( ) 
II. Some dfymarfo on the Allowances to he made in 
Agronomical ObferVations for the <% efraSlion 
of the Air . (By Dr. Edm. Hailey, R. S S. 
Ajlronomer oyal . With an accurate Table of 
T(ef< r a Elions, 
^ /f / E RE the Medium of our Air much more in 
V \ Quantity, or the Force of Gravity much 
greater than it is, or in a word, were the Refradlive 
rower of the Air much more fenfible than we find it, 
nothing could have been a greater Impediment to 
Difcoveries in Aftronomy : For all Objedts appea- 
ring by Refradion higher than really they are, till 
fuch time as the Laws and Quantity of that Re- 
fraction had been afcertained, it would have been 
impoffible to have been fecure of the true obferved 
Place of any Coele/fial Object. But as it falls out to 
be fo little, that none but nice Inftruments can per- 
ceive its Effects, it was not difcovered to be at all, 
till BernardWalthers time, about the Year 1500* 
nor brought to any fort of Rule till Tycho Brahe - 
nor afcertained, till our worthy Prefident made the 
firft accurate Table thereof: The Curve which a 
Beam of Light defcribes, as it approaches the Earth, 
being one of the molt perplext and intricate that can 
well be propofed, as Dr. Brook Taylor in the laft 
Propofition of his Methodus Incrementerum has made 
it evident. By this Table it follows that the ratio 
B b of 
