92 Researches in Absolute Mercurial Thermometry. 



Kote on the above Paper. By Arthur Schuster, F.E.S. 

 Eeceivecl January 4, 1900. 



Mr. Sworn's investigations raise some questions of importance in 

 the behaviour of mercury thermometers. The irregularities which are 

 observed in the behaviour of the mercury thread of a thermometer 

 while descending have led observers to take accurate measurements 

 only in a slowly rising temperature. To avoid inconsistencies, the 

 standard temperatures ought also to be measured under conditions 

 which secure the normal formation of the mercury meniscus, which is 

 that of a rising thread. At the temperature of boiling water it is 

 supposed that this can be done by stopping momentarily the flow of 

 steam, so as to lower the temperature before bringing the mercury 

 thread to its final position. At the freezing point a difficulty has 

 always been felt about the influence of eff'ects of capillarity, and there 

 is no doubt that this is the weakest point at present in the accurate 

 measurement of temperatures with mercury thermometers. 



Mr. Sworn's investigations led him to conclude that if the fall of a 

 thermometer is slow (i.e., when the meniscus travels its own diameter 

 in about one minute), the fall is regular, and not a series of disjointed 

 steps. The difl'erence in the readings of a falling and rising ther- 

 mometer being, according to him, a constant (K), which can be deter- 

 mined by the method described in his paper, it should be possible to 

 reduce readings taken with a rising meniscus to readings with a falling 

 thermometer by simply adding K to the reading. 



Mr. Sworn's contention was that this should always be done, because 

 the freezing point is approached from above, and the boiling point 

 also, according to him, corresponds to a measurement taken with a 

 falling thread. His observations on the behaviour of thermometers in 

 the hypsometer are of considerable importance, but some confirmation 

 is required, because Guillaume describes an experiment which is 

 not in agreement with Mr. Sworn's conclusion, that the difl'erence in 

 the readings between a rising and falling thermometer disappears 

 when the instrument is suspended in steam. On the contrary, 

 Guillaume determines the amount of the difl'erence by observations 

 in the hypsometer, and states it to be between 0*002 and 0*003° with 

 the standard Tonnelot thermometers. I am inclined to think that the 

 truth lies between the two extremes, and that the efiects of capillarity 

 are still appreciable in the steam, but decidedly smaller than at lower 

 temperatures. 1 am led to this conclusion through my observations 

 at the freezing point with Tonnelot thermometers, which have always 

 made me think that Guillaume must have underrated the eflects of 

 capillarity. But e^'en granting for a moment that there is no efl'ect 

 of capillarity in the hj^psometer, I should not be inclined to accept 

 Mr. Sworn's explanation of the fact, which is that the temperature of 



