LIFE IN MARS. 129 



our earth have much to do with the matter of the sun's 

 tide-raising power. Every one knows how the explana- 

 tion of the tides runs in our books of astronomy and 

 geography. The sun is nearer to the water turned 

 directly towards him than he is to the centre of the 

 earth ; he therefore draws that water away from the 

 earth, or in other words raises a wave : but again, says 

 the explanation, the sun is nearer to the earth's centre 

 than to the water on the side turned away from him, 

 and therefore he draws the earth away from that water, 

 or a wave is raised on the further as well as on the 

 nearer side of the earth. If the earth were smaller, the 

 sun would not be so much nearer to the water turned 

 towards him, nor so much farther from the water turned 

 away from him so that both waves would be reduced 

 in dimensions. Applying this consideration to the case 

 of Mars, whose orb is much smaller than the earth's, 

 we see that any tidal wave raised by the sun in Martial 

 seas must needs be of very small dimensions. 



But the existence of ocean currents appears to depend 

 very little on the presence of tidal waves. In the 

 Mediterranean Sea, the Ked Sea, and the Baltic Sea 

 well-marked currents exist, although the tidal wave 

 scarcely affects these seas. Sea-currents would indeed 

 seem to be due to the effects of evaporation taking 

 place extensively over certain portions of the sea sur- 

 face ; and we know that evaporation must proceed very 

 freely in the case of the seas of Mars, since clouds 

 form so marked a feature of his atmospheric economy. 

 We may conclude, therefore, that his seas are traversed 



K 



