NOTES 335 



as the sun reduces the strength of the morning or land breeze, it 

 cannot be through his influence that the Etesian winds then 

 begin to blow. But he does not explain how he would him- 

 self account for their occurrence. They are now known to 

 be further illustrations of the influence of atmospheric pressure. 

 In summer, when the hot region of the Sahara becomes a vast 

 area of low pressure, the air streams into it from the north across 

 the Mediterranean basin. 



The account given of cloud winds (203) is an excellent illus- 

 tration of the utter ignorance of the philosophers of antiquity 

 of the very rudiments of meteorology, and, at the same time, of 

 the confidence with which they offered their explanations of the 

 phenomena of the atmosphere. Even now, after prolonged 

 investigation, the laws that regulate the production of furious 

 winds and gusts connected with clouds are far from being fully 

 understood. The boldest meteorologist of to-day, with all his 

 detailed experience, would hesitate to express his opinion as dog- 

 matically as is done in the text. The idea that air accumulating 

 either above ground or below acquires a vast disruptive force, 

 obtained wide credence in early times. It was this pent-up 

 accumulation which was supposed to burst clouds asunder and 

 produce thunder-storms, while the same energy in caverns under 

 ground led to earthquakes and the eruptions of volcanoes. 



The occurrence of whirlwinds is explained by Seneca from the 

 analogy of eddies in a river. As the water meets with impedi- 

 ments in its flow, it is driven back and made to whirl round 

 before it can continue the onward current, so the wind, as long 

 as it meets with no obstacle, sweeps on, but when it is thrown 

 back by any projection in its course, or is collected together into 

 a highly inclined narrow pipe, it whirls round upon itself like the 

 eddies of a river. But the cause of the vorticose movement 

 where there is no visible impediment is, of course, left un- 

 accounted for. 



In the fifteenth chapter of this Book a story is told of Philip 

 of Macedon, who sent down a party of miners to examine an old 

 mine. The men brought back to daylight a wonderful tale of 

 vast caverns with high over-arching roofs, and filled with huge rivers 

 and vast lakes. If the author's intention was to connect the 

 spaciousness of these underground chambers with the operations 

 of ancient miners, he was sadly mistaken, since at no time has 

 metal-mining led to the excavation of huge caverns ; on the con- 

 trary, it has always been pursued in narrow shafts and passages. 

 If the report brought back to the king was veracious, his emis- 

 saries had only come upon a series of natural grottos and tunnels, 

 such as are of common occurrence in limestone districts, and 



