GEOGRAPHICAL LATITUDE. 7G9 



than the circumpolar ones can be used in deteriuiuiug latitude. Tliis 

 method was to observe certain stars which in one place just graze the 

 horizon and in another appear higher. The altitude in the second place 

 gives the difiterence in latitude of the two places of observation.^ 



Instniments. — No one art or science is so iudepeudeut tliat it can stand 

 or fall, advance or retrograde, without iutlueucingaiid being iudueuced 

 by the others. Accordingly we fiud progress in the knowledge of geo- 

 graphical latitude dependent not only on that of general information 

 regarding the earth's surface, but also on that of astronomy, and even 

 of mechanics, the latter being necessary to increase accuracy of astro- 

 nomical observations, on which all the rest depends. The beginnings 

 with two kinds of rude gnomons have already been mentioned. To 

 the Chinese was known a third sort, more accurate because provided 

 toward the top with a small hole, through which the sun shone, thus 

 terminating the shadow to be measured at the point corresponding to 

 the center of the sun. The Arabs also made use of this sort of gno- 

 mon, but it seems doubtful if the Greeks ever came to a knowledge of 

 it. Eratosthenes employed besides the gnomon the armillary spheres 

 already mentioned, with which he observed the obliquity of the Ecliptic. 

 Further progress was marked by the introduction of the astrolabe, the 

 first mention of which is made by Ptolemy, who says he used one 

 at Rhodes, which however may have been invented by Hipparchus.^ 

 Between this and modern times, the only improved instruments for ob- 

 servation are the similar but more complicated torquetum of llegio- 

 montanus (143G-147G), and the quadratum geometricum, which was 

 known to the Arabs, but is generally ascribed to the invention of Pur- 

 bach^ (1423-1461). In about 1600, observations at sea were much 

 improved by the invention of a portable quadrant by the English sea- 

 captain, John Davis.^ In the mean time the instruments for use on land 

 were being made larger and larger, and, though admitting of more ac- 

 curate division, they became unwieldy. Then came the application of 

 magnifying glasses, to which Huyghens first api>lied the cross-threads 

 to mark the mutual focus of the two glasses of the telescope ; and in 

 1667, Picard first applied (in concert with Auzout) the telescope to the 



1 In recent times it has been customary to observe any selected star, measuring its 

 distance on the meridian from the pole or zenith. The latitude is found by compar- 

 ing the relative position observed with the star's absolute position, wliich will be 

 found in tables prepared for that purpose. On account of atmospheric refraction 

 stars near the zenith are now generally chosen for such observations. 



'^Delambre, Astron. ancienue, i, 184. 



»Wolf,Gesch. Astron., p. 126. 



* Ibid., i>. 278. Encyc. Brit., art. "Navigation," speaking of Davis's The Seaman^s 

 Secrets : " There is a drawing of a quadrant, with a plumb lino, for measuring the ze- 

 nith distance, and one of a curious modification of a cross-stalF, with which the ob- 

 server stands with his back to the sun, looking at the horizon through a sight on the 

 end of the staft', while the shadow of the sun from tiie top of a movable projection 

 falls on the sight box. This remained in common nse till superseded by Hadley's 

 quadrant." . 



H. Mis. 224 41) 



