Note 113 — Chapter VI · THE PHOTOSPHERE AND ITS DUSKY VEIL.
Reading edition
The “solar constant” is the number of units of heat per unit of area which would be received in unit of time by the earth’s surface if its atmosphere were removed. The most approved value is three (small) calories per square centimetre per minute, a “small” calorie being the quantity of heat requisite to raise one gramme of water one degree centigrade.