 In the case of a spherical thermometer, A is very nearly equal 

 to B. The probable error of a single comparison of theory 

 with experiment, in a fairly favourable instance, does not ex- 

 ceed 0°-01 0. 



II. For other temperatures than that of boiling water, other 

 depressions occur ; the connexion between these depressions 

 and the temperature seems to follow a compound-interest law, 

 similar to the preceding. 



III. If, however, a considerable elevation of temperature 

 be effected, then the zero no longer falls, but rises. In various 

 lead-glass thermometers this phenomenon usually commences 

 at 120°-150°, the bulb collapsing to such an extent as to raise 

 the zero sometimes 8°. At some point, which we may take 

 roughly as not less than 100° higher than the last, the zero is 

 again depressed, — as might, in fact, be expected from the then 

 sensible tension of mercury vapour, aqueous vapour, residual 

 air, and other foreign bodies in the tube. 



IV. I have made a large number of comparisons of the 

 mercurial with the air-thermometer. The maximum difference 

 between the two, between 0° and 100°, is at about 33° ; neg- 

 lecting Poggendorff's important correction (as is usually 

 done), it lies at about 50°. It is convenient to use a glass 

 helix, instead of a bulb, for the body of the air-thermometer ; 

 in this way convection of air is avoided. 



* Communicated by the Author. 



