pigafetta’s voyage round the world.

3 8 9

By this figure is feen that the rays of the ftar D are intercepted by the moon at Cfrom the observance of thofe at A, but not from that of thofe at B ; although whenthe moon proceeds to E, the fame ftar will in like manner be concealed from the gazerat B.

“ HI. The compafs may likewife furnilh a much more ready means of difcovering'he longitude. It is well known that the compafs, or the magnetic needle which it con-tains, conftantly points to one fixed fpot, on account of the tendency of the magnettowards the pole. The caufe of this tendency is the magnet finding no point at reftin the heavens except the pole, to which confequently it ever directs itfelf. This, atleaft, is the explanation I propofe of this phenomenon, and that which 1 fhall deem agood one, until experience produce a better.

“ To know, by means of the magnet, the longitude of a place, form a great circle,,in which the compafs may be placed, and divide this circle into three hundred andfixty degrees: turn the compafs until the magnetic needle points to 36o Q , where thear&ic pole is indicated. When the needle is at reft draw a thread from the ar&ic tothe antar&ic pole, which may cut the centre, and let this thread be of greater lengththan the diameter of the compafs. After this take the line of noon, which is to bedetermined by the greateft elevation of the fun : turn then the compafs until the threadwhich traverfes it is on the line of the true meridian, that is to fay, on the lhadow call:by the dial at noon; then, with the remainder of the thread, draw a line from theantar&ic pole of the needle which puffing through the. centre may cut the fleur-de-lis which points out the north, and obferve how many degrees the needle is diftantfrom the meridional line, that is from the true pole; the number of degrees diftant■will indicate the number of degrees of longitude from the fpot where the compafspoints true * : confequently, the more exadl the obfervation of the true meridian, themore exa&ly will the degree of longitude be determined. By what I have laid will befren that the meridian muft never be determined by the compafs, becaufe it north-eafts0r north-wefts f immediately after a departure from the true meridian, but with thea ftrolabe at noon ; and the noon muft be found by afcertainment of the inftant of theS r eateft elevation of the fun.

“ When the height of the fun at noon, cannot be taken, it may be determined by afand-glafs, by reckoning the hours of the night, from the inftant the fun fets to that ofns rifing. When the length of the night is thus known, that of the day is gatheredby fubtra&ing it from twenty-four; by dividing this by two the fum is the time fromfun-rife till noon, at which inftant the gnomen will indicate the meridian. But thefand- g laf s b eing oftentimes incorrect, it is far better to take the height of the pole, bynreans of the aftrolabe and its mediclino j.

The true meridian may alfo be found, or rather the equino&ial line which cuts.*he meridian at right angles, by obferving the points at which the fun rifes and fets,a nd meafuring its deviation north or fouth from the equinox. To effect this, anaftrolabe is made of the earth ; that is to fay, a circle is formed reprefenting the cir-cumference of the earth divided into three hundred and fixty degrees. Fix two pins

is to fay, when the magnetic needle coincides with the meridian-.

T "leaning it declines towards the call or the weft from the north,a 1. cannot find that any aftronomer or allrologer in the days of Pigafetta , for example Regiomontanus,.

pianu S( Gimma Friiius, Panti, Clavius, &c. make any mention whatever of the mediclino; but by whatCe ' r aut ^ or here fays, as well as in other places, it feems the mediclino is the moveable rule fixed on the■itre of tl le aftrolabe, which is fometimes denominated alhidade, at others dioptre, and in Italian"‘‘guardo.

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