342 On Tie fraction. 



tirclv different quantity, viz. 30' in the horizon: but this, 

 according to him, terminated at only 20° of altitude*. 



The following is the manner in which it is related that 

 Tycho made this discoveryf. He had determined with one 

 or two instruments, extremely well made, the latitude of 

 the place, by observations of polaris above and below the 

 pole. He determined it also by the sun's altitude in both 

 solstices, and found it four minutes less by the latter. At 

 first he doubted the goodness of his instrument, and there- 

 fore constructed with the utmost care as many as ten others 

 of different sizes and forms, but they all gave nearly the 

 same result. He could no longer attribute this difference 

 between the two determinations of the latitude to any de- 

 fect in the observations, and therefore endeavoured, by an 

 attentive consideration of the subject, to find out the cause 

 of this curious phenomenon. At length he supp- sed it 

 could only arise from the refraction, which elevated the 

 sun at the winter solstice, having then only 11° of altitude 

 above the horizon. This conjecture agreed very well with 

 the principles of optics ; but still Tycho Brahe could 

 scarcely persuade limself that the refraction was sufficiently 

 large to produce so great a difference : on this account he 

 made other instruments of ten feet diameter, whose axis 

 corresponded exactly with the pole, of the world; and with 

 these he measured the declination of the stars out of the 

 meridian %. He thtn found, that even in summer, the re- 

 fraction, although insensible at the meridian altitude of the 

 sun, was very considerable near the horizon ; and that the 

 defect was about half a degree in the horizon, 



A copy of Tycho Brahe s Table§ of Refrac- 

 tion for a star is given in the margin. 



In this state did the refraction continue for 

 many years. Even Riccioli || in 1665 sup 

 posed it nothing at about 26^ of altitude : 

 but he thought the moon had only 2?/ of 

 horizontal retraction in summer; the sun 30', 

 and the stars 30' 3 7". 



* Mem. de PAcad. av. s. renouv. torn. v. p, 82. Long'* Astronomy, vol. i. 

 p. 254, where a comparison is given of his Table with those of Newton and 

 Flamsteed. f Encycl. Method. 



X The greater part of these very curious and ingenious instruments are 

 given in his j&slronom\ce instaurata Mcchanica, printed at Wandesburg in 

 1598. This work is now become extremely ran- , and to be met with only 

 in a few of the great public libraries : on which account M. Jeaurat had 

 the plates engraved again upon a reduced scale, and published in the Me- 

 moirs of the Academy of Sciences or%>ari5 for the year 17(>3, p 120. 



§ Progymn. p. 79. 104. Street's. Astr. Carol, p. 119. Long's Astr. vol. j. 

 p. 254. Maria Cunitia Urania Propitia. p. 286, fol. 1(350. 



(| Astr. Reformat. Astr. ref. Tabul. p. 47. Iv 



