FOR 
FOR 
FOR 
AB, After having inscribed the two sides 
(tE, GF, iii a circle, draw the diameter Cl), 
so as to be equally distant from the line join- 
ing the points FF that is parallel to it. On 
this diameter set off 100 toises on each side 
of the centre; from these points draw two 
indefinite perpendiculars to the diameter ; 
then if from the points, EF, as centres, two 
arcs are described with a radius of 180 toises, 
their intersections A and B, with the said per- 
pendiculars, will determine the long side 
AB, as likewise the other two FB and EA. 
In like manner may be found the long or 
short side of any polygon whatsoever. 
When a place near a river is to be fortified 
for the safety of commerce, particular care 
should be taken in leaving a good space be- 
tween the houses and the waterside, to have 
a quay or landing-place for goods brought by 
water ; it should also be contrived to have 
proper places for ships and boats to lie secure 
in stormy weather, and in time of a siege; 
and as water-carriage is very advantageous 
for transporting goods from one place to an- 
other, as likewise for bringing the necessary 
materials, not only for building the fortifica- 
tions, but also the place itself, the expences 
will be lessened considerably when this con- 
venience can be had ; for which reason places 
should never be built any where but near 
rivers, lakes, or the sea, excepting in extraor- 
dinary cases, where it cannot be avoided. 
The principal maxims of fortification are 
these, viz. 
1. That every part of the works be seen 
and defended by other parts, so that the ene- 
my cannot lodge any where without being 
exposed to the fire of the place. 
2. A fortress should command all places 
round it ; and therefore all the outworks 
should be lower than the body of the place. 
3. The works furthest from the centre 
should always be open to those that are 
nearer. 
4. The defence of every part should al- 
ways be within the reach of musket-shot, 
that is, from 120 to 150 fathoms, so as to be 
defended both by ordnance and small fire- 
arms ; for if it is only defended by cannon, 
tlfo enemy may dismount them by the supe- 
riority of their own, and then the defence 
will be destroyed at once ; whereas, when a 
work is likewise defended by small-arms, if 
the one is destroyed, the other will still sub- 
sist. 
5. All the defences should be as nearly di- 
rect as possible ; for it has been found by ex- 
perience, that the soldiers are too apt to fire 
directly before them, without troubling them- 
selves whether they do execution or not. 
. 6. A fortification should be equally strong 
on all sides ; otherwise the enemy will at- 
tack it in the weakest part, whereby its 
strength will become useless. 
7. Tire more acute the angle at the centre 
is, the stronger will be the place. 
8. In great places dry ditches are prefer- 
able to those filled with water, because sal- 
lies, retreats, succours, &c. are necessary ; 
but, in small fortresses, wet ditches, that can 
be drained, are the best, as standing in need 
©f no sallies. 
E/c/iAFortification is the art of con- 
structing ail kinds of temporary works in the 
field, such as redoubts, field-forts, star-forts, 
triangular and square forts, heads of bridges, 
and various sorts of lines. &c. An army en- 
trenched, or fortified in the field, produces, in 
marfy respects, the same effect as a fortress ; 
for it covers a country, supplies the want of 
numbers, stops a superior enemy, or at least 
obliges him to engage at a disadvantage. 
The knowledge of a field-engineer being 
founded on. the principles of fortification, it 
must be allowed, that the art of fortifying is 
as necessary to an army in the field as in for- 
tified places ; and though the maxims are 
nearly the same in both, yet the manner of 
applying and executing them with judgment, 
is very different. 
The materials used in the field are such as 
can be readily obtained, viz. sand-bags, 
earth, and fascines ten feet long and one foot 
thick, which are fastened to the parapet, by 
means of five pickets driven obliquely into 
the bank. 
A\ hen wood cannot be obtained for the 
fascines, the parapet must be clothed with 
turf, four inches thick, and a foot and a half 
square. 
The palisades for fortifying the ditch 
ought to be nine or ten feet long and six 
inches thick. 
The beams belonging to chevaux-de-frize 
should be twelve feet long, and six inches 
broad ; the spokes seven feet long, four inches 
thick, and six inches distant from each other. 
Gabions must be three or four feet high, 
and two or three feet in diameter. 
FORTIN, Portlet, or field-fort, a 
sconce or little fort, whose flanked angles are 
generally distant one from another 120 fa- 
thoms. 
FORTS, vitrified, a very singular kind of 
structures found in the Highlands and nor- 
thern parts of Scotland, in which the walls 
have the appearance of being melted into 
a solid mass, so as to resemble the lava of a 
volcano, for which indeed they have been 
taken by several persons who have visited 
them. 
