go Mr. herschel on the 
tion between the two Stars, though no more than the ,01 1 ith 
part of a fecond, becaufe the real diftance between the two Stars is 
not required. It follows then, that thefe obfervations can only 
be affeCted by the difference of the difference ; that is, by an 
alteration in the quantity of refraction occasioned by the change 
of heat and cold, or weight of the atmofphere, and pointed 
out to us by the rife and fall of the barometer and thermome- 
ter. Let us then fee what this difference of the difference may 
amount to. Suppofe a change of 2 2° of Fahrenheit’s ther- 
mometer, that is, from the freezing point to the moderate air 
of a fummer’s night, and a difference of an inch in the height 
of the barometer ; thefe two caufes both confpiring, which 
does not often happen, may occafion an alteration of ,00096th 
part of a fecond in five, at an altitude of 20° ; but this being 
lefs than the thoufandth part of a fecond may fafely be rejected 
as a quantity altogether infenfible. 
Since it may not be always convenient to view thofe Stars at 
the altitude of 20°, it remains to fee what effeCt different alti- 
tudes may have : let us then make the moSt unfavourable fup- 
pofition, that they may one time be feen in a horizontal por- 
tion, having before been feen vertical. In this cafe, as the 
whole difference of refraction in a difference of 5" of altitude 
Is no more than ,0111, provided they are obferved not lower 
than. 20 0 , and the whole difference of the difference of refrac- 
tion is only ,0009; the fum ,012, when both confpire, not 
exceeding much the hundredth part of a fecond, may ff ill be 
rejedted as infenfible. Let us alfo examine how near the hori- 
zon it may be fafe to obferve fuch Stars. At io p , for in Stance, 
the refradtion is 5' 14'', 6 ; the difference for 5" is ,0388 ; the 
joint effeCt of the changes in the barometer and thermometer is 
,0034 ; the fum of the whole together amounts to ,0422, which 
5 is 
