IGNITION FROM COMPRESSED AIR. 221 



They should be employed also in the syringes of air guns*^ Short pistons 

 of fountains acting by compressed air, of the apparatus for ?^ advantage 

 artificial mineral waters, of fire-engines, which are worked machines, 

 with so much labour, and even of air-pumps. As the shorten- 

 ing the piston is an advantage to the pump, we obtain a greater 

 effect with less labour, and in a shorter time, than with long 

 pistons. 



It is essential too, that the instrument does not leak at the The chamber 

 part where the spunk is placed, because there the transient ^^^^ be air- 

 action of inflammation takes place, and a slight emission of air essential for 

 would prevent the effect. But this effect is produced, though the piston to 

 the piston does suffer the air in the tube to pass it. To satisfy ^'^°' 

 myself of this, I made the following experiment, at which they 

 who have seen it were greatly surprised. 



In the length of the piston I made a groove a quarter of a po^r trrooTe« 

 line broad. The spunk took fire as before. Three other ma<'e in the 

 grooves were added successively opposite one another, so as to prevent the"° 

 divide the piston into four equal parts ; and still the spunk took effect, 

 firef. When the grooved piston is moved backwards and 

 forwards in the tube, the air may be heard entering or issuing 

 out ; and the friction is so slight, that the effect of the instru- 

 JTient is easily obtained by pushing it with the hand. This kind 

 of piston would be preferable to those that fit accurately, if a 

 solid substance were employed, hard enough to resist the con- . 

 tinual friction of the air passing through the grooves, if I 

 may be allowed the expression. The grooves in leather pistons 

 soon alter their shape, and spread so as to allov*' the air to pass 

 in too large quantity. 



The piston with four grooves acting very well, I made one ^^"t one of 

 with a single groove, of dimensions equal to the other four, and gtonsdid™^"* 

 what I foresaw actually took place : there was no inflammation. 

 The following are the reasons of this difference. 



The extremity of the grooved piston exhibits the area of a Why the small 

 circle, the peirphery of which touches the interior edge of the plgvent th° "°^ 

 grooves. The column of air contained in the tube rests almost action, 

 wholly on this base. There are only the parts corresponding to 



* In the air-guns of Germany, which are the best we know, the 

 piston of the syringe is e::tremeiy short, 



+ I tried this experiment wiih Mr. Bancks, at his house in the Strand, 

 and we founr) it succeed compietel)' with a common condensing; syringe oi: 

 hii makicg. — C. 



the 



