BOUGUER*S VOYAGE TO PERI?. 29 1 



littte bubbles of the oppofite cloud neceffarily dilate ; their furface enlarging, the thick- 

 nefs of the water contained in them diminillies : and reduced to a thinner fubftance, 

 it is only a greater obliquity, or bubbles more remote from the centre of the fhadow, 

 which can prefent to us, as other experiments of this nature have confirmed, the 

 fame colours. The diameter of the firft circle was commonly nearly five degrees two- 

 thirds, the next nearly eleven degrees, the third feventeen degrees, and thus on ; 

 the white circle was nearly fixty-feven degrees in diameter. The time proper to view 

 this phenomenon, which requires that the (hadow be projected from a cloud, is a fufE- 

 cient excufe for the Peruvians never having beheld it, and why they fliould not be 

 blamed for it. It is an hour not ufual for any but a philofopher to be found upon the 

 fufii'mit of a high mountain : it might probably be noticed fometimes, on our moft 

 elevated fteeples. All of us have feen at a fhort diflance from us. Jogs confined to a 

 very circumfcribed fpace j one thing only was wanting, which was the fun in the op- 

 pofite horizon; but where this latter circumftance has not exadly correfponded, a 

 portion of the white circle is frequently to be diftinguiflied, as I have at different times 

 remarked fince I have paid attention thereto. 



The height of the rocky fummit of Pichincha, is nearly the fame with that of the 

 loweft conllant termination of the fnow on all the mountains of the torrid zone ; and 

 I have found it at two thoufand four hundred and thirty-four toifes above the level of 

 the South Sea. The fnow falls much lower ; it even, though but rarely, fometimes 5 

 fells at Quito, which is more than nine hundred toifes fhort of the height of Pichin- 

 cha, but it may be thawed the fame day : whereas in all the parts of the Cordelier I 

 have gone over, I have obferved it higher to lie undiffolved. Some mountains do not 

 reach this point of termination ; others, as Pichincha, border upon it ; others, and 

 thefe in great numbers, lift themfelves ftill higher, and have their fummits continually 

 covered ; and confequently, from the fiiow being converted into ice, are inacceffible. 

 When the mountains are not enveloped in clouds, their furface muft be a little thawed 

 during the day ; but the fun ceafing to ad, the furface becomes glazed ; the water 

 paffes into the uiterftices of the lower beds, and there freezing, renders the fnow 

 extremely compad, and forms a folid whole. The furface hardens at the fame time, 

 and becomes as fmoothly polilhed as a mirror, fo that it is as it were impoffible to 

 afcend higher. This limit depends upon too great a variety of circumflances not t© 

 be liable to great irregularities. Many mountains in Peru have a difpofition to emit 

 flame, for almoft all of them have been volcanic, or adually are fo notwithftanding 

 their fnows, which much induce a forgetfulnefs of the circumftance : it is befides cer- 

 tain, the larger the dimenfion of the mafs is which conftitutes their bafe, the more 

 fufceptible they muft be of heat, and the limit of congelation farther removed ; 

 as thefe maffes muft be confidered a fecondary foil, every day imbibing the heat of the 

 fun ; on the other hand the part covered with fnow, when very confiderable, produces 

 a contrary effed ; it caufes a greater cold around, capable of congelation, or producing 

 ice a little lower down. In the mean time the difference is not great, inafmuch as I 

 have remarked the lower boundary of the fnow, to form a level line through all the 

 mountains of Peru, in a manner to enable us at a glance to judge of their height. 

 The volcanoes, as I have noticed, create the ftrongeft exception to this rule ; but 

 the exception is fometimes fuch as it may be difficult to forefee. This I have remarked 

 with regard to Cotopaxi, a ftation of our meridian, fituated on the eaftern chain. The 

 fpot on which we were ftationed was between one hundred and fifty and one hundred 

 and eighty toifes below the fnow ; but this mountain, from a recent irruption in 1 742, 

 had caufed the fnow above to thaw. We faw it from below fometimes increafe, and 



1) P 2 fometimes 



