RAI 
Tlie same mode is adopted on the Thames 
in this country, on the Rhine, and on many 
of the large lakes and rivers in North Ame- 
rica. 
RAGG, rowley, in mineralogy, a class of 
silicious stones, of a dark grey colour, with 
many shining crystals, having a granular 
texture, and acquiring an ochry crust, by 
exposure to the air. The specific gravity 
is about 2.8. It becomes somewhat mag- 
netic by being heated in an open fire. In 
a strong fire it melts witliout addition, but 
with more diflficulty than basaltes. It con- 
sists of 
Silica 47.5 
Alumina 32.5 
Iron 20 
100.0 
RAGG stone, in some respects similar to 
the rowley-ragg. The texture is obscurely 
laminar, or rather fibrous, but the laminae or 
fibres consist of a congeries of grains of a 
quartzy appearance, coarse and /ough : it 
effervesces with acids, and strikes fire with 
the steel : it contains a portion of mild cal- 
careous earth, and a small portion of iron. 
It is used as a whetstone for coarse cutting 
tools. It is found about Newcastle, and in 
several other parts of England, where there 
are large rocks of it in the hills. 
RAIA, the ray, in natural history, a genus 
of fishes of the order Cartilaginei. Generic 
character : five spiracles on each side, placed 
beneath, near the neck; mouth beneath 
the head, transverse, beset with teeth ; 
head small, pointed, and not distinct from 
the body; body somewhat rhomboidal. 
These fishes are found only in the sea, where 
they feed on whatever animal substances 
they meet with. They are sometimes of 
the weight of two hundred pounds. They 
conceal themselves for the greater part of 
the winter in the mud or sand of the bot- 
toms, and, indeed, are seldom seen near the 
surface of the water. The female is larger 
than the male, and produces her offspring 
living, and only one at a time ; the young 
extricating itself gradually from its confine- 
ment, and remaining sometime attached by 
the umbilical vessels, after its complete ap- 
pearance. There are nineteen species. 
R. batis, or the skate, is one of the 
largest of the genus weighing sometimes 
two hundred pounds, and one of this size is 
reported to have been served up at St. 
John’s College, Cambridge. It is the most 
esteemed species of the genus, 
VOL. V. 
RAI 
R. clavata, or the thormback, is muck 
inferior to the skate in size and goodness. 
It inhabits the Mediterranean and other 
seas, and is distinguished by its long and 
curved spines, on its upper surface. The 
above are rhomboidal. 
R. pastinaca, or the sting ray, inhabits 
the Indian and Mediterranean seas, and its 
tail is armed with a very long serrated 
spine, with which it can inflict very for- 
midable wounds, and which it casts off every 
year. This was formerly supposed to con- 
tain the most subtle poison, and ancient 
naturalists have been extremely elegant 
and glowing in their descriptions on this 
subject. It injures, however, only by 
piercing and laceration, and, to prevent 
this, the tail is almost always cut off as soon 
as the fish is caught. These fishes often lie 
in ambuscade, and seize their prey by sur- 
prise, and frequently take it by active and 
persevering pursuit. 
R. torpedo, the torpedo, inhabits the 
Mediterranean and the North Seas, and 
grows to the weight of twenty pounds. 
This fish possesses a strong electrical power, 
and is capable of giving a very consider- 
able shock through a number of persons 
forming a communication with it. This 
power was known to the ancients, but ex- 
aggerated by them with all the fables natu- 
ral to ignorance, and it is only recently 
that the power has been ascertained to be 
truly electric. It is conducted by the 
same substances as electricity, and inter- 
cepted by the same. In a minute and a 
half no fewer than fifty shocks have been 
received from this animal, when insulated. 
The shocks delivered by it in air, are nearly 
four times as strong as those received from 
it in water. This power, appears to be 
always voluntarily exercised by the torpe- 
do, which occasionally may be touched and 
handled, without its causing the slightest 
agitation. When the fish is irritated, how- 
ever, this quality is exercised with propor- 
tional effect to the degree of irritation, and 
its exercise is stated, in every instance, to 
be accompanied by a depression of the eyes. 
When the animal exerts that benumbing 
power from which it derives its name, 
and when it operates by separate and re- 
peated efforts, this is always the case, both 
in the continued, and in the instantaneous 
process, the eyes, which are at other times 
prominent, are withdrawn into their sockets, 
a circumstance very naturally attaching 
both to the condensation and discharge 
of the subtle fluid. Specimens have bee» 
M m 
