S94 



APPENDIX. 



from, or admitted to the receiver. M a quicksilver gage ^ 



with a small receiver over it : this is designed to show the ^ 



different densities of the air in the large receiver, when the i 

 machine is at work. There is a communication between 



this and the hole I by a brass pipe. This is not an essen- ^ 

 tial part of an air pump, though it is convenient, as showing 



the degree of exhaustion : the more the air is exhausted the  



higher will the mercury rise in the gage. ? 



Artificial Fountain, jig. 26. A is a strong copper vessel, '* 



having a tube that screws into the neck of it, so as to be air i 



tight, and so long as nearly to reach to the bottom : x is the \ 



handle of a stop. Having poured some water into the ves- ; 



sel, and screwed in the tube, the condensing syringe is to be \ 



adapted, and the air condensed. The stop is to be shut \ 



while the syringe is unscrewed, then, on opening the stop, • 



the air, by its great density acting upon the v/ater in the  

 Yessel, will force it out in a jet to a considerable height, 



LESSON 29. 1 



Sound. Speaking Trumpet, Jig. 20. The voice instead i 

 of being diffused in the open air, is confined within the j 

 trumpet, and the vibrations which spread and -fall against ; 

 the sides of the instrument, are reflected according to the 

 angle of incidence, and fall into the direction of the vibra- \ 

 tions which proceed straight forwards. The wliole of the J 

 vibrations are thus collected into a focus, and if the ear be \i 

 situated in or near the spot, the sound is prodigiously in- > 

 creased. The reflected rays are distinguished from those ] 

 of incidence, by being dotted, and they are brought to a ? 

 focus at F. " \ 



LESSON 30. i 



Musical Sounds. — The line A B, Jig. 17. represents a ; 

 musical string fastened at both ends. Drawn out in the -| 

 situation A C B, and then let go, it will, in consequence of ; 

 its elasticity, not only come back to its position A B, but go ^ 

 to the situation A D B, or nearly as far from A B as A C B : 

 was on the other side. All the motion one way is called one j 

 vibration ; after this, the string will go again nearly as far ^ 

 a;6 C, making a second vibration, then nearly as far as D, 1 

 making a third vibration, and so on, diminishing the extent 



