181 



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 hwer 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 8y 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, suj^ported 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, with 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 ])revent 

 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 

 dimensions of the agitator are so arranged, that no part of it can 

 possibly touch the thermometers when in operation, llie vase, 

 containing water, the stand with thermometers, and the agitator, 

 are mounted upon a wooden revolving stand. The depth of water in 

 the vase is always sufficient to include the whole of the column of 

 mercury, the scales being observed through the water. In taking 

 the observations, the observer, after agitating the water briskly for 

 some time, turns the revolving stand till each thermometer is brought 

 successively opposite to his eye, reading off the scales as quickly as 

 possible to an assistant, who writes down the numbers. Proceeding 

 in this way, I find that six thermometers can be read off and recorded 

 easily in 20 seconds. It is of course desirable to make more than 



* This fact, I find, is also mentioned in Faraday's " Chemical Manipulation," 

 edit. 1827, p. 139. 



Proceedings of the Royal Society. Vol. VI. No. 88. 13 



