420 Mr. Ivory on the Astronomical Refractions. 



Fundamenta Astronomia:, from the zenith to 45° of altitude. 

 The standard quantities of Mr. Bessel are, 48f° Fahr., Bar. 

 29'6 ; the refraction at 45° is 57""49 : now if this number be 

 reduced to 50°, 30 B., there will come out 58"-118 ; the num- 

 ber in the N. A., which does not extend beyond tenths of 

 a second, being 58"' 1. The coincidence is equally exact in 

 every case within the limits mentioned. I have formerly 

 shown that the refractions in N. A. near the horizon are the 

 same with those of the French Astronomers. It is reason- 

 able to conjecture, nay it seems to follow as an unavoidable 

 consequence, that the middle part of the Table in N. A. must 

 be a mean between the other two Tables, or, at least, it must 

 be interpolated by some rule. I throw out these observa- 

 tions for the purpose of showing how necessary it is. to lay 

 before the public, simply and explicitly, an explanation of 

 the real construction of the Table. 



It will readily be conceived that a Table may be constructed 

 in the manner I have been describing, not liable to extreme 

 errors in practice. But it would deserve little praise from the 

 astronomer or the man of science. It would not do much 

 honour to an exciseman *. 



There is another point that stands in need of elucidation. 

 It relates to the corrections for temperature. Without en- 

 tering upon any theoretical discussion, in point of fact the 

 corrections in N. A. are the same with Mr. Bessel's. The 

 differences are altogether trifling. For this we have the au- 

 thority of the Journal of Science f. Now the refractions in 

 N. A. at low altitudes are almost coincident with those in the 

 French Table. But if any astronomer should apply Mr. Bes- 

 sel's corrections to the French Table, he would commit a 

 great inaccuracy. On this ground I asserted in your last 

 Number that the N. A. at low altitudes is more discordant 

 with observation than the French Table. 



The Table in my paper has fully answered every purpose 

 for which it was intended. It has turned out much more 

 correct than I could possibly have expected. I shall now 

 direct your attention to the chief points of my Theory ; 

 briefly mention the proofs already given of its agreement with 

 Nature ; and then add other corroborative evidence that has 

 since occurred to me. 



My investigation sets out with supposing that the heat de- 

 creases equably as we ascend in the atmosphere. This is no 

 new law. It has long been assumed in the Theory of baro- 



* Sup. Enc. Brit., Article Wakefield. 

 f No. £9, p. IS]. 



metrical 



