144 THE ANDES OF SOUTHERN PERU 



exceptions. The physical principles involved are, however, the 

 same everywhere. I take for discussion' therefore the case illus- 

 trated by Fig. 92, since this also displays with reasonable fidelity 

 the conditions along that part of the Peruvian coast between 

 Camana and Mollendo which lies in the field of work of the Yale 

 Peruvian Expedition of 1911. 



Three typical positions of the fog bank are shown in the figure, 

 and a fourth — that in which the bank extends indefinitely west- 

 ward — may be supplied by the imagination. 



If the cloud bank be limited to C only the early morning hours 

 at the port are cloudy. If it extend to B the sun is obscured until 

 midday. If it reach as far west as A only a few late afternoon 

 hours are sunny. Once in a while there is a sudden splash of rain 

 — a few drops which astonish the traveler who looks out upon a 

 parched landscape. The smaller drops are evaporated before 

 reaching the earth. In spite of the ever-present threat of rain the 

 coast is extremely arid. Though the vegetation appears to be 

 dried and burned up, the air is humid and for months the sky may 

 be overcast most of the time. So nicely are the rain-making con- 

 ditions balanced that if one of our ordinary low-pressure areas, 

 or so-called cyclonic storms, from the temperate zone were set in 

 motion along the foot of the mountains, the resulting deluge would 

 immediately lay the coast in ruins. The cane-thatched, mud- 

 walled huts and houses would crumble in the heavy rain like a 

 child's sand pile before a rising sea; the alluvial valley land would 

 be coated with infertile gravel ; and mighty rivers of sand, now 

 delicately poised on arid slopes, would inundate large tracts of 

 fertile soil. 



If the fog and cloud bank extend westward indefinitely, the en- 

 tire day may be overcast or the sun appear for a few moments 

 only through occasional rifts. Generally, also, it will make an ap- 

 pearance just before sunset, its red disk completely filling the nar- 

 row space between the under surface of the clouds and the water. 

 I have repeatedly seen the ship's passengers and even the- crew 

 leave the dinner table and collect in wondering groups about the 

 port-holes and doorways the better to see the marvelous play of 



