CHEMICAL AND PHYSICAL NOTES. 147 
stationary it is dissipating the whole of the heat which it is receiving. 
Now n gives the rate at which it is cooling, in gramme-degrees * 
(gv.°C.) per second; therefore the bundle of sun’s rays of sectional 
area g is supplying heat at the rate of  gr.°C. per second, and 
p= " is this rate for a bundle of rays of 1 square centimetre section. 
g 
Tt will be seen that all the values of p given by thermometer B are 
lower than those given by A. In line ¢ we have the ratio of pa: pp, 
and it will be seen that the values of g agree very well with each other. 
The mean value is 0°873, the extremes being 0°860 and 0°896; that 
is to say, thermometer B indicates 13 per cent. less heat than thermo- 
meter A for 1 square centimetre of sun’s rays. Yet A has a silvered 
glass bulb and B has a plain uncoated glass bulb. It is possible that 
to this difference is due the difference in the results obtained. The 
silvered bulb dissipates by reflection some of the heat which strikes 
its metallic surface, but whatever is not reflected by this surface 
passes into the bulb of the thermometer. In the case of the uncoated 
glass bulb there are two surfaces of reflection, namely that separating 
glass from air and that separating glass from mercury. The heat 
which has passed through the outer glass surface has still to pass the 
inner surface, where some of it is rejected by reflection. 
Taking the maximum value of p, namely 0:00889 gr.°C. of heat 
supplied per square centimetre per second, and multiplying it by 60, 
we have 0°533 gr.°C. per square centimetre per minute. This is the 
heat actually taken up by the silvered bulb of a thermometer from 
1 square centimetre of the rays of the winter sun, which have passed 
obliquely through a glass window. The sky was quite clear and 
cloudless, but the sun’s zenith distance was 57°. In order to allow 
for this, we use the formula given by Sir John Herschel,f from which 
we obtain the value of the solar constant 
0°533 ‘ ‘ 
A= a= 1:1114 er.°C. per square centimetre per minute, 
3 
where 2 is the transmission coefficient of the air for heat and 
1°84 = sec. 57°. 
Two-thirds of this, or 0°7409, would then be the heating power 
of the rays of the vertical sun at the surface of the earth at a height 
of 1850 metres above the sea. This calculation has been carried out 
* It is convenient to give compound names to compound units; they then explain 
themselves. One gramme-degree (1 gr.°C.) is the heat required to raise the tempera- 
ture of one gramme of water by one Centigrade degree. Names such as culorie or 
therm are indefinite, and may be confusing. 
t ‘Meteorology,’ by Sir John Herschel, Bart , Edinburgh, 1861, p. 10. 
y 
Ls 
