288 BOUGUEr's voyage to PERU. 



but we had already obferved the inftrument to have been at feme degrees below the 

 freezing point, and that it varied more than at Quito. It had often varied between 

 morning and the afternoon feventeen degrees, although always in the fhade. 



The mercury which flood expofed on the margin of the fea, was at twenty-eight 

 inches one line, and on the rock one line below fixteen inches ; the elafticity of the 

 air was proved to be, as in the lower countries, and in Europe, exadly in proportion 

 to their condenfations. Thefe obfervations, together with many others made with 

 much care, not only confirm this exaft relation, but proves to us that the intenfity, 

 even of the elaflic force, or virtue of the air, is evidently equal in all the places of 

 confiderable elevation of the torrid zone. The aftual condenfations in every place 

 are there proportional to the weight of the upper columns of air, which caufe the 

 compreflion : and thefe condenfations or denfities alter in a geometrical progreffion, 

 while the heights of places are in arithmetrical progreffion *. Below, this is not the 

 fame, becaufe the intenfity of the elaflic force of the air is there really lefs than 

 at one or two hundred toifes higher, and it mufl neceffarily be confiderably lefs, 

 becaufe it is fo notwithflanding the effeft of the heat which contributes to render it 

 greater. This is not the place to infifl farther on this fubje£l, and to explain the 

 different means I availed myfelf of to afcertain, in every place, the precife degree of 

 this force. To clofe my account of the obfervations made upon Pichincha, the pen- 

 dulum in feconds, when it was flopped immediately for the purpofe of experiments, 

 was fhorter there, than on the fhore of the fea, by thirty-fix hundredths of a line t. 



All our perfeverance was requifite to flruggle againfl the rigour of our fituation for 

 more than twenty days ; and we were at length obliged to acknowledge the neceffity 

 of renouncing fo elevated a flation. The higher, we afcended, the more ground we 

 would explore, our difcoveries dwindled almofl to nothing. A high mountain not 

 only arrefls every cloud that meets it, but thofe alfo at a certain diflance pafling the 

 fide of it, they are thrown behind by the wind, and are there becalmed. Befides, if 

 it happens the point upon which we are flationed fhould be free of them, frequently 

 the others we want to fee, are not ; and the difficulty becomes incomparably greater 

 when the fight of four or five mountains is abfolutely neceffary almofl at the fame 



* This fupplies a very fimple rule which I here explain in favour of fome of my readers. They have 

 only to look into the ordinary table of logarithms for the heights of the barometer, exprefled in lines ; 

 and if they take a thirtieth part from the difference of thefe logarithms, in taking with the charaAeriftic 

 the four firft figures only which follow it, they will have the relative heights of the places in toile*. 

 The mercury flood in the barometer at Carabouron, which is the lowefl of our ftations, at twenty-one 

 inches two and three-quarter lines, or at two hundred and fifty-four and three-quarter lines : whereas on 

 the rocky fummit of Pichincha it flood at fifteen inches eleven lines, or one hundred and ninety-one lines. 

 If we take the difference of the logarithms of thefe two numbers, it will produce one thoufand two hun- 

 dred and fifty, and if a thirtieth part is fubtrafted, it will give one thoufand two hundred and nine toifes 

 for the height of Pinchincha above Carabouron, which correfponds with the geometrical folution. The 

 application of this rule is the more exaft as the heights of the mercury in the barometer vary very little 

 in any place of the torrid zone. The variation below, near the fea, is little more than two and a half 

 or three lines, and at Quito about one line. M. Godin has been the firfl to notice thefe variations at 

 certain hours every day at Quito, which I attribute to the daily dilatation caufed by the heat of the fua 

 upon the atmofphere. On the banks of the fea, this dilatation caufes no alteration in the weight of the 

 air, for be its column higher or lower its weight fhould be equally the fame } but the dilatation caufed 

 during the day takes fomewhat away from a part of the lower column, which adds more to the higher 

 one ; and this varies the diflribiition of the weight with relation to aU places fituate in the Cordelier, and 

 even upon other mountains. *^ 



f I have noticed it on the mountains at thirty-fix inches fix feventy-one-hundredth lines ; at Quito 

 thirty.fix inches fix eighty-three-hundredth lines j and on the fea fhpre thirty-fix inches feven feven-hun- 

 dredth lines. 



Jnftant, 



