300 Analyses of Books. [April, 



the refractions is to find the velocity of the light at any given 

 height in the atmosphere. The only physical principle wanted 

 for this purpose is the refractive power of air according to its 

 density. Hauksbee first determined by experiment, that air 

 refracts light in proportion to its density ; and this result has 

 been confirmed by succeeding philosophers. There is even good 

 reason to think that the conclusion of Hauksbee is nut mate- 

 rially affected by the variable quantities of aqueous vapour con- 

 tained in the atmosphere at different times. Admitting then the 

 principle we have mentioned, we must conceive that the light 

 coming from the sun or from a star, moves in vacuo with a 

 uniform velocity until it reaches the atmosphere. It is there 

 deflected from its course by the spherical and concentric shells 

 of air it meets with, each of which acts upon it with a force per- 

 pendicular to its surface, and directed to the centre of the earth. 

 Now, as all the light enters the atmosphere with the same velocity, 

 and as the deflecting forces are of the same intensity at the same 

 distance from the common centre to which they tend ; it follows, 

 that the new velocities acquired by the action of the forces will 

 be independent of the direction of the light's motion, and will 

 be the same at the same distance above the earth's surface. 



The ultimate deviation of the light of a star from its primitive 

 direction depends upon the augmentation of the velocity which 

 the light acquires in its passage through the atmosphere, and 

 likewise upon the different obliquities with which it crosses the 

 several strata of air. Now the first of these two things is the 

 same for all stars and for all constitutions of the atmosphere, for 

 it is the same when the density of the lowest stratum of air con- 

 tinues the same. But the second is different for stars, that are 

 differently placed with regard to the zenith, and it varies also 

 with the densities of the strata that compose the atmosphere. 

 It is, therefore, certain, that the formula of Laplace is rigorously 

 exact in no case whatever. But when a star is near the zenith, 

 the variations in the obliquity of the light in passing through the 

 several strata of air, are inconsiderable ; and the formula will be 

 nearly true. However, there is always some error, which accu- 

 mulates as the zenith distance increases, and will at length 

 become sensible. Delambre tells us, that in comparing the 

 observations of different days, he found errors arising from 

 refraction that amounted to 6" or 7" at 75° from the zenith ; and 

 the observations of a very accurate astronomer show that similar 

 inequalities are perceptible much nearer the zenith (Dr. Biinkley's 

 paper Phil. Trans. 1821, p. 342). Now these inequalities do uot 

 arise from any thing imperfect in the manner of observing : they 

 are undoubtedly produced by alterations in the remote parts of 

 the atmosphere which do not affect the barometer or thermome- 

 ter placed at the observatory. It appears, therefore, that the 

 peculiar constitution of the atmosphere has a perceptible 

 influence on the refraction at 75° from the zenith ; and when 



