258 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777- 



XXXVII. The Report of the Committee appointed by the Roijal Society to con- 

 sider of the Best Method of Jdjiisling the Fixed I'oirtts of Thermojiieiers; 

 and of the Precautions necessary to be used in making Experiments with those 

 Instruments, p. 81 6. 



It is universally agreed, by all those who make and use Fahrenheit's ther- 

 mometers, that the freezing point, or that point which the thermometer stands 

 at when surrounded by ice or snow beginning to melt, is to be called 32°; and 

 that the heat of boiling water is to be called 212°: but for want of further 

 regulations concerning the manner in which this last point is to be adjusted, it 

 is placed not less than 2 or 3 degrees higher on some thermometers, even of 

 those made by our best artists, than on others. The 2 principal causes of this 

 difference are, firsts that it has never been settled at what height of the baro- 

 meter this point is to be adjusted;* and 2dly, that so much of the quicksilver 

 in the thermometer as is contained in the tube, is more heated in the method 

 used by some persons, than in that used by others. To show that this last 

 circumstance ought by no means to be disregarded, suppose that the ball of a 

 thermometer be dipped into boiling water as far as to the freezing point, and 

 consequently that the length of the column of quicksilver in that part of the 

 tube which is not immersed in the water be 180"; and suppose that the heat of 

 that part of the column of quicksilver be no more tlian 112°. If the 

 thermometer be now entirely immersed in the water, the heat of this column 

 v.'ill be increased 100°; and consequently its length will be increased by —\ gg-^ 

 parts of the whole, as quicksilver expands , , ; „„ part of its bulk by each degree 

 of heat; and consequently the thermometer will stand —-—— or rather more 

 than 1% higher than it did before. Another thing to be considered in adjusting 

 the boiling point is, that if the ball be immersed deep in the water, it will be 

 surrounded by water which will be compressed by more than the weight of the 

 atmosphere, and on that account will be rather hotter than it ought to be. 



We are of opinion, that the quicksilver in the tube ought, if possible, to be 

 kept of the same heat as that in the ball, and that tiie ball ought not to be 

 immersed deep in the water. These 2 requisites may be obtained by using a 

 vessel covered so as to allow no more passage than what is sufficient for carrying 

 off the steam; for then, if the thermometer be inclosed in this vessel in such 

 manner, that the boiling point shall rise but a little way above the cover, almost 



* Fahrenheit found that the heat of boiling water differed according to the height of the baro- 

 meter ; but supposed the difference to be much greater tlian it really is. Mr. De Luc has since, by 

 a great number of experiments made at very different heights above the level of the sea, found a 

 rule by which die difference in the boiling point, answering to different heights of the barometer, is 

 determined with great exactness. According to this rule tlie alteration of the boiling point, by the 

 variation of the barometer from 29h to 30^ inches, is l^sp of Fahrenheit.— Orig. 



