li^jii lion from con:p'cssed Ait. 421 



•F, six lines, the air will no more pass than with a piston of 

 twenty. Accordingly, for a tube of six inches I have re- 

 fluced the piston to six lines, which adds an inch to the 

 column of air, and diminishes the friction two-thirds, so 

 that the oflect of the tinder-box is more certain, and it is 

 more easiiv used. With a lif.lc dexterity you may kindle 

 the spunk by holding the tube in one hund and pushing the 

 piston \\ ith the other, without beina; obliged to rest it on 

 a table, or any other solid body. Mr. Dumotiez, a skilful 

 maker of philosophical instruments, is so fully convinced 

 of the advantage of short pistons, that he now makes them 

 ot these dimensions*. 



They should be employed also in the syringes of air 

 gunsf, of fountains acting bv compressed air, of the ap- 

 paratus for artificial mineral waters, of fire-engines, which 

 are worked wiih so nuich labour, and even of air-pumps. 

 As the shortening the piston is an advantage to the pump, 

 we obtain a o;rcaier 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 ?punk is placed, because tliere the tran- 

 sient action of inflammation takes place, and a slight 

 emission of air would prevent the effect. But this efiect 

 is produced, though the piston does suflVr the air in the 

 tube to pass it. To satifsfy myself of this, I made the fol- 

 lowing experiment, at which they who have seen it were 

 greatly sur-prised. 



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, seas . 

 to divide the piston into equal parts ; and still the spimk took 

 fire. When the grooved piston is moved backward and for- 

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

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

 ilisirunient is easily obtained by pushing it with the hand. 

 This kind of piston would be preferable to those that fit accu- 

 rately, if a solid substance were employed, hard enough to 

 rtsisl the continual friction of the air passing through the 

 grooves, if I may be allowed (he expression. The grooves 

 »n leather pistons soon alter their shape, and spread so a» 

 to allow the air to pass it in loo large quantity. 



• Mr Gill of St. James's itreel, Londoni mxkcs these Instruir.ents of siiU 

 •mailer dinieiisicins. — V.dvy. 



+ In the air pun<) of Gcinany, which arc th« be»t we knvw, the piston of 

 the tyringe is •xtreincly thori. 



The 



