178 The Rev. Dr. Robinson on the Constant of Refraction. 



amounting sometimes to 13", when the intervening valley was overshadowed by 

 clouds, though the meteorological indications at the observatory remained 

 the same. But how much greater would the disturbance of a star have been 

 whose light must have passed through many miles subject to these anomalies ? 

 For we have no reason to suppose that they are confined to the immediate 

 vicinity of the earth's surface ; they must extend as far as the clouds, (whose 

 existence shews an irregular distribution of heat,) or at least six miles high ; 

 more than three times the height of Quito, at which Bouguer found the 

 refraction only two-thirds of what it is at the level of the sea. Some remarkable 

 facts respecting the variation of terrestrial refraction, when the ground is 

 covered with snow, and immediately after sunset, are given by Struve, in his 

 Gradmessung, but one still more in point is mentioned by the Rev. G. Fisher,* 

 in the Appendix to Parry's Second Voyage, page 175. He found, while ob- 

 serving at Igloolik, that at temperatures of from 20° to 30° below Zero, and at an 

 altitude of 3° 8', the refractions of Sirius were about a minute less when observed 

 over open sea to the south-east, than over land covered with snow or ice, to the 

 south-west. The existence of these local anomalies can only be ascertained by 

 low refractions ; and therefore theory is in such cases unavailing. 



But secondly, even were the hypothesis on which the differential equation 

 of refraction is based strictly true, yet that equation cannot be integrated without 

 assuming a relation between its variables, their real relation being unknown. 

 Philosophers have been guided in this, either by supposed conformity to the 

 law of nature, or by facilities of integration ; but in both cases their results cannot 

 be supposed to have any value except as far as they are confirmed by observation, 

 and therefore all must be pronounced alike empirical. But at low altitudes 

 observations are both difficult and uncertain, and therefore it is by no means easy 

 to pronounce on the results of a given hypothesis ; so that besides that lately 

 published by Biot (but which I believe has not yet been applied to construct 

 refraction tables) there are at least four of high authority ; that of Newton, as 

 modified by Bessel, supposing the temperature uniform, but changing the modulus 

 of atmospheric elasticity by an experimental co-efficient; that of Simpson, 



* To whom I am indebted for much valuable information respecting the important observations 

 published there, and indeed for ray acquaintance with the book itself. 



