[KNOWLEDGE, March 21, 1882 



part of each whorl where these ten-day 

 dots meet without an exact ten-day in- 

 terval ; this, however, is unimportant, as 

 in these parts of his geocentric path 

 Mars is invisible. At the proper places 

 along the planet's looped geocentric path 

 are shown the places whore Mars is in 

 perihelion (M), aphelion M', at a rising 

 node (or crossing the plane of the earth's 

 orbit from north to south), ( JJ ), at a 

 descending node (or crossing the plane of 

 the earth's path from south to noi-th), ( ^ ), 

 the place where he attains his greatest 

 distance north (j) and south (I) of the 

 plane of his orbit ; the place where Mars 

 is at the point of his orbit corresponding 

 to the vernal equinox (beginning of spring) 

 of his northern hemisphere, marked (j's<y>, 

 and the corresponding point for the 

 autumn of Mars, marked ,^'s ^. 



The scale of the drawing is the same 

 as that of my picture of the orbits of 

 the terrestrial family of planets (Mars, 

 Earth, Venus, and Mercury), in the " En- 

 cyclopa;dia Britannica," viz., fifty million 

 miles to the inch, and on this scale the 

 lines I, I, &.C., indicate the greatest distance 

 attained by Mars north and south of the 

 plane of the ecliptic. The northerly dis- 

 placement, it will be seen, is the greater. 



The path of Mars must be regarded as 

 passing above the plane of the paper, at 

 a point marked JJ , gi-adually attaining 

 its greatest height (indicated by the 

 length of the " I ") above that plane at 

 the point marked j ; gradually returning 

 towards the plane of the paper, which it 

 crosses again at a point marked i3 ; then 

 attaining its greatest distance below the 

 plane of the paper at the next point 

 marked I ; whence it returns gradually 

 to the plane of the paper at a point 

 marked Q ; and so on continually. 



