188 C. K. Wead—Intensity of Sound. 
2 : : 
(23) hie =S= energy per sec. passing through the unit of 
surface at the limit of hearing; or for 200 ft. 
Suc a ; ps 
= 97 (200 X 30'S)? =2V xx 4'3 xX 10~® ergs. per sec. per square cm. 
The values for S are given in col. 6. They seem to indicate 
that the ear is most sensitive to notes in the middle octave; but 
with regard to Ut, the listeners found it so near the pitch of 
the ever-present noises that it was more easily lost than other 
notes. 
TABLE VI, 
1 2 3 4 5 6 7 
Fork 2V OMe |8os——l Dist 
ork, Zz K K o> ID, Distance. 
10? x | |Eres.per.s.| 19-8 x 
Ut 3-7 62 |°195| 1200 8300 | 50 ft. 
- 70 220 | °20 4400 7600 {100 * 
Ut: 2°5 "7 | °085 650 280 }200 ‘ 
? 3°8 178 | 09 1605 SIO: |3CO * 
Sol, 16 51 | ‘12 610 260 {200 ‘ 
Ut, 2°0 160: }-1% 255 110 {200 “ 
Sol, 15 170 | °30 5100 2200 |200 “* 
Ut; 72 63 | °26 1640 710 |200 “ 
The Ut, fork was mounted before a resonance box, and fitted 
to be driven by an electro magnet; Ut,, Sol, and Ut, were 
mounted on boxes of the usual form open at one end; the Sol, 
and Ut, were open at both ends. The Ut, was heard much 
better in front of the box than behind, but as there were four 
observers, and those ‘before and behind thé box exchanged 
__ places, any error from this difference is eliminated; no difference. 
was detected with the other forks. 
Two in-door experiments may be given: the Ut, was taken 
- near one end of a long hallway, and, being turned in various 
directions, the amplitude was noted when the sound ceased to a 
listener about 50 ft. away; here we cannot apply the law of 
inverse squares; the best thing is to assume that all the energy 
passes through every transverse section of the hall, although 
we know that some was absorbed by the end wall, and some 
escaped by a stairway : 
2Vx=190 and area =19 m*? .-. S<100000 x 1078. 
eo Again -the Ut, fork was heard about 2m. distant when the 
amplitude was less than jj, ; as the fork was near a wall, assume 
that the energy passed out through a quarter of a sphere; and 
