REFRACTION. 



two stars in a position parallel to the li o- 

 rizon measure 3j, it is at most to be 

 reckoned only 29, 59' , 32". 



The quantity of the refraction at every 

 altitude, from the liorixon, where it is 

 gr< atest, to the zenith'where it is nothing-, 

 has. been determined by observavions by 

 many astronomers ; those of Dr. Bradley 

 and Mr. Mayer are esteemed the most 

 correct of any, being- nearly alike, and 

 are now chiefly used by astronomers. Dr. 

 Bradley, from his observations, deduced 

 this general rule for the refraction, r, at 

 any altitude, a, whatever; viz. as rad. 1 : 

 cotang. a-f- 3 r : : 57" : r" the refraction 

 in seconds. This rule is adapted to these 

 states of the barometer and thermometer, 

 77r. either 29.6 inch barometer and 50 

 thermometer, or 30 inch barometer and 

 55 thermometer, for both which states 

 it answers equally the same. But for any 

 other states of the barometer and thermo- 

 meter, the refraction above found is to be 

 corrected in this manner ; viz. if b denote 

 any other height of the barometer in 

 inches, and t the degrees of the thermo- 

 meter, r being the refraction incorrected, 

 as found in the manner above. Then as 

 29.6 : b : : r : R the refraction corrected 

 qn account of the barometer, and 400 : 

 450 t ; : R the refraction corrected both on 

 account of the barometer and thermome- 

 ter ; which final corrected refraction is 



therefore = a ~ '* br. Or, to correct 

 llo4U 



the same refraction, r, by means of the 

 latter state, viz. barometei 30 and ther- 

 mometer 55, it will be as 30 : b : : r : R = 



MR. SIMPSON'S TABLE OF MEAN HE- 

 TRACTIONS. 





b r the correct refraction. 



Mr. Simpson has determined, by theo- 

 ry, the astronomical refractions, from 

 which he brings out this rule, viz. as 1 to 

 .9986, or as radius to sine of 86 58' 30", 

 so is the sine of any given zenith distance, 

 to the sine of an arc ; _2_ of the difference 

 between this arc and the zenith distance, 

 is the refraction sought for that zenith 

 distance. And by this rule Mr. Simpson 

 computed a table of the mean refrac- 

 tions, which are not much different from 

 those of Dr. Bradley and Mr. Mayer, and 

 are as in the following; table. See Simp- 

 son's Dissertations. 



It is evident that all observed altitudes 

 of the heavenly bodies ought to be dimi- 

 nished by the numbers taken out of the 

 foregoing table. It is also evident that 

 the refraction diminishes the right and 

 oblique ascensions of a star, and increases 

 the descensions ; it increases the north- 

 ern declination and latitude, but decreas- 

 es the southern ; in the eastern part of 

 the heavens it diminishes the longitude of 

 a star, but in the western parts of the 

 heavens it increases the same. See QUA- 

 DRANT. 



REFRACTION, terrestrial, is that by 

 which terrestrial objects appear to be 

 raised higher than they really are, in ob- 

 serving their altitudes. The quantity of 

 this refraction is estimated by Dr. Mas- 

 kelyneat one tenth of the distance of the 

 object observed, expressed in degrees of 

 a great circle. So, if the distance be 

 10,000 fathoms, its tenth part, 1000 fa- 

 thoms, is the sixtieth part of a degree of 

 a great circle on the earth, or 1", which 

 therefore is the refraction in the altitude 

 of the object at that distance. But M. 

 Le Gendr'e is induced, he says, by several 

 experiments, to allow only one fourteenth 

 part of the distance for the refraction in 

 altitude. So that, upon the distance of 

 l'i,000 fathoms, the fourteenth part of 



