304 Chronicles of Science. [April, 



shown to be the case. In a cyclone m the Northern Hemisphere, 

 on one side, the wind in its motion round the central area "backs" 

 from south towards south-east, and on the other side it backs from 

 north towards north-west ; whereas the effect of the earth's rotation 

 on a southerly wind flowing polc-\\ ards is to give it westing, in con- 

 sequence of its change of latitude, and the wind therefore "veers" 

 towards south-west and west, forming not a cyclone but an anti- 

 cyclone. 



This carelessness of language is most unfortunate, as it mars 

 much of the valuable work Mr. Buchan is doing. In one of his 

 later paj)ers he asserts that the au- is composed of oxygen and 

 hydrogen, while in the last edition of his ' Handy Book of Meteo- 

 rology' he proposes that each missionary should be instructed in 

 meteorology, in order to issue storm warnings for the South Sea 

 Islanders, " and legitimately increase his influence over them ! " 



9. MINEEALOGT. 



Considering the extensive employment of that large class of orna- 

 mental stones grouped together under the general name of Agate, 

 it is surprising how little is commonly known in this country as to 

 the source whence they are obtained, and the manner in which they 

 are worked. A small district at the southern foot of the Himdsriick, 

 in Western Germany, has for more than four centuries been almost 

 exclusively the seat of the agate trade, and at the present time 

 upwards of 3000 of the inhabitants gain a livehhood by this 

 pecuhar branch of industry. Full information on all that relates to 

 the subject of agates has lately been published by Herr Lange, who 

 writes from the little town of Idar, situated in the very centre of the 

 agate works.* 



Agates usually occur in the form of amygdaloidal nodules, of 

 greater or less size, embedded in a dark-coloured trap-rock or 

 melaphyre. Many are the theories that have been advanced to 

 explain the origin of these nodules, since the time when it Avas first 

 suggested that they were petrified melons ! At the present day it 

 is generally supposed that the cavities now filled with agate were 

 produced by the disengagement of gas or steam when the melaphyre 

 was in a viscous condition. After the solidification of the lava-like 

 mass, the cavities were gradually filled, either partially or com- 

 pletely, by silica held in solution by the water jiorcolatiug through 

 the rock. The precise manner in which the silica was introduced 



* ' Die Ilalbcdclsteiiie aiis dcr Familie dcr Quarze, mid die Geschiclite der 

 Acbatindustric.' Von C. Lungo in Jd.ir. Krciiznadi, 1SG8. Pp. 100. 



