[October, 
570 The Velocity of Light . 
terrestrian centrifugality drives the vapour with an addi- 
tional 1-7-5*56 against the air decreasing with height in 
density. 
To fully appreciate the causal connection of all quantities 
I mention one ought to have read “ On some Properties of 
the Earth.” I quote from page 103 : — “ We get — 
(11200' x 2978) — H20o' x (2978^57) = 275,108 feet 
as height of the atmosphere.” 
“ But this is not the true height. We must account for 
the increase of centrifugality through the 275,108'. It is 
14-40 (of the 14333). This loss of gravity is diminished by 
the mass itself of the atmosphere between its limit and the 
earth, adding to the attraction of the latter. The whole 
loss so becomes— 
14-40- ( 1 - 333 ) = 1-44*63. 
and the height of the atmosphere— 
275,018' + (275, oi 8'444*63) = 280,913', or 6162' more.” 
The 6162' are thus reduced 1 42978, to 5956", &c. 
And page 107 : — “ The height of the atmosphere is 
278,172', with the imaginary equivalent 281,065'.” . . . . 
“ If the outer atmosphere were composed like the inner 
one it would be— 
(5956+281, 065') 2 = 142186 
of the mass of the atmosphere ; as it is constituted we find 
it to be 141655. It is 141655, not 141713, because at its 
distance from the earth the attraction of the sun has 
increased and that of the earth decreased, so as to produce 
a difference of 14(1713 x 2978).”* 
Knowing, then, the height of the atmosphere and the 
velocity of light censed in vacuo, we know the time it takes 
for the light vibration to travel through that height, and we 
find the proportion : — As the height of the atmosphere is to 
the distance of the generator (or reflector) in vacuo, so is the 
time light requires in travelling through the atmosphere, as 
it is in the mean here below, to the time censed to be spent 
on the whole distance. When distance changes time 
changes. 
* 278,172' is about the figure of Bradley and Delambre. Certain modern 
astronomers suppose the height much greater. They seem not aware that 
their observations were made, and their reasons propounded and refuted, in 
the first quarter of this century. 
