196 PLEASANT WA YS IN SCIENCE. 



We can scarcely doubt then that if our earth were so situated 

 at any time when one of the great waves generated by Peru- 

 vian earthquakes in traversing the Pacific, that the hemi- 

 sphere containing this ocean were turned fully illuminated 

 towards Venus (favourably placed for observing her), the 

 disturbance of the Pacific could be observed and measured 

 by telescopists on that planet 



Unfortunately there is little chance that terrestrial ob- 

 servers will ever be able to watch the progress of great waves 

 athwart the oceans of Mars, and still less that any disturb- 

 ance of the frame of Venus should become discernible to us 

 by its effects. We can scarce even be assured that there 

 are lands and seas on Venus, so far as direct observation is 

 concerned, so unfavourably is she always placed for obser- 

 vation ; and though we see Mars under much more favour- 

 able conditions, his seas are too small and would seem to 

 be too shallow (compared with our own) for great waves to 

 traverse them such as could be discerned from the earth. 



Yet it is well to remember the possibility that changes 

 may at times take place in the nearer planets the terres- 

 trial planets, as they are commonly called, Mars, Venus, 

 and Mercury such as telescopic observation under favour- 

 able conditions might detect Telescopists have, indeed, 

 described apparent changes, lasting only for a short time, in 

 the appearance of one of these planets, Mars, which may 

 fairly be attributed to disturbances affecting its surface in no 

 greater degree than the great Peruvian earthquakes have 

 affected for a time the surface of our earth. For instance, 

 the American astronomer Mitchel says that on the night of 

 July 12, 1845, tne bright polar snows of Mars exhibited an 

 appearance never noticed at any preceding or succeeding 

 observation. In the very centre of the white surface appeared 

 a dark spot, which retained its position during several hours : 

 on the following evening not a trace of the spot could be 

 seen. Again the same observer says that on the evening of 

 August 30, 1845, he observed for the first time a small 

 bright spot, nearly or quite round, projecting out of the 



