474 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



condensed, and, parting with its latent heat, thaws out the 

 ground as indicated. Scientific American, March 9,1872. 



NON-CONDUCTING COMPOSITION FOR ROOFS. 



A non-conducting substance, known as Le Roy's Non-con- 

 ducting Composition, has been used with great success in 

 coating steam-boilers to prevent the loss of heat, and has been 

 applied to another useful purpose in India. In that country 

 corrugated iron is employed as a building material for roofs 

 of houses, on account of its cheapness and freedom from ver- 

 min ; but it becomes very intensely heated in summer, so as 

 to be insupportable, and often injurious to health. This com- 

 position, however, applied to the under surface of these cor- 

 rugated roofs, prevents the radiation of the heat from the 

 iron to the space below, and the house can be kept eight de- 

 grees cooler than when the iron is not covered. The heat in 

 buildings not protected by the composition during the month 

 of December ranged from 74 to 101, while in the protected 

 sheds it ranged from 72 to 94. In one instance the differ- 

 ence between the two was 11. 18^1, March 1, 1872, 625. 



EXHAUSTION OF THE BRITISH COAL SUPPLY. 



Several years ago a royal commission was appointed in 

 Great Britain to inquire into the probable duration of the 

 supply of coal in the British Islands. In view of the depth 

 to which the coal-beds extend, it was, of course, necessary to 

 fix an estimated limit to which mining operations could be 

 profitably carried, and, after due inquiry, this was taken to 

 be 4000 feet, since, although, in some cases, mining is prevent- 

 ed by excess of water, yet in Great Britain the deepest col- 

 lieries are generally the dryest. 



Another point for consideration was the waste in working 

 the mines; but it was assumed that, under a favorable 'sys- 

 tem, the loss should only be about ten per cent., although in 

 many cases it amounts to as much as forty per cent. Taking 

 4000 feet, therefore, as the maximum depth to which work 

 might be expected to extend, and excluding all seams less 

 than one foot in thickness, it is estimated by the committee 

 that there exist in the several coal-fields of Great Britain up- 

 ward of 90,207,000,000 tons, in addition to which there are 

 vast tracts of coal lying beneath the Permian, New Red, and 



