MR. JOULE ON THE iVIECHANICAL EQUIVALENT OF HEAT. 
65 
b, b, &c., affixed to a framework also in sheet brass. The brass axis of the paddle- 
wheel worked freely, but without shaking, on its bearings at c, c, and at d was divided 
into two parts by a piece of boxwood intervening, so as to prevent the conduction of 
heat in that direction. 
Fig. 3 represents the copper vessel into which the revolving apparatus was firmly 
fitted : it had a copper lid, the flange of which, furnished with a very thin washer of 
leather saturated with white-lead, could be screwed perfectly water-tight to the flange 
of the copper vessel. In the lid there were two necks, a, b, the former for the axis to 
revolve in without touching, the latter for the insertion of the thermometer. 
Besides the above I had a similar apparatus for experiments on the friction of mer- 
cury, which is represented by figs. 4, 5 and 6. It differed from the apparatus already 
described in its size; number of vanes, of which six were rotary and eight sets sta- 
tionary ; and material, which was wrought iron in the paddle-wheel, and cast iron 
in the vessel and lid. 
Being anxious to extend my experiments to the friction of solids, I also procured 
the apparatus represented by fig. 7, in which a a is the axis revolving along with the 
beveled cast-iron wheel b, the rim of which was turned true. By means of the lever 
c, which had a ring in its centre for the axis to pass through, and two short arms d, 
the bevel turned cast-iron wheel e could be pressed against the revolving wheel ; the 
degree of force applied being regulated by hand by means of the wooden lever f 
attached to the perpendicular iron rod g. Fig. 8 represents the apparatus in its 
cast-iron vessel. 
Fig. 9 is a perspective view of the machinery employed to set the frictional appa- 
ratus just described in motion, a a are wooden pulleys, 1 foot in diameter and 
2 inches thick, having wooden rollers bh,bb,2 inches in diameter, and steel axles cc, cc, 
one quarter of an inch in diameter. The pulleys were turned perfectly true and equal 
to one another. Their axles were supported by brass friction wheels dddd, dddd, the 
steel axles of which worked in holes drilled into brass plates attached to a very strong 
wooden framework firmly fixed into the walls of the apartment*. 
The leaden weights e, e, which in some of the ensuing experiments weighed about 
29 lbs., and in others about 10 lbs. a piece, were suspended by string from the rollers 
bb, b b ; and fine twine attached to the pulleys a a, connected them with the central 
roller^^, which, by means of a pin, could with facility be attached to, or removed 
from, the axis of the frictional apparatus. 
The wooden stool g, upon which the frictional apparatus stood, was perforated by 
a number of transverse slits, so cut out that only a very few points of wood came in 
contact with the metal, whilst the air had free access to almost every part of it. In 
this way the conduction of heat to the substance of the stool was avoided. 
* This was a spacious cellar, which had the advantage of possessing an uniformity of temperature far supe- 
rior to that of any other laboratory I could have used. 
MDCCCL. 
K 
