TOUCHWOOD TINDER-BOX. 101 



<{ They should be employed also in the syringes of air-guns, of 

 fountains acting by compressed air, of the apparatus for artificial 

 mineral waters, of fire.en-ines, which are worked with so much 

 labour, and even of air-pumps. As the shortening 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 

 part where the spunk is placed, because there the transient action 

 of inflammation takes place, and a slight emission of air would 

 prevent the effect, but this effect is produced, though 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 hare seen it 

 were greatly surprised. 



" In the length of the piston I made a groove a quarter of a 

 line broad. The spunk took fire as before. Three other grooves 

 were added successively opposite one another, so as to divide the 

 piston into four equal parts ; and still the spunk took fire. 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 instrument is easily obtained by push, 

 ing 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 continual 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 allow 

 the air to pass in too large quantity. 



*' The piston with four grooves acting very well, I made one 

 with a single groove, of dimensions equal to the other four, and 

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

 The following are the reasons of this difference. 



il The extremity of the grooved pistons exhibits the area of a 

 circle, the periphery of which touches the interior edge of the 

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

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

 the grooves, that are continued through the length of the piston, 

 and communicate with the external air. When the piston is 

 pushed with sufficient velocity to kindle the spunk, the parts of 

 the column corresponding to the grooves rush into them with equal 

 velocity ; but the friction they experience in passing through such 

 narrow tubes occasions a resistance to their passage, a kind of 



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