578 PROFESSOR TAIT ON MIRAGE. 
planation as from his. VINcr’s observations were by no means precise enough 
to make this point certain ; besides, he speaks of the top-masts and not of the 
hulls ; and, from the diminution of the image as the distance increases, it may 
be quite true that the top-masts appear to rise in the inverted image while the 
hull really sks. At any rate it isassuredly not so in the majority of ScORESBY’s 
careful figures. In fig. 1 several examples are shown of multiple images of 
ships at different distances in nearly the same direction ; and in all it will be 
observed that the inverted image of the hull is lower as the vessel is farther 
off. Also that in the upper direct image the hull appears to rise as the vessel 
recedes. 
[#eb. 10, 1883.—I have to acknowledge the kindness of Mr. J. W. L. 
GLAISHER in verifying, and in some important instances correcting, the numerical 
values given in {$10 and 15. My own original calculations, made for the most 
part with four-place logarithms only, were insufficient to give accurately the 
values of 6 close to the critical point. The reason is obvious from the form 
of the expression for that quantity as given in § 12, above. | 
