VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 265 



making a correction, we recommend, that the boiling point should be adjusted 

 when the barometer is at 29.8, if the person chuses to do it in steam ; or when 

 the barometer is at 294-, if he chuses to do it in close vessels, with the ball im- 

 mersed to a small depth under the water. Our reason for pitching on this pre- 

 cise height is, that thus the boiling point will differ from Mr. De Luc's boiling 

 point, by a simple fraction of the degrees of his common scale, namely i of a 

 degree higher. 



We are informed by Mr. De Luc, that the method he used in adjusting the 

 boiling point, though he forgot to mention it in the Recherches sur les Modifi- 

 cations de I'Atmosphere, was to wrap rags round the tube of the thermometer, 

 and to try it with the ball immersed in water in an open vessel, of the form des- 

 cribed in the abovementioned book, while boiling water was poured at different 

 times on the rags, in order that the quicksilver in the tube might be heated, if 

 possible, to the same degree as that in the ball. As well as we can judge from 

 the abovementioned experiments in open vessels, and from the few trials we have 

 made of this method, we are inclined to think, that the boiling point adjusted 

 this way will in general differ but little from that adjusted in steam at the same 

 height of the barometer, especially if the thermometer be not very long, and 

 do not extend a great way below the freezing point ; * consequently, as Mr. 

 De Luc's boiling point was adjusted when the barometer was at 27 Paris or 28.75 

 English inches, it will stand lower than that adjusted in the manner recom- 

 mended by us, by 4 of a degree of his scale ; or 80°4 on De Luc's thermometer, 

 will answer to 212° on Fahrenheit's adjusted in the manner proposed. 



Though the boiling point be placed so much higher on some of the thermo- 

 meters now made than on others, yet we would not have the reader think that 

 this can make any considerable error in the observations of the weather, at least 

 in this climate ; for an error of 1"^ in the position of the boiling point will 

 make an error of only half a degree in the position of 92", and of not more than 

 -i- of a degree in the point of 62°. It is only in nice experiments, or in trying 

 the heat of hot liquors, that this error in the boiling point can be of much 

 signification. 



There is another circumstance that we have not yet taken notice of, which, 

 in strictness, causes some error in thermometers, namely, the difference of ex- 



* In order to see how much the quicksilver in the tube of the thermometer would be heated in 

 this method of adjusting the boiling point, we took tlie abovementioned tube without a ball, wrap- 

 ped it round with rags, and poured boiling water on it as above described : the heat of the quicksilver 

 therein was found to be about 2 1 ° less than that of boiling water ; and therefore the boiling point of 

 a thermometer, adjusted in this manner, supposing the thermometer to be dipped into the water as 

 far as to the point of 32°, should stand about i of a degree lower tlian it would do if the quicksilver 

 in the tube was heated equally with that in the ball. — Orig. 



VOL. XIV. M M 



