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344. =. Croll—Professor Newcoml’s “ Rejoinder.” 
great,” “small,” “comparatively small,” and so forth, without 
any statement of the units of comparison relatively to which 
these expressions are employed. No one reasoning on the 
combined influence of a multitude of physical causes could 
well avoid the almost continual use of such terms. Besides, 
my critic forgets that in almost every case in which I use these 
terms numerical exactness is not attainable; and even if it were, 
it would, as a rule, be of little service, seeing that the conclu- 
sion generally depends on the simple fact that one quantity is 
less or greater than another, not on how much less or how much 
greater the one may be than the other. Although my argu: 
ments are logical, few writers, I venture to say, have done more 
than myself to introduce definite quantitative exactness into 
the questions I have discussed. - 
Prof. Newcomb gives his readers to understand that I assume 
Newton’s law of cooling to be correct; and that I apparently 
nowhere adduce the more correct law of Dulong and Petit— 
viz: that if we take a series of temperatures in arithmetical 
progression, the corresponding rates of radiation of heat will 
not be in arithmetical progression, but in a series of which the — 
differences continually increase. If he will refer to the ‘ Rea- 
der,’ Dec. 9, 1865, Phil. Mag., Feb. 1870, ‘Nature,’ April 1, 
1880, and ‘Climate and Time’ (the book he reviewed), p. 87, 
he will there see the question discussed at considerable length. 
He will also find reference made to a remarkable circumstance 
connected with radiation which perhaps may be new to him. 
It is this: the law of Dulong and Petit (that as the temperature 
of a body rises the radiation of the body increases in a muc 
higher ratio) holds true only of the body considered as a mass- 
The probability is, as has been shown by Prof. Balfour Stewart, 
that the individual particles composing the body obey N ewton’s 
aw in their radiation; in other words, the radiation of a mate- 
rial particle is directly proportionate to its absolute temperature. 
Further, in estimating the extent to which temperature 1S 
affected by a change in the sun’s distance, Newton’s law maxes 
the extent too great; while the formula of Dulong and Petit, 
which is an empirical one, makes-it, on the other hand, too 
conclusion from Dulong and Petit’s law favorable to my theory — 
of the cause of the Glacial epoch, which certainly did escape 
