Researches in Absolute Mercioricd Tliermometry. 



91 



momentary alterations in temperature or pressure not registered by 

 the manometers. I am personally inclined to think that we are 

 dealing in the hypsometer with steam under what I might term 

 oscillatory conditions of temperature and pressure, the effect of which 

 is to reduce all the steam indications of the thermometer to what they 

 would be with a receding menisous. Within narrow limits the mercury 

 may advance along the tube, but of necessity there will subsequently 

 be a capillary force erected which will, within the same narrow limits 

 (viz., K), prevent its return. 



Comparison of Thermometers.- — The apparatus in which the compari- 

 sons were conducted consisted of a cylindrical tank surrounded, except 

 at the top, by a jacket kept at constant temperature, by a circulation 

 of water heated in a thermo-regulator. The capacity of the tank was 

 5 litres, and it could be heated or cooled independently, and its 

 temperature set a few degrees above or below that of the jacket. The 

 contained water would then heat or cool at a definite and constant 

 rate. The upper part of the tank has two plate glass sides let in 

 parallel to one another and at right angles to the reading telescope. 

 Two series of comparisons were made. In the first the normal thei- 

 mometers were compared at 20°, 40°, and 60° C. At each tempera- 

 ture the instruments were compared, two at a time in six pairs, the 

 zeros being taken immediately after the second set of readings for 

 each pair. In the second series the calorimetric thermometers, 

 2220 and 2221 were also utilised, the former in closed series with the 

 normals. These comparisons were made at intervaih of 5° from zero 

 to 55° and at 80°. It is not necessary to refer further to the results 

 obtained with the calorimetric, thermometers, as it was found that 

 the water in the comparison tank was slightly different at different 

 levels according as the tank was at a temperature higher or lower 

 than that of the atmosphere. The bulbs of the calorimetric thermo- 

 meters being placed at different and varying levels as compared 

 with the bulbs of the standard thermometer, the results were vitiated, 

 but this source of error did not affect the comparison of the 

 standards. In the reduction of observations Mr. Sworn reduced all 

 readings to a falling meniscus. Assuming that the actual observations 

 at the freezing and boiling points of water are those corresponding to 

 a falling thread, he adds to such reading in the comparison the con- 

 stant K previously determined by him. From the result of his 

 investigations, Mr. Sworn drew the conclusion that there is no syste- 

 matic difference between the indications of the verre dur and the Jena 

 16'^^ thermometers, and that the flint glass thermometers give indica- 

 tions which are practically identical with those of the hydrogen ther- 

 mometer. 



[The details of the observations are deposited in the Archives of the 

 Society.] 



VOL. LXVI. I 



