HOR 
loam, in Germany and in Ireland. It re- 
ceives a good polish, and serves the pur- 
pose of agate. 
Mr. Jameson observes, on this mineral, 
that, “ at first sight it may appear incon- 
sistent, to consider a petrifaction as a parti- 
cular fossil species; when we reflect, how- 
ever, that woodstone differs in its external 
characters from all other fossils, the justness 
of the Wernerian method will become evi- 
dent. Many other fossils occur in the shape 
of petrifactions, but they are almost always 
identical with some known species, and 
therefore are to be considered only as va- 
rieties of the external shape of the particu- 
lar fossil to which they belong.” 
Horn work, in fortification, an out-work 
composed of two demi-bastions, joined by a 
curtin. 
Horn geld, a tax paid for feeding of 
horned beasts in the forest. See Forest. 
HORNBLENDE, in mineralogy, a spe- 
cies of the clay genus, of which there are four 
sub-species ; viz. the common, the Labrador, 
the basaltic, and the hornblende slate. The 
common hornblende is of a greenish black, or 
raven-black, which in some varieties ap- 
proaches to a greyish and even velvet black. 
It occurs in mass, disseminated and crystalliz- 
ed. Its specific gravity is from 3.6 to 3.8. 
It is opaque, gives a greyish streak, is mo- 
derately hard, and easily frangible ; it ex- 
hales an argillaceous odour. It melts before 
the blow-pipe into a greyish-black glass ; 
the constituent parts, according to Kirwan, 
are, 
Silica ; 37.0 
Alumina 22.0 
Magnesia ..16.0 
Lime .- 2.0 
Oxide of iron 23.0 
100.0 
The common hornblende forms one of 
the essential ingredients of several moun- 
tain rocks; is sometimes accidentally dis- 
seminated in others ; and occurs in beds. 
When in beds, it is frequently accompanied 
with ores, as magnetic iron-stone, iron 
pyrites, &c. It is found in all the northern 
parts of Europe ; and when pure is a capital 
flux for iron ores, (o which purpose it is 
applied in Sweden, where it is obtained in 
large quantities. 
The Labrador hornblende is found in the 
island of St. Paul, on the coast of Labrador, 
is usually of a brownish black, and its spe- 
cific gravity is 3.38. The hornblende slate 
is of a colour intermediate between greenish 
HOR 
and raven black ; it is massive, and is gene- 
rally mixed with mica and felspar. It 
occurs in beds of primitive rocks, parti- 
cularly in clay slate ; also in gneiss and 
mica slate, and is found principally in 
the northern parts of Europe. The ba- 
saltic hornblende is of a velvet black, 
occurs almost always in single imbedded 
crystals, which are small and middle sized. 
The surface is smooth and shining, except 
where it happens to be covered by a thin 
ochery crust. The specific gravity is rather 
less than that of the hornblende slate. It 
melts before the blow-pipe into a brownish- 
black glass. Bergman has analyzed it, and 
found it contained. 
Silica 58.0 
Alumina... 27.0 
Iron 9.0 
Lime 4.0 
Magnesia.., i.o 
9976 
Loss 1.0 
100.0 
It is found in Saxony, Bohemia, Scotland, 
Italy, &c. It resists decomposition longer 
than basalt, hence crystals of it are found 
in clay, formed by the decomposition of 
basaltic rock. Common hornblende is dif- 
ficultly frangible, but the basaltic is very 
easily frangible. 
HORNET. See Vespa. 
HOROLOGY. Horology is that branch 
of science which enables us to measure the 
portions of time. We judge of the lapse of 
time by the succession of sensible events ; 
and the most convenient and accurate mea- 
sures of its quantity are derived from mo- 
tions, either uniform, or else repeated at 
equal intervals. Of the former kind, the 
rotation of the earth on its axis is the most 
exact, and the situation of its surface with 
regard to the fixed stars, or less simply, 
with regard to the sun, constitutes the 
means for determining the parts of time as 
they follow each other. See Astronomy and 
Dialling. Of the latter kind, the rotation 
of machinery, consisting of wheel- work, 
moved by a weight or spring, and regulated 
by a pendulum or balance, affords instru- 
ments of which the utility is well known. 
The term horology is at present more par- 
ticularly confined to the principles upon 
-which the art of making clocks and watches 
is established. A considerable portion of 
this extended subject of research has 
been given under the articles Clock and 
I i 2 
