PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 561 



degrees Fah. at 8,600 feet, being a decrease of 28 degrees, or about three 

 degrees for every 1,000 feet. This agrees with other results when rising 

 8,000 or 10,000 feet from the earth's surface. 



Mean Temperature of the Earth. 



The depth at which the annual changes of temperature disappear varies 

 considerably, not only with latitude but with changes in the nature of soil 

 and rocks in the same place. 



Mr. Quetelet, the eminent European physicist, in his recent work " Phy- 

 eique du Globe" gives some interesting statistics illustrative of this fact. 

 Thus this depth is found at Zurich at 83 French feet; Strasburg at 81 feet; 

 Heidelberg, in compact clay, 83 feet; Bonn 72 feet; Paris, in the observa- 

 tory garden, 68 feet; Leith, England, 55 feet; Edinburgh in trap 55, in 

 sand 66, and in sandstone -96 feet. The mean of these is 73 feet. Only 

 below this mean depth do we encounter the central heat of the earth, which 

 corresponds to about one degree for each forty-eight feet descent, and the 

 effect of which on the diurnal and annual variations above the plane of no 

 variation must be inappreciable. 



The above facts satisfactorily account for the cold and hot springs which 

 are found in different parts of the earth's surface. The hot springs rise to 

 surface from a great depth while the fountains or source of the cold springs 

 lay nearer the earth's surface. 



The coal mine of Monkwearmouth, England, the deepest coal mine in the 

 world, has a depth of 1,900 feet from the surface, and the working of coal 

 underneath extends to a distance of two miles from the shaft. About three 

 hundred persons are emplo3'ed in it and six hundred tons are mined daily. 

 The heat at the bottom varies from 84 degrees to 90 degrees Fah., and the 

 miners working in an almost nude state often suffer from extreme heat. 



Mr. George Bartlett. — It seems singular, at first sight, that there should 

 be so few facts known with regard to meteorology; but when we reflect 

 upon the number and diversity of the forces that tend to bring about re- 

 sults in this branch of natural philosophy, we can then comprehend tho 

 difficulty of arriving at accurate conclusions. On the west side of this 

 continent the controlling forces are fewer and more uniform. In San Fran- 

 cisco, California, during the summer from May to October, there is no rain, 

 and the sudden changes of temperature which we have here are seldom if 

 ever witnessed there. During the summer nights the temperature is very 

 uniform and the mornings are very pleasant, the average temperature of 

 which is about 54° fah.; but about noon the sea breeze sets in when a 

 change takes place and the thermometer falls to near 50°, where it remains 

 for several hours; this makes the afternoons very pleasant; the sea breeze 

 lasts till about sunset. During its continuance, however, there are immense 

 clouds of sand and dust which sport through the streets in such a manner 

 that a clean collar would be completely soiled in an hour. Those who are 

 out doors button up their coats and hurry home. I have seen the ther- 

 mometer over 100° in the shade, and yet the effects of the sea breeze is such 

 that this heat is not oppressive. The explanation of this is that the land 

 is heated more by the sun than the ocean, which causes the heated air 



[Am. Inst.] K* 



