184 The Planet Mars : a Fragment. 



we have to contend with the peculiar difficulties presented by 

 spherical perspective ; difficulties that would hardly be ima- 

 gined by an inattentive observer. Our eyes are apt to be 

 misled by being accustomed in youth to the common projec- 

 tion of the earth in two hemispheres,, till we get to suppose 

 that such would be a real view of our planet at a distance ; but 

 this impression is highly erroneous. In such maps, where the 

 half of the globe is attempted to be as it were squeezed fiat, 

 the marginal regions are necessarily enlarged, and the central 

 parts reduced, to effect some kind of conventional correspon- 

 dence, and render the map of any use ; and we gain no idea 

 from it how extreme, in nature, is the foreshortening towards 

 the edges of a globe in a perspective view ; how small a portion 

 of it is seen in its true dimensions ; and how soon, in every 

 direction, the surface rounds off and falls away from the sight. 

 This however, great as it is, is not all the difficulty. Were the 

 position of the axis of the globe invariable with respect to the 

 observer, though much of the surface towards the poles would 

 be very ill seen, the perspective of what is seen would be con- 

 stant, and readily intelligible, as we notice in the case of 

 Jupiter, whose polar regions are foreshortened to obscurity, 

 but whose equator lies always alike straight before the eye. 

 But where, as in Mars, the axis is at different epochs very 

 differently presented to our sight, though more of the surface 

 is in the end rendered visible, yet the effect of perspective is 

 much more complicated, and the apparent changes of form 

 resulting from it much more perplexing. All this may be 

 readily illustrated by means of a terrestrial globe viewed from 

 a fixed position. If, in the first instance, its axis is placed ver- 

 tically, the effect of rotation will be a wide difference in tin i 

 apparent form of the continents as they occupy the edges or 

 pass across the centre, and we shall be convinced how very 

 unlike must be the hemisphere of the map-makers to that 

 beheld by a distant external eye : these changes however will 

 be regularly recurrent ; all the parallels of latitude will continue 

 sensibly straight ; and the central aspect w ill be successively 

 always the same. Until' we now incline the axis about 23£°, 

 and place ii in various directions towards the eye, so that at 

 one time both poles shall be in the limb, at others each in 

 turn shall be more or less advanced towards the spectator, the 

 surface will be presented in aspects continually varying; the 

 parallels of latitude will be at one time straight, at others pro- 

 jected as semi-ellipses, with a curvature turned sometimes one 

 w.iy, sometimes the other, according to the position of the axis ; 

 even i he central representations will bo quito unlike at different 

 times, though the same side, in a general sense, may be op- 

 posite to us; and the effect of rotation will be a change, in 