These walls were taken notice of by Mr. 
Williams, an engineer, who wrote a trea- 
tise on the subject, and was the first who sup- 
posed them to be the works of art ; other na- 
turalists having attributed them to a volcanic 
origin. These works are commonly situated 
on the tops of small hills, commanding an ex- 
tensive view of the adjacent valley or low 
country. The area on the summit, varying, 
as is supposed, according to the number of 
cattle the proprietor had to protect, or the 
dependants he was obliged to accommodate, 
is surrounded with a high and strong wall, of 
which the stones are melted, most of them 
entirely ; while others, in which the fusion 
has not been so complete, are sunk in the 
vitrified matter in such a manner as to be 
quite inclosed with it ; and in some places the 
tusion lias been so perfect, that the ruins ap- 
pear like masses of coarse glass. Mr. Wil- 
liams has not only determined the walls in 
question to be the works of art, but has even 
hazarded a conjecture as to the manner in 
1 which they were constructed, and which, ac- 
cording to him, was as follows. Two paral- 
lel dikes of earth or sod being raised, in the di- 
rection of the intended wall, with a space be- 
tween them sufficient for its thickness, the 
fuel was put in, and set on fire. The stones 
best adapted for the purpose, called the plum- 
pudding stone, are every where to be found 
in the neighbourhood. These were laid on 
767 
Uie fuel, and, when melted, were kept by the 
frame of earth from running off; and b}' re- 
peating the operation, the wall was raised to a 
sufficient height. This opinion of the stones 
being thrown in without any order, is thought 
to be confirmed by the circumstance of there 
not being any where a large one to be seen, 
nor a stone laid in any particular direction, 
nor one piece which has not in some degree 
been affected by the fire. Mr. Williams 
mentions a fact tending to confirm his hypo- 
thesis, viz. of a brick-kiln situated on the de- 
clivity of an eminence, so as to be exposed to 
the wind, which, happening to rise briskly 
one time when the kiln was burning, so in- 
creased the heat, that the bricks were melted, 
and ran like a lava, for a considerable way 
down the hill. 
This opinion of Mr. Williams lias been em- 
braced by several other authors ; particularly 
Mr. Freebairn and Dr. Anderson, the latter 
having published two treatises upon the build- 
ings in the Archa-ologia. In the same work, 
however, we meet with a paper by the lion. 
Dailies Barrington, in which the author ex- 
presses quite different sentiments. lie ob- 
serves, that Mr. Williams and the other an- 
tiquaries, who suppose the walls in question 
to be the works ot art, imagine that the rea- 
son of their being constructed in this manner 
was the ignorance of cement, which in these 
remote ages prevailed in Scotland : but with 
respect to this circumstance he says, that if 
one side of the wall only was heated, and that 
to any considerable height, the matter in fu- 
sion would in all likelihood drop down to the 
bottom, without operating as any cement to 
the loose stones thrown in amongst it. T'liis 
circumstance of the walls being vitrified only 
on one side is indeed remarkable, and 
takes place in most of the forts of this kind to 
be met with at present; but with regard to it 
Mr. Barrington observes that he himself has 
been twice in the Highlands of Scotland, and 
lias found very few hills of any height which 
were clothed with wood : the trouble there- 
fore of carrying it up to the top of such a 
mountain would be very considerable. 
According to Mr. Cardonnel, the largest 
of these vitrified forts is situated on the bill of 
Knockfarril, to the south of the valley of 
Strath peffer, tw r o miles west from Dingwall 
in Ross-shire. The inclosure is 120 feet long 
and 40 broad within the walls; strengthened 
on the outside with works at eacli end. The 
fort next in consequence to that of Knockfarril 
is situated on the hill of Craig- Phadric near 
Inverness. 
Besides these fortifications, the hill of Noth 
affords a remarkable appearance of the same 
kind: of which Mr. Gordiner gives the fol- 
lowing description, not from his own obser- 
vation, but those of a gentleman of credit 
who visited the place. “ On the top of tl:.* 
hill there is an oblong hollow, as I could guess, 
of about an English acre, covered with a line 
sward of grass: in the middle towards the 
east end of this hollow' is a large and deep 
well. The hollow is surrounded on all sides 
with a thick rampart of stones. On three 
sides of this rampart, from eight to twelve 
feet thick, is one compact body of 
stones and minerals which have been iri 
a state of fusion, resembling a mixture of 
stone and iron-ore, all vitrified, calcined, 
and incorporated. Or the north side. 
