The only way to compare the proportional temperatures of bodies, whether on the earth or in space, is therefore by means of a scale beginning at this natural zero, instead of those scales founded on the artificial zero of the freezing point of water, or, as in Fahrenheit’s, 32 deg. below it.  Only by using the natural zero and measuring continuously from it can we estimate temperatures in relative proportion to the amount of heat received.  This is termed the absolute zero, and so that we start reckoning from that point it does not matter whether the scale adopted is the Centigrade or that of Fahrenheit.

The Complex Problem of Planetary Temperatures.

Now if, as is the case with Mars, a planet receives only half the amount of solar heat that we receive, owing to its greater distance from the sun, and if the mean temperature of our earth is 60 deg.  F., this is equal to 551 deg.  F. on the absolute scale.  It would therefore appear very simple to halve this amount and obtain 275.5 deg.  F. as the mean temperature of that planet.  But this result is erroneous, because the actual amount of sun heat intercepted by a planet is only one condition out of many that determine its resulting temperature.  Radiation, that is loss of heat, is going on concurrently with gain, and the rate of loss varies with the temperature according to a law recently discovered, the loss being much greater at high temperatures in proportion to the 4th power of the absolute temperature.  Then, again, the whole heat intercepted by a planet does not reach its surface unless it has no atmosphere.  When it has one, much is reflected or absorbed according to complex laws dependent on the density and composition of the atmosphere.  Then, again, the heat that reaches the actual surface is partly reflected and partly absorbed, according to the nature of that surface—­land or water, desert or forest or snow-clad—­that part which is absorbed being the chief agent in raising the temperature of the surface and of the air in contact with it.  Very important too is the loss of heat by radiation from these various heated surfaces at different rates; while the atmosphere itself sends back to the surface an ever varying portion of both this radiant and reflected heat according to distinct laws.  Further difficulties arise from the fact that much of the sun’s heat consists of dark or invisible rays, and it cannot therefore be measured by the quantity of light only.

From this rough statement it will be seen that the problem is an exceedingly complex one, not to be decided off-hand, or by any simple method.  It has in fact been usually considered as (strictly speaking) insoluble, and only to be estimated by a more or less rough approximation, or by the method of general analogy from certain known facts.  It will be seen, from what has been said in previous chapters, that Mr. Lowell, in his book, has used the latter method, and, by taking the presence of water and water-vapour in Mars as proved by the behaviour of the snow-caps and the bluish colour that results from their melting, has deduced a temperature above the freezing point of water, as prevalent in the equatorial regions permanently, and in the temperate and arctic zones during a portion of each year.