114 Analysis, tj-c. of Cordier''s Essay upon the 



breath. In some mines, all these causes of error exist 5 in 

 mines long abandoned, very few of them are found ; but 

 making every possible allowance, with all the accuracy that 

 the present state of our knowledge admits, the general fact 

 of the increase of heat in proportion as we descend from the 

 surface, remains untouched. 



From page 10 to page 38 of this memoir, the author is 

 occupied in examining the defects to which former experi- 

 ments are liable. He reduces many of these experiments 

 into a tabular form ; exhibiting the authors, the places, the 

 dates of the observations, the mines, the depths, the tempe- 

 rature of the mine, the mean temperature of the country, and 

 the depth corresponding to an increase of one degree of the 

 centigrade thermometer. On these experiments he makes 

 the following remarks : 



"1. The imperfection of the thermometers; from original 

 inaccuracy, and from inaccuracy proceeding from the effect of 

 time on the mercury, and on the bulb. On the necessity of a pre- 

 vious verification of the mercurial scale. 



" 2. The difficxilty of ascertaining the mean temperature of the 

 place. In this, (and indeed in all other respects,) the experi- 

 ments made in the abandoned quarries under the Observatox'y at 

 Paris, are more free from objection than any others that have 

 been made elsewhere ; they are also more accurate, and longer 

 continued. 



" 3. Doubts may arise on the absolute depths of the mines 

 subjected to observation. The openings of mines are usually be- 

 low the medium level of the country. 



" 4, Observations made on the temperature of springs of fresh 

 water, of artificial fountains, of large streams sufficient to turn 

 a mill at their first exit, may be used, provided care be taken 

 to observe whether they are not influenced by surrounding moun- 

 tainous regions, and other local circumstances. 



" 5. The old opinion of the effect of decomposing pyrites, is 

 now abandoned by those who possess the modern geological in- 

 formation. Pyrites is not so abundant any where, even in the 

 roofs of coal measures, as to produce any notable effect : nor is the 

 decomposition of pyrites easily effected, unless they be broken 

 in pieces ; for their decomposition proceeds by one surface after 

 another." 



He instances this, from the pyrites in the coal mine of St. 

 George Lavencas in Aveyron. 



Cordier then proceeds to the conclusions that may reason- 

 ably be drawn from the experiments passed under review. 



