s8o 
Dr. Brinkley on the 
tions have hitherto been computed, differ from the French 
refractions as follows. 
The French refraction at 45°==57, "5 1 therm. 50, and 
Bradley’s refraction at 45=56, 9/ barom. 29,60 
and the mean diff. = o ",6 x tan. zen. dist. nearly. 
But this will not affect the index error, as it equally affects 
r, &c. 
But the effects of the change of temperature as computed 
by Bradley’s, and by the French refractions, have an im- 
portant concern in this enquiry. 
To deduce the actual refraction from the mean refraction 
for height of thermometer = t, the mean refraction is multi- 
plied by ~~ according to Bradley. 
According to the French tables, the multiplier is ~~ 
at least sufficiently nearly so for the 30 standard stars of 
Mr. Pond. 
The difference then between Bradley’s refraction and the 
French refraction from the change of temperature, is nearly 
x mean refraction. 
Now Procyon is one of the standard stars ; and when this 
star passes the meridian in June, soon after mid-day, we may 
suppose Fahrenheit’s thermometer at 70° ; and when this 
star passes the meridian in December, near midnight, the 
thermometer may be at 30°. The mean refraction of this 
star at Greenwich, is 58" nearly. Therefore the refraction 
computed by the French table may, in summer, exceed that 
computed by Bradley’s table by o/'gS, and the contrary may 
