TRANSACTIONS OF THE SECTIONS. 35- 



scale being equivalent to equal volumes, although their lengths may vary very consi- 

 derably. Mr. Welsh described a raodification which he had made in M. Regnault's 

 process, by which he has been enabled to divide the scales of the thermometers gra- 

 duated at Kew at once into degrees, the readings being afterwards subject only to the 

 small errors of manipulation, and such errors as arise from the unavoidable changes 

 ■which take place in the zero-points of all thermometers. The freezing-points are de- 

 termined in the ordinary way by immersion in well-pounded ice, from which the water 

 is drained off as it melts. The boiling-points are determined by the apparatus 

 devised by M. Regnault, in steam, whose elastic force is exactly equal to that of the 

 atmosphere at the time, a correction being made for the difference of the barometric 

 pressure from the adopted standard pressure. The boiling-points, besides being 

 determined for the usual position of a thermometer, with the stem vertical, are also 

 observed in a similar apparatus with the stem horizontal ; so that, if the instrument 

 should ever be used in any other than the vertical position, the proper correction may 

 be applied. The difference between the boiling-point of a thermometer, in the two 

 positions, is found to be from 0°"2 to 0°5 Fahr., according to the thickness of the 

 glass and the form of the bulb. After the graduation of a thermometer has been 

 completed, its accuracy is examined by a subsequent calibration with a longer column 

 of mercury. If the length of the column, with reference to the scale divisions, 

 is everywhere the same, the graduation is considered good ; but if any difference is 

 found to exist, a more complete examination is made by using columns of different 

 lengths, each of which is nearly an aliquot part of the range of the scale, the remain- 

 ing errors being deduced from these measurements by the method adopted by Mr. 

 Sheepshanks for the thermometers used in connection with the national standard 

 yard. It is, however, seldom that any appreciable correction is found needful. It 

 has long been known that the freezing-point of a thermometer is not constant, but 

 rises by a considerable amount during the first year after its construction. There is, 

 however, another peculiarity in thermometers which is less known. If a thermo- 

 meter, after having been for some weeks exposed to the ordinary temperature of the 

 air, is placed in melting ice, its freezing-point will be, for example, 32°'2 ; if the bulb 

 is then put for two or three minutes into boiling water, and soon afterwards again 

 placed in ice, the reading will no longer be 32°'2, but will have fallen to nearly 32°'0 : 

 if in a day or two it is again placed in ice, the freezing-point will have risen a little 

 — about 0°'l; and if again tried, after two or three weeks, the freezing-point will 

 be found to have acquired exactly the original position of 32°"2. This has been found 

 to be the case with every thermometer examined at Kew, whatever was its age ; the 

 difference in the freezing-point, before and after boiling, being about 0°'17 Fahr., and 

 varying inappreciably in different instruments. This peculiar displacement of the 

 freezing-point seems to be owing to a temporary alteration in the dimensions of the 

 bulb caused by a considerable change of temperature ; the glass, after having been 

 expanded by heat, requiring a week or two to contract to its original size. It appears, 

 therefore, that the alteration in the freezing-point of a thermometer depends upon two 

 separate causes, the one being a slow contraction of the bulb, continuing for many 

 months but ultimately ceasing, and the other a temporary alteration in the dimensions 

 of the bulb, produced by a sudden and considerable elevation of temperature, which 

 disappears in two or three weeks. The rise in the freezing-point of ordinary ther- 

 mometers is probably due to a combination of both these causes ; for if a thermo- 

 meter has its freezing-point set off' soon after being blown and filled, there will be, 

 first of all, the comparatively rapid contraction of the bulb due to the great heat to 

 which it has lately been exposed, and afterwards the more gradual contraction which 

 continues for several months. The author recommended opticians, instead of 

 " pointing off" their thermometers immediately after being filled, to allow them to 

 rest for a month or six weeks, so as to avoid at least the first great change which 

 occurs ; but of course the longer they are kept the better. Mr. Welsh mentioned 

 another fact which he had observed in thermometers. He took about fifteen ther- 

 mometers, and, after carefully ascertaining their freezing-points, kept them exposed 

 to the temperature of boiling water for about 60 hours, allowing them afterwards to 

 cool very slowly. It was then found that the freezing-point had been permanently 

 raised in all of them by about 0°'3 to 0°'4 Fahr. The effect of a subsequent suddeii 

 elevation of temperature was exactly as before, to lower the freezing-point by nearly 



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