liSG Ignition Jrom compressed Air. 



The piston with four grooves acting very well, I made on«! 

 with a single grddve, of dimensions equal to the other four, 

 and what I foresaw actually took place : there was no in- 

 iiamaiation. The following are the reasons of this dif- 

 ference. ; 



The extremity of the grooved piston 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. Only the parts 

 corresponding to the grooves, that are continued through 

 the length of the piston, communicate with the exter- 

 nal air. When the piston is pushed with sufficient ve- 

 locity to kindle the spunk, the pans of the column cor- 

 responding to the grooves rush into them with equal velo- 

 city ; but the friction they experience in passing through 

 such narrow tubes occasions a resistance to their passage j 

 a kind of choking, that suffers only a pari to escape, whilfe 

 the column resting on the areji ot the piston is pushed en- 

 tirely toward the extremity of the tiibe, where the spunk 

 to be kindled lies. 



In the piston with a single broad groove, the area of the- 

 circle, on which the Column of air rests, is much smaller, 

 consequently the column itself is less. The resistance the 

 air experiences in passing through the groove is next td 

 nothing ; for we hear no noise on moving the piston back- 

 ward and forward; and as air expands in all directions^ 

 when the piston is moved, the colunm resting on the arei 

 of the circle, resting at the same time laterally on that 

 which answers to the groove, it recedes from all the points 

 of contact, and flows entirely through the channel it find! 

 open. It is so true, that it wholly floWs out, that the pis- 

 ton, when it touches the extremity of the tube, remain^ 

 there 5 While with other pistons a sufficient quantity of ait" 

 is retained to occasion a spriiig and repel them. 



I think it proper to say a word or two on the quality of 

 the spunk. The driest, softest, and least impregnated with 

 tiitre should be chosen. In thjit of the best quality a piece 

 Ivill not always be found equally good throughout. Some 

 contains a great deal of nitre, and is kindled with more 

 difficulty. 1 his may be known by the cool taste it leaves 

 on the tongue ; or by kindling it: for when it has taken 

 fire the nitre melts, and sometimes throws out sparks, that 

 Jnay be dkngerous when they spirt out of the instrument, 

 particularly if made with a cock. As it is usual to blow 

 On the spunk, to try whether it be kindled, a spark may be 



thrown 



