-V 

 BOOK III. CHAP. Vir. 66i 



The general theory of the operation is built on this principle ; " That 

 " the column of air upon high mountains is much fliorter; and, confe- 

 ** quently, lighter than that of the plains." 



Suppofing a hill, fuch as Snowdon Hill, where the mercury fettles nt 

 four inches below the mean altitude at the level of the Tea, and whofc 

 hei'^ht is about 3,720 feet ; the column of air, at fuch an height, and 

 of one inch diameter, it is fiippofcd, would be found lighter by 14,320 

 grains, or 29 oz. 6 dwt. 3 fcfup. than belo-w. 



Hence it follows, that, in fuch places, the blood, having a load taken:: 

 from it, will fwell and diftend the veflels ; and, at leafl, occafion a 

 Ihortnefs of breath. 



The like dilcitation happens to bottled liquors carried to that height; 

 the air contauied in the liquor rarefies to the fame pitch with the ex- 

 ternal air, cracks the bottle, and makes it fly into a thoufand pieces. 



From the like caufe, perhaps, ludden changes of weather in this 

 climate, from very dry and hot to very wet, and again to hot and dry, 

 produce a variety of difordeis in the human body. 



In the year 1761, after very heavy rains, for feveu or eight days in-. 

 May, fucceeding dry fultry weather j and followed by a return of dry 

 hot weather, a bad fpecies of fever appeared in Kingfton ; the firrt lym- 

 ptoms of which were afudden vertigo, and deprivation of fight; inflant 

 bleeding was found moft ferviceable; and, if neglected, the diforden 



lovvs 14 toifes, or about 90 Englilh feef, to every tall of 1 line of mercury ; but he thinks, the al- 

 lowance ufually made, particularly in great elevations, where the air is cxceedinL;ly thin and lij;ht,. 

 is much tjo fmall. His conjefture is probably jull : and it To, the Englilh mcniuration, given in 

 the former tables, may he tound more exail than any formed by Callini, and other luieigneis. The 

 foUowincr are the ihiuons ot the thermorueter and barometer obferved by JNIr. Br^done in his 

 afcent : 



Height of Fahrenheit's Thermometer. Deg. 1 In. Tin. Height of Barometer. 



At Cutanea, mid-day, May 26 7.6 



Ditto 5 in the morning 27 72 



Nicolofi, mid-day 73 



Spdoma rlel Capriole, where there Was.] , 



fnow (7 at night), _ j' 



Irtthe fame cave, half part n at night, 52 



Torre del Phdofopbo, 3 in the morning, 34 



Foot of the crater, 33 



About half way up the crater, 29 



On the fummit, a little berore fun-rife, 27 



39 8 {-Sea- fide at Citama, 



27 8 Picdinotitc, in the firft region of jEtna, 



27 i\t^icolojii in the fame region.. 



26 ^\ Cpfiagm de Cento Cavalli, ft cond region*. 



2,4 2 Sprlancti di'l C.aprrioh, fame. 



20 ^ Torre del Phllcjhpbo, third region. 



20 4j Foot of the crater. 



19 6| Within about 300 yards nf the fummit., 



19 4 Byfujipofitionat thefummitol the mountain. 

 According to this regiller, and the former tables, the height of /Etna, above the Ten's level, fliouM 

 be about 11,630 teet ; Mr. Brydone fuppofcj it not to exceed 12000. But, at the tormer compu- 

 tation, it is upwards of 4000 teet higher, than the higheft part ot our Blue Mountain Rid;^e in 

 Jamaica. The thermometer being at 33, or 1 degree above the freezing paint, when the barop 

 jneter was at 20 inches 4I lines; we may fuppofe the freezing pointt to have been at the elevation 

 «f near two miles, or about 10,368 feet, ia the mondi of May, 



general 1^> 



