184 



LETTERS ON NATURAL MAGIC. 



the spiral spring will be compressed or condensed, 



as shown at m A, No. 2, while at the other end 



it will be dilated or rarefied, as shown at m B, 



Fiff.il. 



-8*2. JL 



ooonry 



and in the middle of the tube it will have the 

 same degree of compression as in No. 1. When 

 the string is drawn to the other end of the tube 

 B, the spring will be, as in No. 3, condensed at 

 the end B, and dilated at the end A. Now when 

 a column of air vibrates in a pipe AB, the whole 

 of it rushes alternately from B to A, as in No. 2, 

 and from A to B as in No. 3, being condensed at 

 the end A, No. 2, and dilated or rarefied at the 

 end B, while in No. 3 it is rarefied at A and con- 

 densed at B, preserving its natural density at the 

 middle point between A and B. In the case of 

 the spring the ends AB are alternately pushed 

 outwards and pulled inwards by the spring, the 

 end A being pushed outwards in No. 2, and B 

 pulled inwards, while in No. 3 A is pulled inwards 

 and B pushed outwards. 



That the air vibrating in a pipe is actually in 

 the state now described, may be shown by boring 

 small holes in the pipe, and putting over them 

 pieces of a fine membrane. The membrane 

 opposite to the middle part between A and B 



