RESONANCE. 25 



sound be produced by a direct blow, but a thin board may be set vibra- 

 ting and be made to give a tone by merely producing a suitable tone 

 in its vicinity. Tbe vibrations of the air, caused by the motion of the 

 strings of the piano, communicate themselves to the board, which 

 vibrates in the same intervals as the string and reeuforces the note. 

 The note which a given piece of wood may emit varies in pitch directly 

 with the elasticity, and indirectly with the weight, of the wood. The 

 ability of a properly shaped sounding board to respond freely to all 

 the notes within the range of an instrument, as well as to reflect the 

 character of the notes thus emitted (i. e., whether melodious or not), 

 depends, first, on the structure of the wood and next on the uniformity 

 of the same throughout the board. In the manufacture of musical 

 instruments all wood containing defects, knots, cross grain, resinous 

 tracts, alternations of wide and narrow rings, and all wood in which 

 summer and spring wood are strongly contrasted in structure and vari- 

 able in their proportions, is rejected, and only radial sections (quarter 

 sawed, or split) of wood of uniform structure and growth are used. 



The irregularity in structure, due to the presence of relatively large 

 pores and pith rays, excludes almost all our broad-leaved woods from 

 such use, while the number of eligible woods among conifers is limited 

 by the necessity of combining sufficient strength with uniformity in 

 structure, absence of too pronounced bands of summer 

 wood, and relative freedom from resin. 



Spruce is the favored resonance wood; it is used for 

 sounding boards both in pianos and violins, while for the 

 resistant back and sides of the latter, the highly elastic 

 hard maple is used. Preferably resonance wood is not FlG> i 5 ._cross 

 bent to assume the final form; the belly of the violin is section of a 

 shaped from a thicker piece, so that every fiber is in the f^erl ° 

 original as nearly unstrained condition as possible, and 

 therefore free to vibrate. All wood for musical instruments is, of course, 

 well seasoned, the final drying in kiln or warm room being preceded by 

 careful seasoning at ordinary temperatures often for as many as seven 

 years or more. The improvement of violins, not by age but by long 

 usage, is probably due, not only to the adjustment of the numerous com- 

 ponent parts to each other, but also to a chauge in the wood itself; years 

 of vibrating enabling any given part to vibrate much more readily. 



II.— WEIGHT OF WOOD. 



A small cross section of wood, as in fig. 15, dropped into water, sinks, 

 showing that the substance of which wood fiber or wood is built up is 

 heavier than water. By immersing the wood successively in heavier 

 liquids, until we find a liquid in which it does not sink, and compar- 

 ing the weight of the same with water, we find that wood substance is 

 about 1.6 times as heavy as water, and that this is as true of poplar as 

 of oak or pine. 



