CONDITIONS OF HABITABILITY OF A PLANET. 



95 



As he tells us, the measurement of these markings shows them to 

 be many miles in width, and thousands of miles in length ; the 

 explanation that the sharp edges of the markings show them to be 

 channels of artificial construction must therefore be abandoned, and 

 has been abandoned. As Mr. Maunder tells us they are now 

 explained as "straths of vegetation springing up along the banks" 

 of such channels. But this second explanation of the markings 

 seems to me plainly inconsistent with the observed facts. These 

 are that the edges of the markings are (1) sharp, and (2) parallel. 

 But anyone who has seen, in India, cultivation carried on along the 

 banks of channels by means of the water contained in them must have 

 observed that the edges of such cultivation are not sharp, but very 

 ill-defined, and are never parallel. The reasons are plain : there is 

 always water enough for keeping the crop alive close to the bank, 

 but as you go further back from the bank the supply of water 

 diminishes, and it more and more frequently happens that the 

 cultivation at the outer edges has water enough to begin with and 

 therefore starts to grow, but as the season goes on and the water 

 supply falls, the growth at the other edges withers and dies for 

 want of water. In the second place, unless the supply of water at 

 the head of the channel is absolutely uniform from year to year, the 

 strip of cultivation is wider in a year of abundant supply and 

 narrower in a year when the supply is smaller. But the supply of 

 water produced by melting snow-caps at the poles of Mars is very 

 unlikely to be absolutely uniform from year to year, and if the 

 markings were due to cultivation (or vegetation) produced by such 

 melting snow, we ought to see the markings vary in width from year 

 to year. This has never been observed. 



Finally, the edges of such cultivation (or vegetation) are nevei 

 parallel. The reason is quite plain. Near the source of supply, at 

 the head of the channel, the water is abundant, and owing to the 

 fall of the ground along the banks can be carried by the necessary 

 subsidiary channels to a great distance. As you go lower down the 

 channel, the drawing-ofF of much of the water has greatly diminished 

 the supply to start with, and the decrease in level of the point from 

 which the subsidiary channels start greatly diminishes the distance 

 to which the water can be got to flow along them. The consequence 

 must be (and, as every observer can testify, actually is) that the 

 cultivation (or vegetation) along the banks of a channel tapers down 



