Researches in Absolute Mercurial Thermor)ietry. 89 



that the two varieties of ice would give the same results, control 

 determinations were made with Nos. 2220 and 2221, the indications of 

 which could be relied upon to show differences exceeding 0*001° C. 

 The apparatus for taking the zeros did not differ materially from that 

 generally used and described by Guillaume. 



The thermometer was plunged into the ice within one or two minutes 

 after removing it from the hypsometer, whilst the bulb was still at 

 40 — 50°. The thermometer was held vertically in the hand until the 

 mercury had fallen sufficiently for the bulb to be immersed with 

 safety into the ice. 5 — 10 mm. of the stem above 0° C. were exposed 

 to the ice, adjustment to the vertical made, the thermometer raised so 

 that the image of the meniscus was just clear of the ice, and the 

 readings taken.- The stem was always well tapped. 



The indications of thermometers at the temperature of saturated 

 steam were investigated in a form of rotary hypsometer which pre- 

 sents some slight difference from that used at the Bureau Inter- 

 national. The difference consists in an improvement of the position 

 and construction of the manometer which measures the pressure excess 

 of the steam. In the Breteuil instrument the manometer keeps its 

 vertical position while that part of the hypsometer which holds the 

 thermometer may be placed in either a vertical or horizontal position. 

 This construction renders it necessary for the manometer opening to be 

 placed at some distance from the thermometer bulb, the two being 

 separated by a narrow passage through which the steam has to pass. 

 The manometer will, therefore, register too high a pressure. To correct 

 for this the steam in passing into the condensers when it is at atmo- 

 spheric pressure, is forced through an exactly similar passage, so that 

 the hypsometer pressure may be assumed to be half way between the 

 pressm-e indicated by the manometer and the atmospheric pressure. 

 Mr. Sworn gave up the convenience of having the manometer in a 

 fixed position and secured thereby greater certainty in measuring the 

 actual steam pressure at the thermometer bull). Arrangements had 

 to be made, of course, for the manometer to turn so as to keep the 

 water column vertical when the tube is placed in the horizontal posi- 

 tion. Two manometers were used, one connected with the inner 

 chamber holding the thermometer, and the other with the outer steam 

 jacket, but no difference in pressure could ever be detected. In order 

 to prevent the formation of troublesome water drops in the ma- 

 nometers, short and wide glass chambers were interposed between 

 them and the steam. It was thus ascertained that the pressure excess 

 of the steam could be kept within 0-02 mm. of mercury. 



A distillation of mercury was avoided by leaving the ast two 

 degrees of the thread unexposed until it was thought that the depressed 

 zero had attained a constant position. Tapping was always resorted 

 to, but the author has been, unable to satisfy himself that it makes any 



