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of convexity increasing as the squares of the distances, limits the sight of a ship within moderate scope. Thus, in ascending from a place like Boston harbor, the scene becomes very interesting. As you gradually go up, so come up the ships behind the horizon. It looks like magic. Of a clear day you can see ships a sea a hundred miles off when the sun is in the opposite direction. With cloud fields between the observer and the ships, they have the appearance of sailing above and over the clouds. So the meandering of a river is sometimes seen convoluting itself over and under the clouds in the distance. These unique sights are of course optical illusions, but without a knowledge of the science of optics, would be deemed mysteries. They prove, however, how subject we are, to be misled by our senses, in cases where science is not availible to correct their errors.

The most marked difference between an earth-view and a sky-view occurs in the storm cloud. The nimbus, or thunder cloud, when viewed from the earth level looks like an agitated and confused mass of leaden colored vapor. When viewed from a little above its level and from a few miles distance, it looks symetrical. Bulged out above and below, and contracted in its middle, it trails along over the earth like a huge smoking fuming engine, dragging its lower part slightly behind, like the trail of a Court Lady's garment. The electrical cannonading as it passes along gives it quite a grand and imposing effect. It is quite practicable to sail above, behind, or in the midst of these imposing meteors. Sailing behind one, and between its upper and lower cloud, I saw a beautiful prismatic colored grotto, and, apparently from within this grotto, came terrific peals of thunder. This grotto was no doubt produced by the refractive power of the gas in the balloon, as the sun was shining in between the upper and lower cloud, and through the balloon, and the grotto appeared on the opposite side of it; that is, the grotto was on the