Remarks on Stones of a regular Figure found near Bagneres in Gascony: With other Observations
communicated by Monseur Secondat de Montesquieu, of the Academy of Sciences of Bordeaux, in a Letter to Martin Folkes, Esq; Pr. R. S.
Philosophical Transactions. London 1750.
However different the preceding Subject of my Discourse may be from what I am next going to speak of I beg Leave to set down One Observation more, which I have made on the Pic du Midy, and of which the Result may prove curious.
It is well known, that the greatest Degree of Heat in common Water is that which it acquires by boiling; that is to say, if Water is put upon the Fire, it grows by degrees hotter and hotter, till it quite boils; but, after that, though there be never so much Fire added, and it stand never so long upon it, it will never grow hotter than it was on the first Infant, when it began to boil. Hence the Degree of Heat of boiling Water is looked upon as fixed and invariable.
Fahrenheit, that ingenious Master in Mechanics, so well known by his Mercurial Thermometers, is the first who has remarked the contrary. He observed, that the Heat of boiling Water was greater when the Air was heavy (that is to say, when the Mercury stood higher in the Barometer); and, on the contrary, the Heat was less when the Air was lighter.
Mr. Le Monnier the younger, who has obliged us with a Translation of Mr. Cote's Lectures in Natural Philosophy, with excellent Notes upon the said Work, has put Fahrenheit's Discovery past all Doubt, and has very much improved it.
On the 6th of October 1739, being provided with a Barometer, and a Mercurial Thermometer of M. Delisle, he climbed up to the highest Top of the Canigou, a Mountain in Roussillon, which passes for the highest among the Pyrenees: There he found his Barometer to stand at 20 Inches 2 ½ Lines; whilst at Perpignan it stood at 28 Inches 2 Lines. The Difference between the Heat of the Water which he boiled there, and that which he boiled at Perpignan, was 15 Degrees of his Thermometer.
The same Thermometer being surrounded with Snow, the Mercury fell down to the fame Degree as pounded Ice had made it do at Paris.
Hence he concludes, that the Heaviness of the Air has a sensible Influence on boiling Water; but that it in no way alters the Term of Congelation.
All these Particulars may be seen, p. 408. of Cote's Experimental Lectures; and in the Memoirs of the Academy of Sciences of Paris, Anno 1740.
This is the same Experiment which I have repeated on the Top of the Pic du Midy; thinking that so singular a Fact ought to be observed more than once.
I carried Two Barometers, the Tubes of which the Reverend Father Francis had been so good as to fill for me with great Care. I had likewise with me Two Mercurial Thermometers, upon which I set the Degrees at Bagneres: I took the fixed Terms of the Graduation; that is to say, that of Congelation, and that of boiling Water, afterwards putting nought to the Term of Congelation. I marked 180 degrees Difference between this Term and that of boiling Water.
Being come to the highest Top of the Pic du Midy on the 9th of last July, the Mercury rose in one of my Barometers to 20 Inches 2 Lines; and in the other, to 20 Inches 1 ½ Line. I surrounded my Thermometer with Snow, and the Mercury fell exactly to the same Degree as the Snow had made it fall to at Bagneres. Afterwards I plunged it into boiling Water; whereupon the Mercury rose to 165° of my Graduation: So that the Difference between the Heat of boiling Water on Pic du Midy, and that at Bagneres, consisted of 15°.
At my Return to Bordeaux, I observed, that I had marked the Term of boiling Water at Bagneres less high by 3 ½, than at the Term of boiling Water at Bordeaux, taken at the time when the Barometer was at 28 Inches 2 or 3 Lines: Therefore having anew graduated my Thermometer the 165th, the Degree of the former Graduation fell now upon the 162nd; so that the complete Difference between the Term of boiling Water on the Top of the Pic du Midy, and that of the same at Bordeaux, the Barometer being at 20 Inches 3 Lines, amounts to 18. Degrees on the Thermometer of Fahrenheit.
Now the Conformity between the Observation made by M. Le Monnier, and this Repetition of the same Observation, can hardly be greater; seeing the Heights of the Barometers are almost the same; and the 15 Degrees of Difference, found by M. Le Monnier on De Lisle's Thermometer, amount precisely to 18 Degrees on the Thermometer of Fahrenheit, which I made use of.