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extent of the prospect. In other words, the base and hypothenuse of the
supposed triangle would be so long when compared with the perpendicular,
that the two former may be regarded as nearly parallel. In this manner
the horizon of the æronaut would appear to be on a level with the
car. But, as the point immediately beneath him seems, and is, at a great
distance below him, it seems, of course, also, at a great distance below
the horizon. Hence the impression of concavity; and this impression
must remain, until the elevation shall bear so great a proportion to
the extent of prospect, that the apparent parallelism of the base and
hypothenuse disappears--when the earth's real convexity must become
apparent.
337
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