CHEMICAL AND PHYSICAL NOTES. 149 
130 x 10% square metres. Therefore the working value of the sun to 
the earth is at least 113 x 10” horse-power, This figure depends on 
the amount of steam actually generated in a particular instrument ; 
but no instrument is perfect, therefore the above figure falls short of 
the truth. One horse-power per square metre has been taken as a 
probable work-value of the sun’s vertical rays at the level of the 
sea, This is equivalent to 1°06 gr.°C. per square centimetre per 
minute. In accepting these values of the solar heat constant at the 
sea level we are assured that we are not exaggerating. 
It is impossible to determine, or to estimate exactly, how much 
of the sun’s heat is absorbed in its passage through the atmosphere. 
We have seen that Herschel estimates the amount transmitted to be 
two-thirds of the amount which arrives at the earth’s orbit, leaving 
one-third to be absorbed. The true amount absorbed is probably 
rather under than over this figure. 
Taking 1 horse-power per square metre as the total work-value 
of the sun’s rays, and remembering that the mean distance of the 
earth from the sun is 212 times the length of the sun’s radjus, we 
find that the rays emitted by 1 square metre of the sun’s surface are 
spread over 212?, or in round numbers, 45,000 square metres of the 
earth’s surface. Therefore, the probable work-value of 1 square metre 
of the sun’s surface is at least 45,000 horse-power. 
It is useful to note that the sun’s heating power at the distance 
of the planet Mercury is 64 times, and at that of Venus it is twice 
its value at the Earth’s distance. 
The Necessity of the Knowledge of Calorimetric Factors in Counec- 
tion with the Use of the Barometer.—lIt is an interesting historical fact 
that Fahrenheit, to whom we owe the thermometer as a physical 
instrument of precision, got the idea of filling his thermometer with 
mercury by observing and being troubled by the irregularities of the 
barometer due to change of temperature. It is also probably not 
altogether accidental that the length of one degree of his thermometric 
scale corresponds to cne ten-thousandth of the volume of the inass of 
mercury used in the thermometer.* If mercury expands by yo)oo 
for every Fahrenheit’s degree, then an error of 1° F. in the estimation 
of the temperature of the barometer introduces an error in the baro- 
metric pressure of yg)yq of the whole height of the column of 
mercury. If that height is 30 inches the error is three-thousandths 
of aninch. If the height is 760 mm. the error is 0-076 mm. It is 
obvious, therefore, that unless we can be perfectly certain that the tem- 
* This fact gives Fahrenheit’s thermometer a genuine title to the name centigrade, 
which Celsius’ scale lacks. 
