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rOPULAR SCIENCE REVIEW. 
preceded and followed by two variable states. The former is that which 
various experimenters have sought to measure. The resistance of the air 
attaining its maximum during the initial variable state, it is clear that the 
wing of a bird would find in the air a more solid fulcrum if, throughout its 
descent, it were placed in these initial conditions. Now, through trans- 
lation, the wing at each instant of its descent comes to act on a new column 
of air, which it tends to depress. But, from the short duration of the pres- 
sure, each of these columns has not time to acquire the velocity of the wing; 
it is thus compressed, and presents the maximum resistance of the initial 
variable state. To test this theory M. Marey gave his artificial birds a hori- 
zontal movement of translation : attaching one, e.g ., to the end of a long 
arm which was driven round while the wings were made to beat by means 
of a steam-driven air-pump. When the arm was stationary, the wing 
described between its extreme positions an angle of about 60°. On driving 
the arm 10 metres per second the amplitude was reduced to 30° and even 
20°, showing the effect of resistance of the air on the velocity of the 
strokes. 
METALLURGY, MINERALOGY, AND MINING. 
The Fibrous Quartz of the Cape. — This, which is a pseudomorph after 
Krokydolite, is described by Herr Dr. F. Wibel, whose paper on this subject 
is noticed in the u Geological Magazine ” for March 1874. It says that it is 
a curious fact that whilst quartz so commonly occurs crystallized, it has 
rarely been observed in distinctly fibrous forms. The best-known example 
is that of the so-called fibrous quartz of South Africa. The object, how- 
ever, of the paper of Dr. Wibel is to show that this substance is not an 
original form of quartz, but is merely a product of pseudomorphism, in 
which the fibrous structure of a pre-existing mineral has been retained. Dr. 
Wibel has examined two varieties of this African mineral — the one brown 
and the other blue. The brown variety occurs in the form of bands in a 
highly siliceous brown ironstone. Analysis of the fibrous mineral showed 
that it contains — silica, 57*46; ferric oxide, 37*56; water, 5*15. Treated 
with hydrochloric acid, the iron is removed, and a white fibrous siliceous 
material is obtained. Hence he concludes that the brown mineral consists 
of a mixture of white quartz and ferric hydrate in the form of Gothite 
(Fe 2 0 3 *H 2 0). 
What is Brochantite ? — This question is fully answered by Dr. A. Schrauf, 
in a paper in the 11 Proceedings of the Vienna Academy of Science,” and is 
noticed at length in the March number of the u Geological Magazine.” It 
seems that several minerals, differing from one another both chemically and 
morphologically, have hitherto been grouped together under this specific 
name; and, although our knowledge of many of these varieties is still 
imperfect, yet the author feels justified in referring them to four distinct 
types, namely: — (1) The Brochantite of Rezbanya in Hungary, of which 
two varieties (a and b ) are recognised; and some of the Cornish and 
Russian Brochantites. (2) The Waringtonite of Cornwall, and a third 
variety (c) from Rezbanya. (3) The Brochantite of Nischne-Tagilsk, in 
