34 ^ GKOow&KiiiGE*s further Observations 
than the mean, negative. These factors of different values 
will be nearly the same for the thermometer witiun and 
without, in summer; when the temperatures approximate: 
but in winter, when the temperatures may differ 6® or 8°, the 
factors will vary accordingly. It may therefore not be material, 
when the quantity of refraction is small, not exceeding i-*', 
whether the correction is applied for the thermometer within 
or without ; but when the temperature within is supposed to 
be affected by the horizontal current from without, I should 
recommend, in all such cases, the use of the latter correction. 
To make a direct comparison of these refractions with the 
French tables, it may be objected, that the latter being com- 
puted for the metrical barometer 0,760 m., which is equal 
to 29,93 inches of our barometer, the mean refraction from 
the proposed formula should be increased by the factor ,0 1 1 1 ; 
since we reckon the mean state at 29,60 inches : but, the me- 
trical thermometer at -f 10 being equal to ours at 50°; and 
the mean state being determined from observation, to be 45® 
of Fahrenheit; the factor for 50** will be — ,0100; which 
will nearly compensate the factor for the barometer. I have 
therefore compared the mean refractions resulting from my 
formulae, with those of the French tables, from 80° to 90°: 
excepting the two last, the difference is not considerable ; and 
whether these arise from defect in the formula, or from local 
causes, can only be determined by observation. 
On comparing the refractions, now proposed, with my 
former deductions, they will not differ one second at 81°. At 
72® the present is ■— of a second less than the former ; and at 
74° zenith distance of the winter solstice, o,"i8 must 
be deducted as a correction. It will appear on inspection of 
