REFRACTION, terrestrial, is that by which terrestrial objects appear to be raised higher than they really are, in ob serving their altitudes. The quantity of this refraction is estimated by Dr. Alas kelyne at one tenth of the distance of the object observed, expressed in degrees of a great circle. So, if the distance be 10,000 fathoms, its tenth part, 1000 fa thoms, is the sixtieth part of a degree of a great circle on the earth, or 1", which therefore is the refraction in the altitude of the object at that distance. But M. Le Gendre is induced, he says, by several experiments, to allow only one fburteenth part of the distance for the refraction in altitude. So that, upon the distance of 1'000 fathoms, the fourteenth part of which is 714 fathoms, he allows only 44" of terrestrial refraction, so many being contained in the 714 fathoms. See his Memoir concerning the trigonometrical operations, &c. Again, M. de Lambre, an ingenious French astronomer, makes the quantity of the terrestrial refraction to be the eleventh part of the arch of dis tance. But the English measurers, Col. Edward Williams, Capt. Mudge, and Mr. Dalby, from a multitude of exact obser vations made by them, determine the quantity of the medium refraction to be the twelfth part of the said distance. The
quantity of this refraction, however, is found to vary considerably, with the dif ferent states of the weather and atmo sphere, from the fifteenth part of the dis tance to the ninth part of the same, the medium of which is the twelfth part, as above mentioned. Some whimsical ef fects of this refraction are also related, arising from peculiar situations and cir cumstances. Thus, it is said, any person standing by the side of the river Thames, at Greenwich, when it is high water there, he can see the cattle grazing on the Isle of Dogs, which is the marshy meadow on the other side of the river at that place ; but when it is low water there, he cannot see any thing of them, as they are hid from his view by the land wall or bank on the other side, which is raised higher than the marsh, to keep out the waters of the river. This curious ef fect is probably owing to the moist and dense vapours, just above and rising from the surface of the water, being raised higher or lifted up with the surface of the water at the time of high tide, through which the rays pass, and are the more re fracted.