& Newcomh—CroWs Clir 



Art. XXXIL—Beview of CroWs Climate and Time with especial 

 reference to the Physical Theories of Qimaie maintained therein;* 

 by Simon Newcomb. 



The present notice of Mr. Croll's work is confined to an ex- 

 amination of his physical theories of climate, avoiding all those 

 portions which have a geological bearing. The physical the- 

 ories propounded have two distinct applications; the one to 

 the present climate of the earth ; the other to the changes of 

 that climate during past geological ages. In the laiter depart- 

 ment of the work the principal object is to account for the glacial 

 epoch or epochs, the author conceiving that there may have 

 been several such epochs. The data from which his conclusions 

 respecting the past are derived are necessarily founded on his 

 theories of the causes of present climate, since it is only by a 

 thorough discussion of the way in which all climatic causes ope- 

 rate, and by tests of all the conclusions by a comparison with 

 the present climate of the globe, that any safe rules can be formed 

 forjudging of the climate of the past. 



We are forced to say at the outset that the physical data for 

 forming a reliable estimate of the separate eflfects of various 

 causes on climate are almost entirely wanting. The physical 

 theory of cosmical heat is, at the present time, in a state nearly 

 approaching the chaotic, a circumstance all the more surprising 

 when we consider the advanced state of other departments of 

 the theory of heat. Cournot and his successors have devoted 

 to the mechanical theory of heat an amount of profound research 

 which has made it a branch of the most exact of the sciences. 

 On the other side, Melloni and his successors have done a great 

 deal for what we may call the chemical theory of heat. Between 

 these two lie the physical theory, as affecting climate and cos- 

 raical temperature, "which has, comparatively speaking, been 

 neglected entirely. To illustrate what we mean let us consider 

 ^e temperature of the earth from the widest point of view, 

 -practically, there is but one source from which the surface of 

 the earth receives heat, the sun, since the quantity received from 

 all other sources is quite insignificant in comparison. There is 

 but one way of losing heat, by radiation into space. The tem- 

 perature of the surface being in a state of permanent equilibrium, 

 the quantity of heat radiated and reflected must be equal to the 

 total quantity received from the sun. It is this equality winch 

 determines the mean temperature of the surface of the globe. 

 If the earth were not surrounded by an atmosphere, if. con- 

 sequently, the amount of heat radiated from each square foot of 

 the land, as well as from th6 whole surface, were equal to that 

 „/.5"^^^*® *°<i Time in their geological relations: a theory of secular changes 

 Of the Earth's Climate. By Jamea CrolL New York, D. Appleton & Co., 1875. 



