BOOK II. CHAP. VII. 55: 



afliffed by the native warmth of the marje below, and the acliou 

 of the Iblar heat above ; which enable thofe globules to penetrate 

 the furfice in form of vapour. In a vale encompafied with luch 

 prodigious mounds, the foiar rays muft: ftrike with confiderable 

 impreffion, and fupply the earth to a certain depth with a large 

 flock of heat, which doth not wholly leave it for many hours 

 after fun-fet. The ingenious Dr. Hales remarks, that fo great a 

 heat as the fun occafions,. at two feet depth under the eai th's fur- 

 face, mufl. needs have ftrong influence in railing tiie moifture at 

 that and greater depths; whereby a continual reek muft always be 

 afcending,- during a hot feafon, by niglit as well as by day; for the 

 heat at two feet depth is nearly the fame night and day. The im- 

 pulfe of the fun-beams giving the moifture in the earth a brifk. 

 undulating motion, thefe acjueous particles, when feparated and ra- 

 refied by heat, afcend into the atmofphere. In the day-time, the. 

 rarefaftion of thefe particles is fo great, that they pafs from the 

 earth imperceptibly : after fun-fet, the cool air, rufliing down- 

 wards from the mountains, condenfes, and renders them vifible.. 

 In this ftate the fog refts brooding over the vale, for want of heat 

 to raife it higher, or of wind to difpel it; for the land-wind does. 

 not ulually blow here with an impetuofity fufficient to drive it over 

 tliefe lofty barriers that hem in the vale: but it is obferved, that 

 ftrong Norths in the winter-months force it vehemently through 

 the opening of the Southern chain, through which the Cobre flows, 

 and difperle it for fcveral miles, even to Spanifli Town, and fome- 

 times beyond it ; but, whenever this happens, no fog is to be fcen 

 in that quarter of the vale bordering on the Northern range of 

 mountains from which the wind then fets. Another lingularity is, 

 that, ou the approach of a rainy day, this fog does not appear the 

 antecedent evening ; the reafon of which may be, that fuch even- 

 ings being always clofe and ildtry, it is probable the rarefaiRion 

 continues as well by night as by day. and, the ufual condenfation 

 not taking place, the particles are not rendered obvious to the 

 fight, although perb.aps the reek at fuch times is rather more co- 

 pious than at others. So in the low lands, on the evening pre- 

 ceding rain, the atmofphere feels unufually dole and moift, the 

 thermometer does not fmk after fun-fet, no perceptible vapours are 



noticed,., 



