ncide with the centre of the eye-pupil.
Such an instrument could be readily made to show the distance of the
sea-horizon, which at once determines the height of the observer above
the sea-level. For this purpose all that would be necessary would be a
means of placing the eye at some definite distance from the small
mirror, and a fine vertical scale on the mirror to show the exact
depression of the sea-horizon. For balloonists such an instrument would
sometimes be useful, as showing the elevation independently of the
barometer, whenever any portion of the sea-horizon was in view.

The mention of balloon experiences leads me to another delusive argument
of the earth-flatteners.[52] It has been the experience of all
aeronauts that, as the balloon rises, the appearance of the earth is by
no means what would be expected from the familiar teachings in our books
of astronomy. There is a picture in most of these books representing the
effect of ascent above the sea-level in depressing the line of sight to
the horizon, and bringing more and more into view the convexity of the
earth's globe. One would suppose, from the picture, that when an
observer is at a great height the earth would appear to rise under him,
like some great round and well-curved shield whose convexity was towards
him. Instead of this, the aeronaut finds the earth presenting the
appearance of a great hollow basin, or of the concave side of a
well-curved shield. The horizon seems to rise as he rises, while the
earth beneath him sinks lower and lower. A somewhat similar phenomenon
may be noted when, after ascending the landward side of a high cliff, we
come suddenly upon a view of the sea--invariably the sea-horizon is
higher than we expected to find it. _Only_, in this case, the surface of
the sea seems to rise from the beach below towards the distant horizon
convexly not concavely; the reason of which I take to be this, that the
waves, and especially long rollers or uniform large ripples, teach the
eye to form true conceptions of the shape of the sea-surface even when
the eye is deceived as to the position of the sea-horizon. Indeed, I
should much like to know what would be the appearance of the sea from a
balloon when no land was in sight (though I do not particularly wish to
make the observation myself): the convexity discernible, for the reason
just named, would contend strangely with the concavity imagined, for the
reason now to be indicated.

The deceptio