Lesson XIII. 





157 



I IONS ON LESSON XII. 



1. Describe the principle of the diop- 

 trical : 



-' H.,w i> of this telescope 



calculated ? 



1 >escribe the camera obscura. 



4. \v < .tnitra lucida? 



ACOUSTICS. 



;ODDcnox. The science of Acoustics forms a very considerable branch of 



.it part which treats of the nature of sound, and the law* 

 . the origin, propagation, and perception of sound. The term acoustics is of 

 origin, being derived from akouo ('Airouw), to hear, and sometimes 



: ined Phonics, which is also a word derived from the Greek word 

 pkone (*u>-j;). a voice, or sound. 



LESSON XIII. 



- atmospheric vibration takes place, we experience the sensation of sound. If 

 a bell rings, or a bird sings, the knowledge that such is taking 

 place is conveyed to our brain by means of the vibrations pro- 

 duced in the atmosphere, which cause wave-like motions in ih 

 air. These waves of sound are collected by the concha, or 

 \f external ear, from which they are conveyed along the auditory 

 canal or external auditory passage, and impressed upon a fine 

 nervous membrane, called the drum of the ear or 

 which is situated at the bottom of the auditory canal, 

 tympanum augments the vil : i-s i \tnnu- 



and transmits them to the internal ear partly by means of a 

 chain of little bones, which are placed behind it, and partly by 

 means of the air striking another membrane, and causing vibra- 

 tions there. The vibrations are then admit: lal \rinth, which is a bony 

 maze, consisting of three portions, viz., the vestibule, the semicircular canals, and the 

 ;i, in fact, the true auditory or^an. The audifon ni-rve h iving received 

 the necessary impressions, conveys them to the brain. I ha\v thought it necessary to 

 inform you thus much of the structure of the ear, before i-ntering upon the science of 

 acoustics ; but when we consider the anatomy of our bodies, we shall learn more 

 about the internal arrangements of this curious structure. 



QUESTIONS. 



143. T. Is air necessary to tin- ! 



uration of sonorous vibn- 



/' N >. It is the usual medium; hut 



(iii'l, or n< rial body will answer, 



it form* :i 



Etween the sounding body and the ear. 

 Pijr. 12. a, the tgmp**m or <n ..,Ht,,,. or llttlr l,.v 



f. the comrk*. or external er ; / 1, tb* radttOfjr eawO, or xlwnal 



144. T. Why is it necessary to have 

 a oonti; i 



/ I ,1 use sound cannot be conveyed 



145. T. I low ,lo you know thataooRd 

 cannot be conveyed in a vacuum ? 



/' Ik-cause if a small bell be aw 



