POPULAR MISCELLANY. 



427 



this singular circumstance, a special form 

 of parasitism, which he calls reciprocal par- 

 asitism. It may, perhaps, be illustrated by 

 the case of the leaf of the maple, which is 

 frequently attacked at the end of the sum- 

 mer by an erysiphus, that occupies the lower 

 surface and fruits there. In the fall the 

 leaves change color and drop off, but the 

 spots occupied by the erysiphus remain 

 green, and so continue, a spot distinct from 

 all the rest of the leaf, for weeks. A simi- 

 lar reciprocation may take place between 

 the algoid and fungoid parts of the lichen. 



A Mechanical Rock-Excavator. By the 



aid of perforators, worked by compressed 

 air, it has been possible to complete in a 

 few years works of subterranean excava- 

 tion like the tunnels of Mont Cenis and 

 Mont St. Gothard, which would formerly 

 have occupied several generations of miners. 

 Hand-drilling can be dispensed with when 

 such perforators are used, for the perforator 

 itself makes the hole destined to receive the 



powder by which the rock is blown up, and 

 the miner has nothing to do but to pick up 

 the pieces. A full solution of the mechan- 

 ical operation of boring is thus obtained, 

 and a self-acting ventilation of the shafts 

 and the removal of all noxious vapors are 

 secured at the same time. Till recently it 

 was not practicable to appl} 7 the perforators 

 in coal-mines charged with explosive gases, 

 for such mines had to be worked without 

 using powder. Two French engineers, Mil. 

 Dubois and Francois, have resolved this 

 difficulty by the construction of an appa- 

 ratus which they call a bosseyeuse, or, as we 

 might call it in English, a pioneer-drill. Its 

 principal feature is a kind of needle-wedge 

 which is made under the action of the per- 

 forator to enter the hole made by the drill 

 and break the rocks up by the force of 

 pressure, without throwing out the frag- 

 ments as powder does. This apparatus has 

 been used in several gas-infected mines in 

 the Belgian coal-basin, with a considerable 

 consequent diminution in the frequency of 



n '- 2 - jig . s in* 



w i 



i 



-c^^ J>" IL L HOLE 



FiG3. 1 to 4. Application of the Pioneer-Needle to break up the Eock in the Hole 



THAT HAS BEEN ALREADY MADE BY THE FOIL OF THE PERFORATOR. 



explosions. The powder, if that is used, is 

 introduced when the hole excavated by the 

 perforator has reached a depth of about 

 twenty-seven inches ; or, if the mine con- 

 tains explosive gases, the needle-wedge is 

 substituted for powder. This apparatus, 

 which is represented in the figures (1 to 4), 

 consists of two bars of iron, shaped like 



segments of a circle, and touching by their 

 ends, the union of which constitutes a cylin- 

 der hollowed along its axis with a conical 

 hole. The section of the two segments is 

 of the same size as the drill-hole, so that 

 they fill it completely when inserted in it. 

 As soon as the drill-hole has been hollowed 

 to a sufficient depth, these two bars are in- 



