Account of Mr. Smith's Air-pump Vaponr-hatb. 295 



tofned preflure, do not, as ufual, refift the impetus given to 

 the blood by the contra6tion of the heart, which has been 

 found to occafion difficulty of reipiration, violent retching, 

 and even vomiting of blood. 



I need not tell you, that the prefTure of the atmofphere 

 in its mean ftate is equivalent to a weight of 15 pounds to 

 every fquare inch. It has thence been computed, that the 

 preflure of the circumambient fluid upon the furface of the 

 whole earth is equivalent to the weight of a globe of lead 60 

 miles in diameter. If every fquare uich of the human body, 

 therefore, fuftains a preflTure of 15 pounds, every fquare foot 

 mud fuftain that of 3160 pounds; and according to.this cal- 

 culation, every ordinary fized man has to iupport a weight of 

 between 15 and 16 tons for his common load. Such an 

 enormous preflure, if not counterbalanced by the elaflicity ot 

 the air within our bodies, would crudi us in a moment. If 

 a portion of this preflure be removed from any part of the 

 body, as is the cafe, for inftance, when a leg or arm is cn- 

 clofed in the cylinder of this Apparatus, and the air partially 

 extrafted, the efi'ecl of this unequal prefliue loon becomes 

 perceptible to tlie patient. Should the exhaultion be carried 

 to its utmofl; extent, it requires no ftretch of imagination to 

 conceive the eft'ecls that would be produced ! 



The quantity of atmofpheric preflure on the human body 

 fometimes varies near a whole ton, from natural caufes. 

 When it is diminiflied fo much, many people, parlicularlv 

 the nervous, find themfelves inailive and irritable. It is 

 fomewhat furprifing that the fpring of the internal fluid, 

 mentioned above, a» counterpoiflng atmofpheric preflure, 

 fliould exadtly balance it when artificially augmented, and 

 even wlicn naturally diminifhcd, but not when artifcliUy 

 diminiflied. 'i'iuis, though we perceive no evident diflcn- 

 fion of the blood-veflTels'on the furface of the body from 

 the natural diminution of atmofpheric preflure; yet the cafe 

 is widely diflcrent when the diminution is artificially pro- 

 duced, as can be cfteoled to any extent within the cylinder 

 of this Apparatus. Thus animals in the receiver of an air- 

 pump become inmiedialely uneafv, and can live but a very 

 ihort time in air artificially deprived of no more of its prefliire, 

 than by the barometer, appeared to take place on the top of 

 Mount Blanc. I am aware that this plucnomcnon mav bo 

 accounted for on other principles ; namely, that a certain 

 quantity of oxygen gas diflufcd tlirough a given Ipacc, will not 

 luflaiu animal life half fo long as twice the (]uanlity dilVuled 

 through the fame fpaee. 'I'here may be fometliing in this; 

 but, I humbly think, it is not fuflicienl alone to account for 

 T 4 the 



