VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. ] C)5 



X. Of an improved Thermometer. By Mr. James Six. p. 72. 

 Attempting some time before to ascertain the greatest degree of heat and cold 

 that happened in the atmosphere each day and night, or during the course of 24 

 hours, Mr. S. experienced the inconvenience which attends thermometers 

 commonly used for that purpose ; viz. the necessity of the observer's eye being 

 on the instrument the very instant the mercury stands at the highest or lowest 

 degree : for, since the time when that may happen is utterly uncertain, tf it be 

 not immediately noticed, it can never after be known. The sultry heat of the 

 summer's days, and freezing cold of the winter's nights, which is commonly 

 most severe at a late unseasonable hour, render it very unpleasant to be abroad 

 in the open air, though it is absolutely necessary for the thermometer to be 

 placed in such a situation. Ingenious men of our own country, as well as 

 foreigners, have, it seems, long ago, endeavoured to remedy this inconvenience ; 

 and several thermometers of different constructions have been invented for that 

 purpose. Van Swinden describes one, which he says was the first of the kind, 

 made on a plan communicated by Mr. Bernoulli to Mr. Leibnitz. Mr. Kraft, 

 he also tells us, made one nearly like it. A description of those by Lord 

 Charles Cavendish and Mr. Fitzgerald may be seen in the Philos. Trans, vol. 50, 

 p. 501, and vol. 51, p. 820. Though much ingenuity appears in the invention 

 of those curious instruments, Mr. S. thought that a thermometer might be 

 constructed more conveniently to answer the purpose, and show accurately the 

 greatest degree of heat and cold which happened in the observer's absence. Mr. 

 S. then gives a description of another thermometer, rather of a complex form ; 

 and then adds, thus far our thermometer resembles in some respects those of 

 Mr. Bernoulli and Lord Charles Cavendish ; but the method of showing how 

 high the mercury had risen in the observer's absence, the essential property of 

 an instrument of this kind, is wholly different from theirs, and effected in the 

 following manner. Within the small tube of the thermometer, above the sur- 

 face of the mercury on either side, immersed in the spirit of wine, is placed a 

 small index, so fitted as to pass up and down as occasion may require : that sur- 

 face of the mercury which rises carries up the index with it, which index does 

 not return with the mercury when it descends ; but, by remaining fixed, shows 

 distinctly, and very accurately, how high the mercury had risen, and conse- 

 quently what degree of heat or cold had happened. Towards evening, says Mr. 

 S. I usually visit my thermometer, and see at one view, by the index on the left 



circumstance of tbe unavoidable change of the absolute direction of the ray at entering the water 

 telescope. The author there thinks that he was the first who ever detected that important circum- 

 stance ; not then adverting that the same was fully pointed out, and reasoned from as the leading 

 principle of Mr. Wilson's present paper, published in the London Philos. Trans., 5 years before the 

 time the paper in question was read at Edinburgh in 1788. 



c c 2 



