ON" THE MECHAEHCAL EQUIVALENT OE HEAT. 
323 
In this design account had to be taken of the requisite head of water necessary to 
force some 20 lbs. a minute tbroimh the coil. It was estimated that this would 
O 
require some 30 lbs. on the square inch, which, together with the 5 lbs. excess of 
pressure in the brake above the atmosphere, and a margin of some 25 lbs. in order 
to secure steadiness of flow, made a total of GO lbs. on the square inch, or 122 feet of 
head. 
The Circulating Pump. 
20. It was essential that this head should be approximately steady, and under 
control during the trials, also that the water should be drawn as directly as possible 
from the town’s mains, in order to secure both the low temperature and great purity 
of this water. This precluded the direct use of the water from the large tank in the 
tower, which would otherwise have just aftbrded this head. It also precluded the 
use of such head as there might be in the town’s mains, as this was insufficient and 
continually varying, so that some special means of imparting the steady head to the 
water after drawing it from the tower mains was necessary. This involved pumping 
the water through the ice-cooler and brake. It might be done by pumping it from 
the service tank in the laboratory into an. accumulator under a constant load, or by 
passing the water through a centrifugal pump, running at a steady speed on its way 
to the brake. 
The facilities in the laboratory decided this question. There already existed the 
quadruple vortex turbine, with four three-inch wheels in series, worked from the 
water in the tower, which would work steadily up to 1 h.p., in a j^osition which would 
be convenient for driving a centrifugal pump in the in-cmcuit of the pipe leading to 
the brake ; I also had a quintuple centrifugal pump with flve l|-inch wheels in series 
which was adapted to the purpose. It was decided, therefore, to lead the water from 
the surface tank, 9 feet above the floor, into the quintuple pump, driven b}^ the 
turbine under a constant and controllable head, so that the head would be raised to 
the required mnount. Then, to lead the water through tlie cooling coils to a pressure 
gauge close to the brake, and thence through a regulating valve into the passage with 
the thermometer leading into the brake. (See Plates 6 and 7.) 
The Outlet from the Condenser. 
21. In order to prevent the formation of steam, owing to the presence of air in the 
water, before it had passed the outlet thermometer, it was necessary to maintain a 
certain pressure in the effluent water as it passed the bulb of this tliermometer. At 
first it was thought that a head of 5 or 6 feet would suffice. In oi'der to secure this, 
the level of the condenser being some 3 feet above the bulb, the pipe leading from 
the condenser was carried up vertically about 3 feet higher, then turned over and 
led down again to an orifice immediately over the switch, while from the top of the 
bend a vertical branch extended upwards about 3 feet, with its mouth open, to the 
air. Tills was subsequently raised. (See Plate 4.) 
2 T 2 
