cal. 4.197 4.188 4.181 4.176 4.175 4.177

These are expressed in terms of the hydrogen scale, but the difference
from the nitrogen scale is so small as to be within the limits of
experimental error in this particular case. Rowland himself considered
his results to be probably correct to one part in 500, and supposed
that the greatest uncertainty lay in the comparison of the scale of
his mercury thermometer with the air thermometer. The subsequent
correction, though not carried out strictly under the conditions of
the experiment, showed that the order of accuracy of his work about
the middle of the range from 15 deg. to 25 deg. was at least 1 in
1000, and probably 1 in 2000. At 30 deg. he considered that, owing to
the increasing magnitude and uncertainty of the radiation correction,
there "might be a small error in the direction of making the
equivalent too great, and that the specific heat might go on
decreasing to even 40 deg. C." The results considered with reference
to the variation of the specific heat of water are shown in the curve
marked Rowland in Fig. 6.

[Illustration: FIG. 5.]

S 10. _Osborne Reynolds and W.H. Moorby (Phil. Trans._, 1897, p. 381)
determined the mechanical equivalent of the mean thermal unit between
0 deg. and 100 deg. C., on a very large scale, with a Froude-Reynolds
hydraulic brake and a steam-engine of 100 h.p. This brake is
practically a Joule calorimeter, ingeniously designed to churn the
water in such a manner as to develop the greatest possible resistance.
The admission of water at 0 deg. C. to the brake was controlled by
hand in such a manner as to keep the outflow nearly at the
boiling-point, the quantity of water in the brake required to produce
a constant torque being regulated automatically, as the speed varied,
by a valve worked by the lifting of the weighted lever attached to the
brake.

[Illustration: FIG. 6.]

The accompanying illustration (fig. 7) shows the brake lagged with
cotton-wool, and the 4-ft. lever to which the weights are suspended.
The power of the brake may be estimated by comparison with the size of
the rope pulley seen behind it on the same shaft. With 300 pounds on a
4-ft. lever at 300 revolutions per minute, the rate of generation of
heat was about 12 kilo-calories per second. In spite of the large
range of temperature, the correction for external loss of heat
amo