GENERAL MEETING. 25 
The air temperature (t,) was found from the indications of the 
bright and black bulbs (t, and t,) by the empirical formula 
t= t, + C (tr —t) 
where C is a small coefficient, to be determined experimentally, 
and is nearly constant. This arrangement of bright and black 
bulbs can be used by meteorologists and physicists without a 
screen, and even in the sunlight, if the theory of the action of the 
bright and black bulbs is perfectly understood. A similar for- 
mula will give the temperature (T) of a single radiating body whose 
effect is equal to the total effect that is shown by the black bulb: 
T=t,+C (,—t..) 
He then stated that the theoretical basis of this method has quite 
recently been further elucidated by Professor Ferrel, who has shown 
that the approximate nature of the relation between the above con- 
stant C, the radiating, absorbing, and conducting powers of the 
_thermometers, and the velocity of the wind is given by the following 
equation : 
Br, 
1+ = pr 
B’/+B 
ae eT y 
p Dea 
1 
where r, and r, are the radiating (and absorbing) powers of the 
blackened and silvered bulbs, respectively, v is the velocity of the 
wind or currents flowing past the bulbs, and B B’ B” are constant 
coefficients depending on the size, conductivity, and specific heat 
of the substance of the bulbs. 
In reply to a question of Mr. GiLBert, he stated that the differ- 
ence between the bright and black bulbs had rarely exceeded a 
few tenths of a degree in the delicate shelter made of oiled paper, 
as used by him at Poulkova, the maximum occurring February 22, 
1866, at 10 a. m., when, the louvre box being in the full sunshine, 
the bright bulb was at 14°.9 Cent. and the black bulb at 14°.3, 
showing that the latter had been slightly warmed by the warm sides 
of the box. 
In reply to a question of Mr. Harxness, the author explained, 
that although it was conducive to accuracy that these thermometers 
should be placed within a shelter, yet this was not necessary; if 
we take advantage of the more accurate method of determining 
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