444 Dr. J. J. Hood on Retardation 



number of observations on the temperature of the sea and air 

 have been collected and published by the Government of the 

 United States of America. From these observations the 

 result has been deduced that the difference of temperature 

 between the two hemispheres is insensible, and probably 

 slightly in favour of the higher mean temperature of the 

 hemisphere which possesses the largest water-covering*. In 

 this way my theoretical views have been fully confirmed by 

 the crucial test of leading to conclusions which did not seem 

 likely to be true at the time I originally placed them on 

 record. 



Some further illustrations of the question under consider- 

 ation may be obtained from the results of observations on 

 the distribution of a well-known class of plants. The condi- 

 tions most favourable to the growth of the larger ferns were 

 recognized by Robert Brown and other eminent botanists to 

 be humidity, shade, and uniformity of moderately elevated 

 temperature. These conditions exist in their most perfect 

 form among the smaller islands of the great oceans. In 

 islands like New Zealand and others of inferior size the tribes 

 of plants alluded to are widely spread and highly developed. 

 The relative distribution of these plants in the northern and 

 southern hemispheres is highly instructive. The extratropical 

 regions of the northern hemisphere contain thirteen times as 

 much land as the corresponding portion of the southern hemi- 

 sphere, and in the latter arborescent ferns are known to grow 

 much further from the equator than in the former. 



Professor Lindley remarked long since that at the time of 

 the deposit of the Lias formation, a geological epoch of 

 somewhat higher general temperature than the present, the 

 vegetation was similar to that of the southern hemisphere in 

 the Pines as well as in the Cycads. 



LIII. On Retardation of Chemical Change. By John J. Hood, 

 D.Sc. (Lond.), Assoc. Royal School of Mines]. 



IN a short paper published in this JournalJ some time ago, 

 on Retardation of Chemical Action, it w r as shown that 



* Dr. J. Hann has published in the Proceedings of the Academy of 

 Vienna a good resum6 of the facts as to the distribution of temperature 

 in the two hemispheres of the earth, and he concludes that their tempera- 

 tures are almost equal. See Ferrel, American Journal of Science, August 

 1882, page 89, u The Relative Temperature of the two Hemispheres of the 

 Earth." 



t Communicated by the Author. 



\ Phil. Mag. [5] xiii. 



