A t M 



fomctimes fee in porters. If the prcfTare be fiibdivided, fo 

 that it lie equally all around, upv«vds, I'.owir.vards, and fide- 

 ways, and no part of the ikiii be exempt therefrom, it is 

 impofiible any fradlure or luxation (hould follow. The fame 

 may be obfer'ved of the niufcles and nerves ; which though 

 foft, yet being compofed of folid fibres, do naturally fuftaiii 

 each other, and refill the common weight. The fame holds 

 of blood and other luimours; and a^ water does not admit 

 anv manifeft condcnfation, fo the aniinal humours contained 

 in their vcfiels may fuffer an attrition from an impulfe made 

 in one or more particular places, but can never be forced out 

 of their ve-llels by an univerfal compreirion. It follows, that 

 as none of the parts undergo either fe-paration, luxation, 

 contufion, or any other change of fituation ; it is impoffi- 

 blc any fenfe of pain fliould eiifue, which can only be the 

 cfl'eit of a folution of continuity. This is confirmed by 

 what we fee in divers," &c. See Diving. 



The fame is farther confirmed by Mr. Boyle, who, in- 

 cluding a young frog in a vcffel half full of water, and in- 

 truding fo much air that the water might fudnin eight times 

 the wci<^ht itotherwife would ; yet the animalcule, notwith- 

 ftanding the great teudernefs of its lliin, did not feem at all 

 affefted thereby. 



Befides, it ou;^ht to be confidered that the preffure of tae 

 atmofphere is uniform and equal on all parts of the body; 

 and that we h-ive been accuftomed to it by long experie-nce. 

 It fliould alfo be recollefted, that when the ordinary weight 

 of the atmofphere is augmented, the weather is commonly 

 dry and ferene; the circulation is promoted^ the blood is 

 driven to the internal parts; a more abundant fe-cretion of 

 the juices takes place; and the tonic tenfion of the folid 

 parts is increafed; and thefe circuniJlances combined pro- 

 duce an additional flow of fpirits, and render us more lively 

 a;id aftive. The fame beneficial effecl is obfervable even in 

 brute animals. On the contrary, when the weight of the 

 air is diminifiied, the weather is ufually moift and foggy, 

 and the animal frame becomes fenfible of opprefTion, liftleff- 

 nefs, and inadivity. Thefe changes in the Hate of the at- 

 mofphere, which are felt more or Icfs by perfons of all 

 defcriptions, and of which valetudinarians frequently com- 

 plain, would be more fenfibly experienced, if they occurred 

 by very fudden tranfitions : for to this circumftance the fenfa- 

 tion ofuneafinefs and indifpofition is chiefly to be attributed; 

 and accordingly great and fudden changes in the ftate of the 

 barometer and atmofphere, are generally accompanied with 

 a correfponding alteration in the corporeal frame and animal 

 fpirits. But when a change of this kind occurs gradually, 

 and when the fame (late of the atmofphere continues for 

 feme time, its effcft is lefs fenfibly perceived; as the body 

 pofleffes a power of accommodating itfelf to fuch change. 

