THE MECHANICAL EQUIVALENT OP HEAT. 
379 
imperative. The form of regulator devised, although it did not insure that absolute 
uniformity which we could have wished, was yet so effective that we venture to 
describe it at length, in the hope that it may prove useful to others who have met 
with the same difficulty. 
AB (see Plate 4, fig. 2) was a flexible diaphragm made of india-rubber of one ply. 
This was bolted between two brass saucer-shaped discs with tinned surfaces. The 
lower of these discs was connected with a tube leading to the motor ; the upper, by 
means of E, F, G, with an iron reservoir (a mercury bottle) placed on the floor. A 
thin metal rod was attached to a plate in the centre of the diaphragm, and at its lower 
extremity were fixed two cylinders placed so as to form a balance valve. The ends 
of these cylinders were grooved in such a manner that any upward movement 
diminished, and any downward movement increased the size of the channels through 
which the water had to pass. The water from the main entered at (7, and after 
passing through the grooved channels followed the path indicated by the arrows, 
until it arrived at the injector of the motor. An iron tube about 6 feet high was 
supported by a vertical plank, which reached from the floor to the roof of the labora¬ 
tory, The lower end of this tube entered the iron reservoir G at the bottom, and to 
the upper extremity was fastened about 4 feet of high pressure tubing of which the 
free end was lashed to a mercury jar H, similar to the hand one used in a Geissler’s 
pump. This jar rested on a movable shelf which, by means of a rope and pulley, 
could be raised from about 4 feet to 10 feet above the vessel G. Sufficient mercury 
was placed in this jar to fill the flexible tube and the lower 3 or 4 inches of the iron 
vessel G. The taps at K, F, and L, were then opened and water passed into the tube 
E, K, and the vessel G. As the pressure increased, the mercury was forced up the 
tube il/, A until its surface became visible in H. The tap at F was then closed and 
the screw plug at E opened to allow any air in the pipes, or above the diaphragm, to 
escape. E was then closed and first the tap L was opened, then the tap F, until the 
mercury again appeared in H. F wms then finally closed and would not again be 
used unless it was necessary to refill the apparatus. The pressure above AB could 
now be adjusted at will by altering the elevation of H, and the effective pressure 
could thus be changed from 20 lbs. to 60 lbs, per sq. inch. When the tap L was 
opened, if the pressure due to the tap-water on the lower side oi AB exceeded the 
pressure on the top, the valve was lifted and the flow to the motor diminished until 
the pressure of the water which had passed the valves became the same as that above 
the diaphragm. Conversely, if the water pressure diminished, the flow to the motor 
increased. The tap at K was only used to remove the pressure of the mercury 
column from the top of the diaphragm wdien the instrument was not in use. Within 
certain limits, this regulator acted excellently; but if the water pressure fell so low 
that the diaphragm assumed its lowest position, it ceased to be effective. By altering 
the elevation of the movable shelf at H, the rate of the motor could be adjusted at 
will, and a scale being fixed to the vertical plank, the extent of the adjustment 
3 c 2 
