74 ANNUAL EECORD OF SCIENCE AND INDUSTRY. 



turn by a trellis -work, such as is ordinarily employed in 

 summer-houses. A slight covering of straw is even more 

 effective than such trellis-work. In general, in the climate 

 of Cornwall, it may be stated that in sharp, dry, clear weath- 

 er the soil that is covered with vegetation will be from 8 to 

 10 warmer if it is secured from the sky by straw matting, or 

 other moderately thick material, spread horizontally at the 

 hei2:ht of three or four feet from the gjround. Another ele- 

 ment of greater moment in relation to temperature is the 

 movement of the air. The blight caused bv a local drauoht 

 of icy wind may be frequently traced in well-defined lines in 

 a gap in a northeastern hedge along the rows of early plants. 

 A high wind will rob any horizontal screen of its protecting 

 influence, but the placing of wattled hurdles or other effect- 

 ive vertical screens, about four feet high, on the windward 

 side of a tender crop, in rows twenty or thirty feet apart, 

 must be recommended as a preservative measure, even more 

 important than a provision of horizontal screens. An um- 

 brella carried by a person at night serves at times for protec- 

 tion against some 10 of cold, quite as efficiently as it at 

 other times protects against the snow and the rain. Jour- 

 nal Royal Institution of Cornimall^ 18'74, 111. 



THE INTERNAL HEAT OF THE EARTH. 



According to Nature^ Professor Mohr, of Bonn, has made 

 a very important contribution to our knowledge of the 

 causes of the internal heat of the earth. He starts with the 

 proposition that if the interior is still fused, then, as we ap- 

 proach this furnace, with every increasing depth a less space 

 must be necessary to produce the same increase of heat. In 

 other words, the increase of heat per hundred feet must be- 

 come greater and greater in proportion as we descend. This, 

 however, is directly opposite to the results of the most care- 

 ful thermometric measurements, which show that at a depth 

 of five or six thousand feet we come to a resfion in whicli the 

 temperature ceases to increase; and even supposing large er- 

 rors in these measurements, we must conclude that the re- 

 gion of constant temperature will be reached before we de- 

 scend to a depth of fifteen thousand feet. Professor Mohr 

 finds, from all the facts available to him, evidence that the 

 cause of the increase of heat in the interior of the earth 



