HEAT. 683 



and it has been uniformly found that after passing the limit of 

 surface-action (p. 682) the heat quite regularly increases. The 

 rate is 1° F. for 50 or 60 feet of descent. At the Artesian well of 

 G-renelle, a temperature of 85° F. was obtained at 2000 feet, equi- 

 valent to 1° F. for every 60 feet of descent. In Westphalia, at 

 Neusalzwerk, in a well 2200 feet deep, the temperature at the 

 bottom was 91° F., or 1° F. for 50 feet of descent. At Pregny, near 

 Geneva, a depth of 680 feet gave 63° F. 



It has been proposed to make a tropical climate in the Garden 

 of Plants by taking the heat from the earth's interior. Arago and 

 Walferdin have estimated that at a depth of 3000 feet the water 

 would have a temperature of 200° F., " sufficient not only to cheer 

 the tropical birds and monkeys of the Zoological Gardens and the 

 hot-houses and green-houses of the establishment, but to give 

 warm baths to the inhabitants of Paris." 



At Yakutsk, Siberia, Magnus found a gain of 15° F. in descending 

 407 feet, equal to 1° F. for 27 feet. 



The rate 1° F. for 50 feet of descent in the latitude of New York 

 would give heat enough to boil water at a depth of 8100 feet ; and 

 3000° F., the fusing-point of iron, at a depth of about 28 miles. 

 As the ratio would not be an arithmetical one, since the fusing- 

 point of any substance is increased by pressure, the depth of fusion 

 would exceed this amount. But the facts still prove that the earth 

 has a source of heat within. 



4. The warm climate over the whole globe in the early ages, and 

 the serial diminution of heat with the progress of time until now, 

 when there is a frigid Arctic, corresponds with the idea of a cooling 

 globe. This cause appears to have acted conjointly with that con- 

 nected with the geographical distribution and height of the land, 

 referred to on the preceding page. 



At present, very little of the interior heat reaches the surface. According to 

 Poisson, the amount is only one-seventeenth of a degree Fahrenheit ; and to reduce 

 this amount one-half, or to one-thirty-fourth, would now require 100,000,000,000 

 years. Mr. Hopkins, of England, has stated that, supposing this the only 

 mode of cooling, it must have required as long a time as this to have dimi- 

 nished the temperature the last two or three degrees of its decrease. 



5. The wide distribution of volcanoes over the globe is evidence 

 of internal heat. Volcanoes, extinct or active, border the Pacific 

 from Fuegia to the Arctic ; through the Aleutian Archipelago to 

 Asia ; down the Asiatic coast, through Kamtchatka, Japan, and the 

 Philippines, to New Guinea, New Hebrides, and New Zealand ; and 

 they constitute half of the islands of this ocean, two of which, in 

 Hawaii, are nearly 14,000 feet high. This volcanic region is equal 



