ACOUSTICAL INSTRUMENTS. 101 



these groups has distinct characteristics. The stopped pipes 

 have generally a soft and massive tone; the half stopped one, 

 sweet and piercing ; those entirely open, allowing a freer action 

 of harmonics and setting more air in motion, give the more 

 strident, although massive, effect peculiar to open diapasons. 



These three groups are again subject to endless modifications 

 of tone by the proportions of pipes ; the diameter affecting those 

 that are cylindrical, and the depth, from front to back, those that 

 are rectangular. Moreover, the size of the sheet of wind at the 

 mouth, the force of the blast, and its direction, can be made to 

 evoke any peculiarity which the proportions of the pipe afford, 

 and to give prominence to a certain set of harmonics, or to allow 

 a relative succession of harmonics and fundamental, as in the 

 case of the " Viol di gamba " pipe. 



Reed-pipes, on the other hand, have their representative in the 

 trumpet, where the vibration of the human lip, through which the 

 wind passes into the tube, are the exciting cause. On examining 

 a reed-pipe it will be seen how the " thrilling " lip is replaced by 

 a metal tongue known as a " beating " reed. 



Considering that any form of open or " shaded " tube can be 

 applied to the reed in an organ, the great opportunity of 

 varying tone, as compared with the trumpet, is evident, even if 

 the reed were always the same ; for, according to the proportions 

 of the tube, and the gradual or rapid divergence of the cone, it is 

 possible to give predominance either to the fundamental, or to a 

 set of harmonics, or even to particular harmonics alone. But the 

 reed can also be varied either as regards its pliancy, or its curve, 

 or its freedom of motion. 



In organs the reed employed is almost always a "beating reed," 

 i.e., the tongue cannot pass through the slit over which it is set, 

 but strikes against it. 



In free reeds the tongue can pass through without striking. 

 Free reeds were introduced into Europe from China by Kratzen- 

 stein, and brought to perfection under Sir Charles Wheatstone, 



