204 GEOGRAPHY AND ASTRONOMY. in summern s, and the remainder is the sun's summer declina- tion E s. Subsfract the meridian altitude in winter et v from the cola- titude n e, and the remainder is the sun's winter declination E v. Or in short, if the meridian altitude and colatitude be given, substract the less from the greater, and the remainder is the sun's declination. Problem X. 00 To find the latitude of a place by the meri- dian altitude of a star, when it is on the south meridian." Find the declination of that star in some table or scale of the star's declination. If it has declination northward, (as the sun has in summer) substract the declination from the meridian altitude, and it gives you the colatitude. If thestar's declina- tion be southward (as the sun's is in winter) add its declina- tion to its meridian altitude, and it gives you the colatitude. Note, When I speak of north and southward in relation to winter and summer, in many of these problems, I mean in nor- thern latitude such as ours is in Great Britain. When the star is on the north meridian see how to findthe latitude by it in problem XXXII. Problem XI. " By what methods is the longitude of places to be found." Though the latitude (which lies northward and southward) may be determined with the utmost certainty by the methods be- fore proposed, yet the longitude of a place (which is the distance of any two places from each other eastward or westward) is very hard to be determined by the sun or stars, because they always appear moving round from east to west. The longitude there- fore of places is usually found by measuring the distance on earth or sea from west or east. The map makers who describe countries, provinces or king- doms, measure the distances on the earth by au instrument made on purpose, with a wheel so contrived, that a certain number of its revolutions is equal to a pole, a furlong, or a mile ; it bath also a mariner's compass and needle touched with a loadstone fastened to it,' to shear how much their course varies from the -north or south. In this last age they have also invented a way to find the difference of longitude between two towns that are some thou- sandsof miles asunder in distant nations ; and that is by a nice and exact observation of the moment when the eclipses of the moon begin or end, made by mathematicians at those distant places. And thus by the difference of time in those eclipses they compute the distance of place. This invention is still further improvedby observations of the eclipses of the four moons or little secondaryplanets, which roll round the planet Jupiter as our moon does round our earth.
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