VOL. XXXV.] PHILOSOPHICAL TRANSACTIONS. 26/ 



absolute degree of certainty, is yet in many respects preferable to the trigono- 

 metrical one, as it has also been found by experience to come nearer the truth, 

 and leads us, by a new and singular scale, from the very horizon of the sea, 

 to the tops of the highest mountains, a distance far beyond the reach of geo- 

 metrical instruments. This new method is grounded on that essential quality 

 of the air, its gravity or pressure. As the column of mercury in the barometer 

 is counterpoised by a column of air of equal weight, so whatever causes may 

 make the air heavier or lighter, its pressure will be thereby increased, or 

 lessened, and consequently the mercury rise or fall. Again the air is more 

 or less condensed or expanded, in proportion to the weight or force which 

 presses it : hence it is, that in England, Holland, the maritime provinces of 

 France, and in general all those countries which border on the sea, the mer- 

 cury stands highest; that the higher you remove from the sea into the mid- 

 land countries, the lower the mercury will descend, because the air also be- 

 comes more rarefied and lighter; and that on the tops of the highest mountains 

 it falls lowest; and these heights of the mercury, in different places, are reci- 

 procally as the expansions of the air. From these principles, supported by a 

 competent number of observations, it has been attempted by several learned 

 men, to derive proper tables, by which the height of any place may be deter- 

 mined, if the height of the barometer be given, or the height of the barome- 

 ter determined from the given altitude of the place; and likewise the expan 

 sions of the air settled, as they answer to every inch, or part of an inch, in 

 the barometer. 



Dr; S. passes over the first experiment of this kind, which wa* made in the 

 year l648, but a few years after the invention of the Torricellian tube was made 

 public in France by Father Mersenne, by Monsieur Perier, according to the 

 directions of the celebrated Mcmsieur Pascal, his brother-in-law, on the high 

 mountain Puy de Domme, near Clermont in Auvergne, the height of which 

 was thereby determined to 500 French toises, or 3000 Paris feet. (See the 

 Appendix to M. Pascal's Traite de I'Equilibre des Liqueurs.) Nor will the 

 Dr.'s present purpose admit a particular enumeration of those made some time 

 after, in 1661, 1665, and 1666, by George Sinclair, professor of philosophy 

 in the university of Glasgow, on the cathedral of that university, and on several 

 high mountains in Scotland, as also in some wells and coal-pits, a particular 

 account of which he inserted in his Ars magna gravitatis et levitatis. Only ob 

 serving, that these experiments of Sinclair, as well as that of Monsieur Perier, 

 were intended not so much to lay the foundation of a calculation, to determine 

 the differing heights of places, as to prove the gravity and pressure of the air, 

 a problem very much controverted at that time, and to show that the same is 



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