IlOH 
«f the crust of this globe, has 'been here 
adopted, from Professor Jameson’s perspi- 
cuous description of it, on account of its 
so exactly corresponding \vith the appear- 
ances which masses of rocks every where 
present to our view. The present outline, 
though perhaps sufficiently correct, is, how- 
ever, by no means pretended to supersede 
the study of the work alluded to ; since an 
accurate knowledge of the subject can only 
be yielded by the study of the more highly 
finished performance itself. 
ROCKET. See Pyrotechny. 
ROD, a land measure of sixteen feet and 
a half; the same with perch and pole. 
ROELLA,in botany, so named in honour 
of William Roell, a genus Of the Pentandria 
Monogynia class and order. Natural order 
of Campanacese. Campanulaceae, Jussieu. 
Essential character : corolla funnel-form, 
with the bottom closed by staminiferous 
valves ; stigma bifid ; capsule two-celled, 
cylindrical, inferior. There are five spe- 
cies, all natives of the Cape of Good Hope. 
ROHRIA, in botany, so named in honour 
of Julius von' Rohor, a genus oftheTrian- 
dria Monogynia class and order. Essential 
character : calyx bell-shaped, five-parted ; 
corolla five-petalled, unequal ; stigmas 
three, revolute; capsule. There is but 
one species, viz. R. petioliflora, a native of 
the woods of Guiana. 
ROGUE. ' See Vagrant. 
ROHAULT (James), in biography, 
a French philosopher, was the son of a 
rich merchant at Amiens, where he 
was born in 1020. He cultivated the 
languages and belles lettres in his own 
country, and then was sent to Paris to 
study philosophy. He seems to have been 
a great lover of truth, at least what he 
thought so, and to have sought it with much 
impartiality. He read the ancient and mo- 
dern philosophers ; butDes Cartes was the 
author who most engaged his attention. 
Accordingly he became a zealous follower 
of that great man, and drew up an abridge- 
ment and explanation of his philosophy 
with great clearness and method. In the 
preface to his Physics, for so his work is 
called, he makes no scruple to say, that 
“ the abilities and accomplishments of this 
philosopher must oblige the whole world to 
confess, that France is at least as capable 
of producing and raising men versed in all 
arts and branches of knowledge, as ancient 
Greece.” Clerselier, well known for his 
translation of many pieces of Des Cartes, 
conceived such an affection for Rohault, on 
account of his attachment to this philo- 
ROL 
sopher, that he gave him his daughter in 
marriage against alt the remonstrances of 
his family. 
Rohault’s Physics were written in French, 
but have been translated into Latin by Dr. 
Samuel Clarke, with notes, in- which the 
Cartesian errors are corrected upon the 
Newtonian system. The fourth and best 
edition of Rohault’s Physics, by Clarke, is 
that of 1718, in 8vo. He wrote also “ Ele- 
mens de Mathematiques,” “ Traiffi de Me- 
chanique,” and “ Entretiens sur la Philoso- 
phic.” But these dialogues are founded 
and carried on upon the principles of the 
Cartesian philosophy which has now little 
other merit, tlian that of having corrected 
the errors of the ancients. Rohault died 
in 1675, and left behind him the character 
of an amiable, as well as a learned and phi- 
losophic man. 
His posthumous works \vere Collected and 
printed in two neat little volumes, first at 
Paris, and then at the Hague, in 1690. The 
contents of tliem are, 1. The first six 
books of Euclid. 2. Trigonometry. 3. 
Practical Geometry. 4. Fortification. S. 
Mechanics. 6. Perspective. 7. Spherical 
Trigonometry. 8. Arithmetic, 
ROLANDRA, in botany, so named in 
honour of Daniel Rolander, a pupil of Lin- 
Timus, who travelled to Surinam ; a genus of 
the Syngenesia Polygamia Segregata class 
and order. Natural order of Composit® 
Capitat®. Cinarocephal®, Jussieu, Essen- 
tial character : florets bundled into a head 
with scales interposed ; calyx partial, two- 
valved, one-flowered ; corollets hermaphro- 
dite ; down none. There is but one species, 
viz. R. argentea, a native of the West In- 
dies. 
ROLLING mill, in mechanics, a ma- 
chine for working metals into plates, or 
bars, which are required of an even thick- 
ness. In the Plate, Rolling-Mill, are three 
elevations of a machine for this purpose, 
A B, D E, in all these figures is a massive 
frame of cast iron, consisting of two distinct 
cheeks, A B and D E, which are connected 
together by being both affixed to an iron 
plate, F F, bolted down upon two ground 
sills, G G, supported on masonry, and form- 
ing the foundation for the whole machine ; 
each cheek has an oblong mortice, a h, 
through it ; a strong iron screw, d, is screw- 
ed through the upper end of each cheek, 
and has a wheel on the top of it, with teeth, 
to receive a handspike to turn it by. I K 
are the two rollers made of cast iron, and 
very truly turned in a latli^jg^j(»Jf have 
pivots, il,k m, at each end, turned at the 
