I 



Royal Society. 309 



desirable, before doing so, to allow the freezing-point to have 

 attained a permanent position. A few divisions have been cut on 

 the tubes near the freezing-point, and the reading with reference to 

 this short arbitrary scale taken from time to time in melting ice. 

 The period elapsed since the construction of the thermometers has 

 been too short to afford as yet much information as to the probable 

 constancy of the freezing-points. They have, however, already 

 shown generally a tendency to rise, in some cases to the extent of 

 nearly 0°-3 Fahr., but in most of them it does not yet exceed 0°-l 

 or 0°-2. Another peculiarity in connection with the freezing-point 

 has shown itself in almost all the thermometers yet tried. After a 

 thermometer has been exposed for some weeks to the ordinary tem- 

 perature of the air, if its freezing-point be ascertained, and it be 

 then suddenly exposed for a short time to the temperature of boil- 

 ing water, and again immediately placed in ice, it is found that the 

 latter determination of the freezing-point will be lower than the 

 former by a very appreciable amount, generally between 0°-l and 

 0°-2 Fahr. The freezing-point does not recover its previous posi- 

 tion for some time, probably two or three weeks. This peculiar dis- 

 placement of the freezing-point has been found to take place also in 

 the case of a standard by Troughton and Simms belonging to the 

 Royal Society, The freezing-point of this instrument, before being 

 raised to the temperature of boiling water, was 32-25, afterwards it 

 had fallen to 32-15. This displacement of the freezing-point has 

 been remarked by Mr. Sheepshanks in the course of his experiments 

 on standard thermometers*. From the experiments now in progress, 

 it is to be hoped that, after a time, some approximation may be 

 made to the laws of these perplexing phsenomena. 



The apparatus employed for comparing the indications of differ- 

 ent thermometers, consists of a cylindrical glass vase 15 inches deep 

 and 8\ inches in diameter, — a stand for supporting the thermometers 

 under comparison, and a means of agitating the water in such a way 

 as completely to assimilate the temperature throughout the vessel. 

 The stand for the thermometer is a vertical rod, supported by a 

 small tripod resting on the bottom of the vase. The thermometers 

 are suspended from hooks sliding on this rod, and adjustable to any 

 height ; they are arranged, M'ith their bulbs at the same height in 

 a circle 3 inches diameter round the rod, and kept fixed with suffi- 

 cient firmness below by being strapped with elastic bands against a 

 projecting six-rayed frame attached to the supporting rod. Six 

 thermometers of almost any form and length can thus be compared 

 at once. The agitator is a flat ring of tinned iron, about 2 inches 

 broad, fitting easily within the vase, and connected by four light rods 

 with a similar ring at top, which serves as a handle. A packing of 

 india-rubber is placed on the outer rim of the plunger to prevent 

 jarring against the glass. The flat tin ring is cut half across at several 

 places, and the corners bent in various ways, so that when moved 

 upwards and downwards the water is driven in all directions. The 



* This fact, I find, is also laeiitioiied in Faradav's " Chemical Manipulation," 

 edit. 1827, p. 13'J. 



