98 SCIENTIFIC APPARATUS. 



Pipes and reeds present the same difficulty ; the former, how- 

 ever, in their various forms, are shown ; and the latter, as " free 

 reeds," form the foundation of the " seraphine," harmonium, and 

 American organ principle. Their " beating " modifications are to 

 be seen in the organ, and in several orchestral instruments, such 

 as the clarionet. 



Membranes are chiefly met with in the form of drums, of which 

 three varieties the side drum, the bass drum, and kettle-drum 

 are in use. The first speaks a high, but undefined, note, the 

 penetrating power of which is enhanced by the " snare," a loop of 

 catgut stretched tightly across the lower membrane. The bass 

 drum gives a similarly undefined, but extremely low, note. Both 

 act chiefly in marking time and accent. The kettle-drum used 

 in orchestras is a more strictly musical instrument than the 

 two former. It gives a definite note of 8 or 1 6 foot tone. Two 

 tuned to the tonic and dominant of the scale, or exceptionally 

 to some other interval, are commonly employed, but a larger 

 number is occasionally used with effect. Berlioz proposed 

 to extend their employment, and in one of his works, the 

 " Requiem," writes for a chromatic octave of twelve, tuned in 

 semitones. 



Modes of propagation in Air were studied by a French Commis- 

 sion in 1822. The observers were divided into two groups, 

 placed respectively at Mont Lhery and at Villejuif, and twelve 

 alternate cannon shots were fired from each station. By measur- 

 ing the interval between the appearance of the flash and the 

 arrival of the sound, the rapidity of the latter was fixed, for 

 1 6 C, at 340-9 metres per second. 



In Water, Colladon and Sturm made excellent observations on 

 the Lake of Geneva. The sound was produced by the stroke of 

 a hammer on a submerged bell, some gunpowder being at the 

 same instant ignited. It was received by a kind of speaking 

 trumpet, shown in the Exhibition, the bell being covered by a 

 sheet of metal. The distance between the two stations, 13,487 



