A glance at the Table will show that this was to be expected. In size, Mercury comes between the Moon and Mars, and the atmospheric veil ought therefore to be, as it evidently is, very slight and transparent; offering little or no hindrance to an observer scanning it from another world. The other necessary consequences of small size and mass will follow; the feeble force of gravitation, the languid atmospheric circulation, the extreme range of temperatures, the low temperature at which water will boil. But the heat to which Mercury is exposed far transcends our terrestrial experience. In the mean it receives nearly seven times as much heat from the Sun as the Earth does, but this supply is not maintained uniformly, for Mercury moves round the Sun in a very eccentric orbit, so that when in aphelion it receives, surface for surface, only about four times as much heat as the Earth, but some six weeks later when in perihelion it receives more than eleven times. The great range of temperature due to the thinness of the atmosphere must therefore be further increased by the varying distance of the planet from the Sun. A reference to Prof. Poynting's figures shows that the mean temperature of Mercury must approximate to 194° C., while water will boil at 40° C. or even lower. Here, then, is a condition the exact reverse of Mars. Water as a liquid will be rare on Mercury, not because it is congealed, but because it is evaporated; on the dark side of the planet it may, indeed, pass into ice, but on the side exposed to the Sun it must exist normally as a constituent of the atmosphere. Water in a liquid state, water that flows, must be almost unknown. But we have good reason to believe that that which is the dark side of Mercury at one time is always dark; that which is exposed to the Sun is always exposed to it.