Mevieivs — Sir R. Ball's Ice-age. 231 



a Thammotettix, a Dawsonites, and a Stenecphora from the Tertiary 

 of British Columbia. 



At pages 178-181 Mr. Buckton refers to geological speculations 

 as to the changes of land and climate affecting Insect-life in late 

 Tertiary and Quaternary times ; also to possibilities of development 

 and of degeneration among Insect forms in geologic times, and he 

 hesitates to offer any outline of the phylogenetic descent of the 

 Homoptera in particular. T. E. J. 



I^E^VIE■WS. 



I. — The Cause of an Ice Age. By Sir Eobkrt Ball, LL.D., 

 F.R.S., Koyal Astronomer of Ireland. Pp. 180. (Kegan Paul, 

 Trench, Trtibner & Co. 1891.) 



THIS little book contains a very clear and agreeably M^ritten ex- 

 position of the commonly received Astronomical theory of Glacial 

 periods. But it goes further than that, because it offers an explana- 

 tion of a difference of the mean temperatures of either hemisphere 

 during the summer and winter seasons, reckoning from equinox 

 to equinox, which has not hitherto been taken due account of in 

 estimating the effects of the earth's position with reference to the 

 sun. The author proves, by a short calculation given in an Appendix, 

 that owing to the obliquity of the ecliptic the quantity of heat, 

 received from the sun upon one hemisphere during its summer, 

 bears to the quantity received during its winter the invariable pro- 

 portion of 63 to 37. This will be the case always, whatever be 

 the position of the equinoctial line with respect to the major axis of 

 the orbit, and whatever be the eccentricity of the orbit. He points 

 out that, in consequence of an inadvertence in a statement in 

 Herschel's Outlines of Astronomy, the proportion has hitherto been 

 regarded as one of equality ; and it is obvious how great a difference 

 this consideration will make in estimating climatic effects. 



The greatest eccentricity which the earth's orbit can have is about 

 07. When with this eccentricity winter in the northern hemi- 

 sphere occurs in aphelion, taking the mean daily heat for the whole 

 year received by that hemisphere, as unity, the mean daily heat 

 received by it in a short summer of 166 days will be represented 

 by 1-38 ; and the mean daily heat received in its long winter of 199 

 days will be only 0'68. This, the author says, will produce a severe 

 glacial epoch, when the summers will be short and very hot, and 

 the winters long and very cold. While the eccentricity remains the 

 same (for it changes very slowly) when the axis of the earth is next 

 carried round by precession until the winter occurs in perihelion, the 

 mean daily heat received in a long summer of 199 days will be 1"16, 

 and in a short winter of 166 days will be 0-81. This he believes 

 will produce an interglacial period — interglacial because two or 

 three such reverses may occur before the eccentricity is sensibly 

 altered. It must be remembered that the unit of heat here used is 

 a very large one, being that which raises the mean temperature of 



