202 - BOUGUER's voyage TO PERU. 



fometimes dimlnifh, in thicknefs ; but, at the fame time, the boundary of the beginning 

 of congelation likewife lowered ; and fell below the ftation on which we were encamped 

 to work at our triangles. I gave myfelf the trouble, at the beginning of 1743, again 

 to vifit the mountain, to fatisfy myfelf of this, and various other circumftances, fo that 

 I could not be deceived. This Angularity feems to have fome conneftion with thofe 

 acknowledged operations, in which congelation is haftened by the aiTiftance of fire. In 

 the meantime, the examination led me to a difcovery, that the fmgularity depended 

 upon a very different caufe. I difcovered, that what at a diftance I had taken for fnow 

 was not, but water, which, falling from above, and gufhing from out the mountain on 

 every fide, froze as it run. It is certain, that the leaft degree of heat is fuflicient to 

 thaw particles fo delicate as fnow, when it falls upon a furface interiorly heated. B^t 

 when a body of water of a certain thicknefs runs over the fame ground, the heat below 

 may be fo weak, as not to communicate with the upper furface ; and if this furface is 

 found expofed to an excefs of cold, nothing can prevent it being converted into ice. 

 The fnow on the higher parts of Cotopaxi thawing, from its vicinity to the fire, conti- 

 nually produces new w^ater, .and this water freezing below after being divided into an 

 infinity of ftreams, forms, as it were, when received at a certain diflance, ringlets of 

 ice on the mountains, but, looked at from a certain diftance, it appears a perfect cover- 

 ing. The fame effect may have place upon all the other mountains ; the fnow only 

 maintaining itfelf to a certain limit in defcending ; whereas a body of water will, in 

 . proportion as it decreafes in bulk, freeze on its furface, lower on the mountain. Such 

 is the elucidation of this phenomenon, derived to me from my vifiting the places. If 

 regard is had to the exception it furniflies, and to others lefs confiderable, we repeat it, 

 the lower limits of the fnow conftitutes a fufHciently exad level line through all the 

 country in the environs of the equator. 



But if we examine the circumftance in a more general manner, if we diredl our re- 

 gard to the globe, this line will not be found exaftly parallel with the earth : it is evi- 

 dent it mufl, in its direction, gradually fall, as it retires from the torrid zone, or ad- 

 vances towards the poles. In the middle of the torrid zone this line is at two thoufand 

 four hundred and thirty-four toifes above the level of the fea ; at the entrance into the 

 temperate zone, it will be found but two thoufand one hundred, pafling by the fummit 

 of Theyde, or the Peak of Teneriff, which is nearly of this height *. In France and 

 Chile, the line will pafs at fifteen or fixteen hundred toifes, and, continuing to defcend 



* The Pcre Feuillee, to whom we are indebted for z. great number of obfervations, afHgns, in a manufcript 

 relation prefented to the Academy on his return from a voyage to the Canaries in 1724, two thoufand two 

 hundred and thirteen toifes to the height of the Peak of Teneriff. But we are induced to beHeve, for 

 reafons we fhall give, that we muft take from the calculation of this height one hundred and forty or one 

 hundred and fifty toifes at leaft. The obferver worked upon a bafe, whofe length, not being more than 

 two hundred and ten toifes, was much too fhort, confidering that he was diftant from the Peak not Ief$ 

 than ten thoufand toifes. This bafe, too, from the badnefs of its fituation, was not equivalent to another 

 much lefs : for, conformably to a method fcarcely ever good but in theory, it was directed up tlie moun- 

 tain, inftead of having a direction nearly perpendicular : infomuch, that the bafe was really reduced to 

 forty toifes, which had been placed in a fituation nearly perpendicular to two vifual rays directed to the 

 fummit of the mountain. Finally, P. Feuillee neglefted the inclination of his bafe, becaufe they told 

 him the fea had formerly covered the ground. Now, were this true, the ground muft have acquired ele- 

 vation fince, and ftill more towards the foot of the mountain where his fecond ftation was fixed : now, as 

 the inclining of the ground was no more than three toifes in two hundred and ten, which is by no means 

 confiderable, the two vifual rays, by reafou of the elevation of the fecond ftation, have met at little dif- 

 tance, and at little elevation in the air ; and regard being had to the ftiortnefs of the reduced bafe, wbicU 

 was not more than forty toifes, we jnuft diminifh the height calculated by P. Feuillee a thirteenth or four- 

 teenth part. I conceive the reader will not confider this note as foreign to a work of the nature of this, 

 which has fo often a relation to mountains. 



6 in 