 The fpring of that elafiic fluid, to which we have already re- 

 ferred, fcrves as a counterpoife to the preffure of the at- 

 mofphere, and when this is diminiflied it becomes more re- 

 laxed, fo that the equilibrium between the one and the 

 other is maintained. Hence it happens, that in moifl; foggy 

 weather, when the preffure of the atmofphere is leafl: con- 

 fiderable, our veins never fwell, nor are we fenfible of any 

 internal expanfion of our bodies ; but, on the contrary, the 

 veffels are more dillended, the circulation becomes more 

 languid, and we feem to be oppreffed with a weight. Upon 

 the whole, we may obferve, that the preffure of the atmo- 

 fphere refembles a kind of bandage, which being drawn 

 tighter, as in the cafe of increafed preffure, confl;ringes the 

 veflels of the body, and accelerates the circvdation ; and 

 which being more relaxed, as in the diminiflied preffure, oc- 

 cafions a diftenfion of the veffels, and is attended by a more 

 flow and languid circulation. But this is a fubjeift, in the 



ATM 



elucidation of which phyfiologills are not agreed. As 

 variations of the atmofpheric preffure in the fame place pro- 

 duce effefts that are fenfibly felt, partieuhuly by perfons of 

 delicate and tender confiitution, whatever explication may 

 be given of thefe effects, and to whatever intermediate 

 caiifes they maybe afcribed; the changes of preffure are 

 alfo perceived in different fituations, as they are m.ore ele- 

 vated or depreffcd. Indeed if the afcent from lower to 

 higher flations, and vice verfa, be gradual, the body adapts 

 itfelf to the changes that attend them, and they are fcarccly, 

 if at all, perceivable; but in the cafe of a more rapid afcent 

 or defcent, or when the difference of height is very confider- 

 able, the effefts are more fenfible and apparent. Many facts 

 and obfervHtlons to this purpofe have been furniflied by thoft« 

 vi'ho have afceuded in balloons, or dcfcended in diving-bells. 

 (See Aerostaticn, and Diving.) The accounts given by 

 perfons who have afcended confiderable eminences above the 

 level of the fea, have been very various; nor is it certain that 

 the effefts they have perceived have been owing w holly or 

 merely to the variation of the atmofpheric preffure. Some have 

 complained of a total laffitude, which they have afcribed to 

 the dilatation of the corporeal veflels, of obilrueT;ions to the 

 funftions of the rcfpiratory organs, of violent reachings and 

 vomitings of blood, and, in fome cafes, of the extrufion of 

 blood through the fine coats of the lungs, and an enfu- 

 ing ha;moptyfis. M. Sauflure, in his alcent to the top 

 of mount Blanc, felt great uneafinefs, as he advanced up- 

 wards. He informs us, that his refpiration was much 

 oppreffed, the circulation of blood accelerated, and the 

 pulfe quickened, that he was feized with other fymptoma 

 of a fever; and that his flrength was alfo very much ex. 

 haufted. Thefe fyraptoms of ojiprefllon and debility, how- 

 ever, did not begin to appear till he had afcended to ths 

 perpendicular height of li miles above the level of the fea ; 

 and upon an additional afcent of J of a mile, he found the 

 fymptoms above recited. To fome other concurring caufe, 

 befides the rarity of the atmofphere, it is natural to afcribe 

 fome of thefe fymptoms; and, indeed, he hinifelf fays, that 

 the atmofphere at the top of the mountain was much im- 

 pregnated with carbonic acid, which is known to be per- 

 nicious to animals, and to be pioduiflivc of fome of the 

 above-mentioned effefts. In other cafes, perfons in elevated 

 fituations have experienced no effeCls like thofe which M. 

 Sauffare has related, antl which the mechanical theory of 

 diminifhed preffure u'ould lead us to cxpeft. Mr. Brydone 

 and M. Howel mention no inconvenience of this kind to 

 which they were fubjetl on the top of mount JEtna; nor 

 do the French malhematicians, who were for fome time on 

 the fummit of a very high eminence of the Andes, make 

 any other complaint befides that of the difficulty of refpira- 

 tion. (See Andes.) But Dr. Heberden, who afcended to 

 the top of Teneriffe, a mountain higher than jEtna, makes 

 no mention even of this circumftance. It has alfo been 

 allegul, that no inconvenience has been experienced by a 

 gradui.l defcent^in the diving-bell to cor.fiderable depths in 

 the fea, as long as the perfons who have dcfcended have 

 remained in the air in the bell; though they have found a 

 very matfiial difference on expofing themfeives to the pref- 

 fure of the water. See Diving. 



It is n'lt eafy to aflign the true caufe of the variations of 

 the atmolpheric weight and preffure that occur in the fame 

 fituation. In places within the tropics, where thefe varia- 

 tions are not very confiderable, the chief caufe feems to be 

 the heat of the fun ; and its effefts are regular and uniform, 

 as the mercury in the barometer fubfides about half an incli 

 in the day, and rifes again to its former height in the night. 

 But in the temperate zones the range ic much greater, ck- 

 2 tending 



