Velocity of Sound in Air. 453 
and in either case be careful that the blast is of exactly the 
power to give the desired note without constraint. 
The tubes with which I experimented were of four dia- 
meters — *45, '75, 1*25, and 2 - 08 inches respectively (11\7, 
19 - 5, 32*5, and 54*1 millim.). After detecting the possible 
sources of error which have just been described, and trying a 
few modifications of the ordinary organ-pipe, the speaking 
ends or first quarter-wave lengths of the tubes were modified 
in form from cylindrical to pear-shaped, approximating to the 
shape of Helmholtz's resonators, and by this means a pure tone 
was obtained. The blast was obtained from a fan, the wind 
from which passed through a regulating bellows with automatic- 
valve action, and it was found that great care was necessary 
on this point. The pressure in the bellows was 2*5 inches of 
water, and in the speaking mouth in every case very small. 
The temperature was observed by means of a thermometer 
entering the tube, so that the actual temperature during 
vibration might be recorded. The wet-bulb temperature and 
barometric pressure were also taken for moisture correc- 
tion. The pitch was taken from a carefully tested Koenig 
fork of 256 vibrations, and the tubes were set to give a beating 
rate of about 4 per second, the lengths being read by a micro- 
meter and standard rods. All the notes were exceedingly 
feeble, the pressure in the mouth being less than -^ inch of 
water, much under the lowest which Regnault found to in- 
fluence the velocity. 
The results are given in the following Table: — 
Table III. 
Velocity of Sound in dry air, at 0° C, in tubes. 
Diameter of \ 
tube J 
117 millim. 
19-5 millim. 
32-5 millim. 
54*1 millim. 
327-09 
327-14 
■ 32698 
326-70 
327 09 
326-69 
32699 
326-79 
326-70 
326-85 
328-72 
328-74 
328-78 
328-72 
328-72 
328-89 
328-76 
328-84 
328-84 
328-83 
329-90 
329-82 
329-84 
329-70 
329-95 
329-80 
329-53 
329-56 
329-65 
329-48 
Means 
Differences ... 
324-56 
326-90 
328-78 
329-72 
2-34 1-88 0-94 
I found it very difficult to get observations with the tube of 
