250 Dr. J. Croll on some Controverted Points 
even as things are, as Mr. Ferrel remarks, " the mean tempe- 
rature of the southern hemisphere is the greater of the two," 
the mean temperature of the southern being 60 o, 89 F. and 
that of the northern 59° - 54 F. 
Heat cut off by the Atmosphere. — Professor Newcomb says 
further, "Another idea of the author which calls for ex- 
planation is that solar heat absorbed by the atmosphere is 
entirely lost, so far as warming any region of the globe is 
concerned.'-' This is no idea of mine. My idea is not that 
the heat cut off is entirely lost, but merely that the greater 
part is lost. A large portion of the heat is reflected, and of 
that absorbed one half, perhaps, is radiated back into space 
and lost, in so far as the earth is concerned. 
Tables of Eccentricity. — Referring to my tables of eccen- 
tricity of the earth's orbit, he says : — " That there are from 
time to time such periods of great eccentricity is a well- 
established result of the mutual gravitation of the planets; 
but whether the particular epochs of great and small eccen- 
tricity computed by Mr. Croll are reliable is a different 
question." I may here mention that Professor McFarland, 
of the Ohio State University, Columbus, a few years ago, 
undertook the task of re-computing every one of the 150 
periods given in my tables, and he states that, except in one 
instance, he did not find an error to the amount of •001*. 
"The data for this computation," continues Professor New- 
comb, " are the formulae of Le Verrier, worked out about 
1845f, .without any correction either for the later corrections 
to the masses of the planets or for the terms of the third 
order, subsequently discussed by Le Verrier himself. The 
probable magnitude of these corrections is such that reliance 
cannot be placed upon the values of eccentricity computed 
without reference to them for epochs distant by merely a 
million of years.'"' 
In regard to this objection I may mention that the whole 
subject of the secular variations of the elements of the pla- 
netary orbits has been re-investigated by Mr. Stockwell, taking 
into account the disturbing influence of the planet Neptune, 
the existence of which was not known at the time Le Verrier's 
investigations were made. Professor McFarland, with the aid 
of Mr. StockwelL's formulas, has computed all the periods in 
the tables referred to above; and on comparing the results 
found by both formulas, he states that " the two curves exhibit 
a general conformity throughout their whole extent." And 
his computations, I may state, extend from 3,260,000 years 
* American Journal of Science, vol. xi. p. 456 (1876). 
t Le Vender's formulae were worked out several vears before 1845. 
