224 



♦ KNOW^LEDGE ♦ 



[August 1, 1887. 



perature remains all the year round at tbe mean temperature 

 of the air above ; at gi-eater depths it increases about 1° to 

 60 or 70 feet. In the first case the aii' of the pit will be 

 cooler in the summer and warmer in the winter than the air 

 above. Let us suppose, for example, that the depth of one 

 .shaft from the surface is 600 feet and that of the other 

 100 feet. Both shafts convey to the workings below the 

 general pressure of the gi-eat atmospheric ocean. But the 

 column in the deeper shaft is made up of 600 feet of iinder- 

 gi-ound air supplemented by the outer air, while the shallower 



pit has but 100 feet of underground air, and makes up the 

 equivalent 600 by 500 more of outer air. 



Therefore, if the outer air is denser than the undergi-oimd 

 ail' (as in winter time), the pressure in the 100-feet shaft 

 will preponderate, and vice versa in the summer, when at a 

 given level the column of air pressing through the deeper 

 shaft includes .500 feet of denser underground air in the 

 place of 500 feet of warmer outer air that is pressing through 

 the 100-feet shaft. For reasons presently to be explained, 

 such automatic ventilation is but little used in coalpits. 



THE ONE-SCALE ATLAS. 



MAP I 



The Imear scale radiaJfy ij xcniform. 

 'dvaughout ; Ow fhyfort scale is 



and. byh*- at th£ edge . 



"^>-J« 



Jcale ofMilea at centre of -nujp 



En^ ah Miles 



Sjriall. ar^as are greater hyP^s^luiJtf 

 way toihe edge, o/ui hy /^4,*^ a4t 

 ffie ed^e . 



