Page:Arrian's Voyage Round the Euxine Sea Translated.djvu/169

168 obervation of the tar Canopus

which is the difference between its apparent height at the two places. Five degrees of latitude, at 69.2 5 Englih miles each, = 346 Englih miles, == 302.3 Olympic tadia; which hould he, according to this calculation, the ditance between Alexandria and Rhodes, uppoing them to lie under the ame meridian. But Alexandria is 1° 51′ to the eat of Rhodes, a pace in the latitude of Alexandria equal to 110 Englih miles. for the ditance from Alexandria to Rhodes, = 3177 Olympic tadia. But this ditance is too mall, owing to the proportionally greater refraction at the altitude of 1° 2′, than at 6° 18′, which amounts to 16′ in altitude, and to about 17′ in ditance.

Equal to 3 32.8 Olympic tadia, for the ditance from Alexandria to Rhodes.

Let us now ee how the calculation of Poidonius, repecting the circumference of the earth, would tand, had his obervations of the repective altitudes of Canopus at Alexandria and Rhodes been correct, though without allowing for refraction. The apparent difference of altitude at the two places was, as I before oberved, 5°. Say then 5°: 360::5000 tadia to 360,000 = 41207.4 Englih miles, jut double to his later calculations, being 1000 tadia to a degree. By his other computations, derived from the gnomonic meaurements of Eratolihenes, and which eliminate the ditance only at 3750 tadia, it would tand thus—5:360::3750:270,000, or 750 tadia to a degree.Poidonius, it is evident, made two mitakes, betides that of uppoing Rhodes and Alexandria to lie, under the ame meridian; the firt in uppoing Canopus to have no altitude at Rhodes, whereas it has a real one of, made by himelf. He remarked, 1° 2′,