92 



E. WALTER MAUNDER^ F.R.A.S., ON THE 



has produced a change equivalent to the planet having " neared 

 and neared." 



A telegraph wire against the background of a dull sky can 

 he perceived with certainty at an amazing distance, the limit 

 being reached when the wire subtends a second of arc, or in 

 other words when its distance from the observer is two hundred 

 thousand times the thickness of the wire. But though this is 

 quite unmistakable perception, it is not a defined image that 

 is formed. If a bead be put upon the telegraph wire, the bead 

 must be more than thirty times the breadth of the wire to be 

 perceived, and some sixty or seventy times the breadth of the 

 wire before it could be fully defined, so that the observer could 

 distinguish between a bead that was square, round or any other 

 shape, the area of its cross-section being supposed to be the same 

 in each case. But between the limits of one second of arc and 

 sixty seconds of arc, all minute objects, whatever their shape or 

 discontinuity, must take on, in the observer's eye, the two 

 sinjplest possible geometrical forms, the straight line and the 

 round dot. Here, and not in any gigantic engineering works, 

 is the explanation of the artificiality of the markings on Mars as 

 Mr. Lowell sees them; their artificiality disappears under 

 better seeing with more powerful telescopes. 



The existence of water in the liquid state is the chief 

 condition for habitabihty of a planet ; and this we have seen 

 dei)ends upon the size and density of the planet, on the one 

 hand, and its distance from the sun, on the other. Applying 

 the criterion to the planet Mercury, we find that on the average 

 it receives six and a half times as much heat from the sun as 

 the earth does, but from its small size, its atmosphere must be 

 rarer even than that of Mars. The range in temperature from 

 day to night must be extreme, and water can usually only 

 exist as vapour on the side turned to the sun, and as ice on the 

 side turned from it. But there is little doubt that Mercuiy 

 always turns the same face to the sun, even as the moon 

 turns the same face to the earth, and this condition alone is 

 sufficient of itself to render Mercury uninhabitable. 



In the case of Venus we have a world not very much smaller 

 than om' own. The force of gravity is about seven-eighths thiit 

 on the earth, and the atmospheric density probably about three- 

 quarters. These are not important differences, and thougli 

 Venus receives almost twice as much light and heat per unit of 

 surface, it is possible that the immense amount of cloud with 

 which its atmosphere is filled may make a sufficient screen. The 

 pr<)l)ability is that ice is comparatively rare on Venus, but that 



