254 National Geographic Magazine. 
the exact scale of the general map. It needs but little imagina- 
tion to foretell that houses would be mere specks, roads, faint 
lines, and forests, masses of color, in other, words, that it would 
be more instructive to consult the general map, on which all 
details are magnified to be clearly visible and topographic 
features brought out with great distinctness than to attempt 
to trace with unaided eye, from the image of objects at a dis- 
tance of twenty-four miles, the course of streams or roads 
through forest or moor, or to judge of the relative elevations 
or modeling of the ground from the values of light and shade. 
Without an intimate local knowledge of the county there would — 
be nothing to indicate the name or boundaries of villages, or 
estates or the political and other subdivisions of the land, which 
are most clearly indicated on the map, in unmistakable styles 
of lettering. 
Another and more serious problem which would be lessened as 
the balloon receded from the earth would be the distortion in 
perspective produced by the irregularities of the surface. The 
higher points being nearer the balloon would appear in the image 
on larger scale than the lower, and only in the case of a perfectly 
level country, would it be possible to produce a map without dis- 
tortion by the method proposed, and then only for a limited area. 
As the balloon receded, the relative differences of elevation 
would bear a smaller and smaller proportion or ratio to the dis- 
tance, in other words, the distortion would grow less until at an 
infinite distance it might be neglected. 
We might conceive that the observer was stationed at an infi- 
nitely great distance, and provided with a series of magnifying 
lenses of suitable powers to produce maps of any desired scale, 
yet, beyond a limited area, he would still be confronted with the 
problem of eliminating the distortion produced by the curvature 
of the earth. 
Such is the conception of an accurate map which is an attempt 
to produce on a plain surface or sheet of paper, a horizontal pro- 
jection of objects on the ground, which will show the relative 
positions of every detail on any desired scale with as little dis- 
tortion as possible, and on which distances may be measured in 
any direction, and areas computed with a degree of accuracy only 
limited by the scale. 
When a survey of a small area 1s made, such as an estate or 
parish, which bears but a small proportion in area to the surface 
