Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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ed; therefore tell me how he diſ-ingageth himſelf in the next place
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from that particular of the Stars having conſtantly kept the ſame
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diſtance from the fixed Stars circumjacent to it.</
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<
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>SALV. </
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<
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>He betakes himſelf, in like manner, to two threads, yet
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more unable to uphold him than the former: one of which is
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wiſe faſtened to refraction, but ſo much leſs firmly, in that he
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ſaith, that refraction operating upon the new Star, and ſublimating
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it higher than its true ſituation, maketh the ſeeming diſtances
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tain to be diſtinguiſhed from the true, when compared to the
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cumpoſed fixed Stars that environ it. </
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<
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>Nor can I ſufficiently
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mire how he can diſſemble his knowing how that the ſame
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ction will work alike upon the new Star, as upon the antient one
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its neighbour, elevating both equally, ſo as that ſuch a like
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dent altereth not the ſpace betwixt them. </
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<
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>His other ſubterfuge is
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yet more unhappy, and carryeth with it much of ridiculous, it
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ing founded upon the errour that may ariſe in the inſtrumen
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peration it ſelf; whilſt that the Obſervator not being able to
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conſtitute the centre of the eyes pupil in the centre of the
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tant (an Inſtrument imployed in obſerving the diſtance between
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two Stars) but holding it elevated above that centre, as much as
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the ſaid pupil is diſtant from I know not what bone of the cheek,
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againſt which the end of the Inſtrument reſteth, there is formed
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in the eye an angle more acute than that which is made by the ſides
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of the Inſtrument; which angle of rayes differeth alſo from it
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ſelf, at ſuch time as a man looketh upon Stars, not much elevated
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above the Horizon, and the ſame being afterwards placed at a
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great height; that angle, ſaith he, is made different, while the
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ſtrument goeth aſcending, the head ſtanding ſtill: but if in
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ting the Inſtrument, the neck ſhould bend backwards, and the
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head go riſing, together with the Inſtrument, the angle would then
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continue the ſame. </
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<
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>So that the Authours anſwer ſuppoſeth that
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the Obſervators in uſing the Inſtrument have not raiſed the head,
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as they ought to have done; a thing which hath nothing of
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hood in it. </
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>But granting that ſo it had been, I leave you to judge
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what difference can be between two acute angles of two
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ral triangles, the ſides of one of which triangles are each four
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[
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Italian] Braces
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[
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i.e.
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about three Engliſh yards] and thoſe of the
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other, four braces within the quantity of the diameter of a Pea;
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for the differences cannot be abſolutely greater between the length
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of the two viſive rayes, whilſt the line is drawn perpendicularly
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from the centre of the pupil, upon the plain of the Rule of the
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Sextant (which line is no bigger than the breath of the thumb)
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and the length of the ſame rayes, whilſt elevating the Sextant,
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without raiſing the head together with it, that ſame line no longer
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falleth perpendicularly upon the ſaid plane, but inclineth, making </
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