is an arc of the ecliptic, by which the longitude of a star is increased or diminished by means of the refraction.

TERRESTRIAL REFRACTION, is that by which terrestrial objects appear to be raised higher than they really are, in observing their altitudes. The quantity of this refraction is estimated by Dr Maskelyne at one tenth; by Le Gentre at one fourteenth; by De Lambre at one eleventh; and by others at a twelfth of the distance of the object observed, expressed in degrees of a great circle. But it is obvious that there can be no fixed quantity of this refraction, since it depends upon the state of the atmosphere, which is extremely variable. Hence some very singular effects of it are related, of which the following is worthy of notice. It is taken from the Philosophical Transactions of London 1798; being an extract of a letter, dated Hastings, August 1, 1797.

"On Wednesday, July 26, about five o'clock in the afternoon, while I was sitting in my dining-room at this place, which is situated upon the Parade, close to the sea-shore, nearly fronting the south, my attention was excited by a number of people running down to the sea-side. Upon enquiring the reason, I was informed that the coast of France was plainly to be distinguished by the naked eye. I immediately went down to the shore, and was surprized to find that, even without the assistance of a telescope, I could very plainly see the cliffs on the opposite coast; which, at the nearest part, are between 40 and 50 miles distant, and are not to be discerned, from that low situation, by the aid of the best glasses. They appeared to be only a few miles off, and seemed to extend for some leagues along the coast. I pursued my walk along the shore eastward, close to the water's edge, conversing with the sailors and fishermen upon the subject. They at first could not be persuaded of the reality of the appearance; but they soon became so thoroughly convinced, by the cliffs gradually appearing more elevated, and approaching nearer, as it were, that they pointed out and named to me the different places they had been accustomed to visit; such as the Bay, the Old Head or Man, the Windmill, &c., at Boulogne; St Valery, and other places on the coast of Picardy; which they afterwards confirmed when they..."