222 IGNITION FROM COMPRESSED AIK. 



the grooves, that are continued through the length of the pis- 

 ton, 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 thern 

 with equal velocity ; but the friction they experience in passing 

 through such narrow tubes occasions a resistance to their pas- 

 sage, a kind of choaking, that suffers only a part to escape, 

 while the column resting on the area of the piston is pushed 

 entirely toward the extremity of the tube, where the spunk to 

 be kindled lies, 

 and the large j^j d^g piston with a single broad groove, the area of the cir- 

 cle, on which the column of air rests, is much smaller, conse- 

 quentiy the column itself is less. The resistance the air expe- 

 riences in passing through the groove is next to nothing; for 

 we hear no noise on moving the piston backward and forv/ard j 

 and as a r expands in all directions, when the piston is moved, 

 the column resting on the area 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 finds open. It is so true, that it wholly flows 

 out, that the piston, when it touches the extremity of the tube, 

 remains there ; while with other pistons a sufficient quantity 

 of air is retained to occasion a spring and repel them. 

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

 touchwood. spunk. The driest, softest, and least impregnated with nitre, 

 should be chosen. In that of the best quality a piece will not 

 always be found equally good throughout. Some contains a 

 great deal of nitre, and is kindled with more difBculty*. This 

 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 may be dangerous when 

 they spirt out of the instrum.ent, particularly if made with a 

 Danger of cock. As it is usual to blow on the spunk, to try whether it 

 blowing on it, be kindled, a spark may be thrown from it into the eye. This 

 painful accident once happened t#me. 



* Spunk is prepared from agaric, which is first boiled in water ; beaten 

 well when dry ; steeped in a strong solution of saltpetre ; and lastly dried 

 in an oven. If the solution of nitre be too strong, the agaric is loaded with 

 this salt, which retards its inflammation, 



5 They 



