THE MECHANICAL EQUIVALENT OF HEAT. 
435 
agate point and its bearing, the slightest variation in the position of the shaft of the 
stirrer causing very different quantities of heat to be generated. The quantity of heat 
generated by the stirrer varied from about one-sixth to one-fifteenth of the heat 
generated by the current, according to the electromotive force used ; and as the 
capricious changes above referred to amounted, in some cases, to 5 or 6 per cent, of 
the heat due to the stirring, they might affect our ultimate result by as much as a 
half and, in some cases, nearly 1 per cent. The quantity of heat appeared to vary 
with the rate of revolution in such a manner that was a constant, where t was the 
time of rising 1° C. and r the rate of revolution. 
Our experienee of this stirrer led to the following conclusions :— 
(l.) The ideal stirrer must be able to throw the water from the bottom to the top 
of the calorimeter whatever the mass of the water. 
(2.) The “ work done ” by the stirrer should be the same, whatever the depth of 
the water. 
(3.) The “ work done ” by the stirrer should be, as far as possible, employed in 
throwing the water upwards to the lid, rather than in simply causing a rotation of 
the water near the bottom of the calorimeter. 
(4.) The whole of the energy should, if possible, be employed in lifting the v/ater 
and not in overcoming the friction between solid surfaces, i.e., the bearings should be 
outside the calorimeter. 
Although we found it impracticable to devise a form which would entirely fulfil 
the above requirements, the one adopted by us during our 1892 experiments was 
satisfactory. 
During the spring of 1892, we made a large number of trial experiments with 
different forms of stirrers, and it should be borne in mind the form it ultimately 
assumed was an example of the “ survival of the fittest.” A section will be found in 
Plate 2, fig. 2. 
A cylindrical tube AB, 1 inch in diameter, closed at the lower end, rested on four little 
feet about 1/16 of an inch in length, which were fixed to the base of the calorimeter. 
The plate at the lower end was perforated in the centre, so as to allow the end of the 
revolving shaft to pass through without touching it. Ptound this centre hole were 
four slits through which the water in the calorimeter passed into the cylinder. The 
end of the revolving shaft v/as fitted with a small cylinder of agate, wdiich hung 
loosely within a ring fixed to the base of the calorimeter at C. The end of the 
agate was not in contact with the base of the calorimeter. The bearing by which 
the stirrer was supported was fixed at the top of the central glass tube, and thus any 
increased downward thrust of the stirrer did not alter the friction wdthin the calori¬ 
meter. The stirrer consisted of an inverted cone with nearly vertical paddles at its 
extremities (a section is shown at D). When the stirrer revolved, the water was 
drawn in through the base of the cylinder, thrown outwards by the paddles, and, 
being unable to escape at the lower end, mounted rapidly to A, whence it was ejected 
with considerable violence against the roof of the calorimeter. If the rate exceeded 
3 K 2 
