146 EEPOET — 1882. 



depend upon tlie measurement of tte length of colamns of mercury in 

 various parts of the tube, it is important to decide upon the best method 

 of separating these ' threads ' (as they will hereafter be called) from the 

 main mass of mercury in the tube and bulb. 



In general it is in the first place necessary to make the mercury run 

 from the bulb into the tube, so that a column of any length can be obtained 

 when the thermometer is at the ordinary temperature. To eifect this the 

 instrument should be held in a vertical position with the bulb uppermost, 

 and the other extremity of the tube should be cautiously tapped against 

 the table. If the mercury does not start it is well, as a preliminary, to 

 heat and cool the bulb several times. Tolerably rapid changes of tempera- 

 ture of a few degrees will often suffice. 



The transference of the mercury from the bulb to the tube causes a 

 vacuum bubble to appear in the former, and many writers, among whom 

 A. von Oettingen, in a work often hereafter to be cited (' Ueber die Cor- 

 rection der Thermometer insbesondere liber Bessel's Kalibrir-Methode,' 

 Dorpat, 1865, p. 46) may be mentioned, recommend the separation of a 

 column of the desired length by the proper manipulation of this bubble 

 and a system of dexterous jei-ks. In a wide bore, if the bubble is brought 

 to the junction of the tube and bulb, it is possible by a jerk to effect a 

 disruption between the mei'cury contained in the one and the other re- 

 spectively. It is, however, exceedingly difficult, if not impossible, to apply 

 this method satisfactorily to thermometers with narrow bores, and the 

 violent jerks which are required are extremely dangerous to the instru- 

 ments. 



In all the experiments undertaken by the Committee, therefore, the 

 separation of the thread was effected by heat. The use of a blow- 

 pipe flame for this purpose has been recommended (Wiillner, ' Lehrbuch 

 der Physik,' vol.iii. p. 14), but such a course is unnecessarily violent and 

 risky. Instead of this, a very small flame, four or five millimetres in 

 height, was used, obtained from the gas issuing from a narrow oi'ifice at 

 the extremity of a piece of glass tubing drawn out fine. Into this the 

 thermometer was introduced, care being taken to heat the tube equally all 

 round, and the rupture was effected at the point where the heat was ap- 

 plied. It is easy thus to break ofi" threads to within a millimetre of the 

 leno-th aimed at. When great accuracy is important it is advisable first 

 to break oS" a thread longer than that required, and then to separate from 

 it a portion of the desired length. Greater steadiness of the mercurial 

 column while the thermometer is being heated is thus attained. Many 

 dozens of threads have been broken off in this way within the experience 

 of members of the Committee without a single breakage of the instruments 

 employed. 



(3) Before proceeding further it will be convenient to define certain 

 terms which will be frequently used. 



The upper and loiuer ends of a thread are those furthest from and 

 nearest to the bulb respectively. 



The point at which the lower end of a thread is placed when a measure- 

 ment of its length is made, is called the initial point. 



When by any method the corrections are determined only for a number 

 of selected (and in general approximately equidistant) points, the points 

 so chosen are termed -principal points. If, when the tube has by means 

 of a svstem of principal points been divided into a number of parts the 

 relative volumes of which are known, these parts are further subdivided 



I 



