But as yet the evidence is not conclusive that Venus has this long rotation period. Several observers of high rank believe that our neighbour rotates in nearly the same time as the Earth, but its markings are so faint and elusive that the problem is a difficult one. The spectroscopic method of determining the speed of rotation has been equally indecisive. Until, therefore, the rotation period has been decided, the habitability of Venus must remain in question. If it always turns the same face to the Sun, there can be no more life upon it than upon Mercury; if on the contrary it rotates in much the same time as the Earth, then, so far as we know, it may well be a habitable world. Whether it is actually inhabited is a matter at present entirely beyond our knowledge. A page or two back we touched lightly on the eccentricity of the orbit of Mercury--lightly, because it was not the chief factor in disabling the planet for habitation. But the condition introduced by this eccentricity is one which of itself would be sufficient to put it out of court. In the six weeks in which Mercury moves from aphelion to perihelion, it approaches the Sun by fourteen millions of miles, and the heat received by it is increased 2-1/2 times. Then, in the next six weeks, it recedes as far, and there is a like diminution. In other words, six weeks makes a greater proportional change in this one planet's condition than we should experience if our Earth were transported from its own orbit to that of Mars.