RADIATION IN ABSOLUTE MEASURE. 
601 
by means of the galvanometer, to have risen to the ciesired point, the flame was 
withdrawn, and the cold water, at the temperature of the laboratory, applied as 
before. It was also found advisable for steadiness of temperature to stop the current 
of cold water through the water-jacket, and to warm up the water in the water- 
jacket (which, while the flow continued, was kept at the temperature of the public 
supply) to be roughly the same as the air temperature of the laboratory. 
The Sprengel pump used for exhausting the enclosure and the duplicated MacLeod 
gauge used for determining the residual pressure are fully described, and certain 
difficulties and uncertainties are pointed out, in my former paper, and need not be 
further spoken of here. 
For the purpose of calculation two pieces of numerical information are still 
required. One is the capacity for heat of the copper globes; and the other, the 
cooling surface. 
The capacity for heat was determined very carefully for these particular globes by 
Mr. D. Macfarlane in 1872 ; and my determination has not sensibly varied from 
his result. I propose, however, to make a fresh determination of this constant as 
soon as possible. 
The surface of the globe is obtained by careful measurement with calipers. But it 
will be observed that, besides loss of heat by radiation, there is also a loss by con¬ 
duction along the two thermo-electric ^rires. When a sooted globe is cooling in 
full air pressure the amount of heat conducted along the thin wires used is very 
small in comparison with that lost by radiation and convection. But when the globe 
is cooling in vacuum or, at any rate, when a highly ^^olished silvered globe is cooling 
in vacuum, the heat lost by the wires is by no means negligible in calculating the 
value of the surface emissivity. 
In making use of the results obtained up to the present time I have used for 
calculation an equation of the form 
- c 'I’ = eS {v - Vo), 
ing gave rise to a most distarbing difficulty at the commencement of the corresponding vacuum series ; 
because it was desirable to commence the observations on cooling as soon as the cooling commenced, but 
it was not till ten or fifteen minutes later that a proper vacuum could be secured, even with the most 
vigorous use of the pump. 
* ‘ Phil. Trans.,’ 1887, pp. 444-446. There is just one remark which it seems worth while to make, on 
account of repeated misunderstanding, namely, that the MacLeod gauge does not measure (nor profess 
to measure) pressure in the pump of any gas collapsible by pressure, for any reason whatever, at the 
existing temperature of the gauge. Thus it does not measure the pressure of mex’cury vapour which 
may be present in the so-called vacuum, nor the pressure of vapour of water, for example. Nor does it 
even measure the pressure of any gas which may with the help of a small pressure be caused to collapse 
against the walls of the tube, or by absorption into, or on, the mercury used in the gauge. I have shown 
the possibility of this last-named occurrence in a paper on an “ Improved Mercury Pump,” British 
Association, 1886. 
MDCCCXCIII.-A. 4 H 
