SLU 
serves, that the bottom of the sluice-work 
should be made deeper than either. 
When a sluice is to be placed at the bot- 
tom of an harbour, in order to wash away 
the filth that may gather in it, by means of 
the waters of a river or canal, in this case 
the bottom of the slnice-work should be 
two feet or, eighteen inches higher than the 
bottom of the harbour, that the water may 
run with the greater violence. 
An engineer ought always to have in his 
view, that the faults committed in the con- 
struction of sluices are almost always irre- 
parable. We shall therefore lay down 
some rules, from Belidor, for avoiding any 
oversight of this kind: 1. In order to ad- 
just the level of the sluice-w'ork with the 
utmost exactness, the engiueer ought to de- 
termine how much deeper it must be-than 
a fixed point : and this he should mark 
down in his draught, in the most precise 
terms possible. 2. When the proper depth 
is settled, the foundation is next to be exa- 
mined; and here the engineer cannot be 
too cautious, lest the apparent goodness of 
the soil deceive him: if the foundation is 
judged bad, or insufficient to bear the su- 
perstructure, it must be secured by driving 
piles, oragrate-work of carpentry. 3. There 
should be engines enough provided for 
draining the water; and these should be 
entirely under the direction of the engineer, 
who is to take care that they be so placed 
as not to be an obstacle to the work ; and 
also cause proper trenches to be cut, to 
convey the water clear off from the founda- 
tion. <1. When the sluice is to be built in 
a place where the workmen will be un- 
avoidably incommoded by the waters of the 
sea, &c. all the stones for the mason-work, 
as well as the timbers for that of carpentry, 
should be prepared before-hand; so that 
when a proper season offers for beginning 
the work, there remains nothing to be done, 
but to fix every thing in its place. 5. In 
order to show the state of the work, an 
exact journal should be kept of the mate- 
rials employed, to be signed every week by 
tbe chief engineer and undertaker ; observ- 
ing to distinguish the different pieces of 
materials, and the places where they were 
employed. 6. When an undertaker is 
found, who is not only able to be at the ex- 
pense of providing all the materials, but 
1 likewise vigilant and active to execute 
whatever is judged necessary for the perfec- 
tion of the work, it would be the worst of 
policy to give the preference to others, 
who, through' ignorance, or dishonesty, 
SMA 
bring in estimates lower than it is possible 
to execute the work as it ought. However, 
that the conditions of the contract may be 
properly executed, the chief engineer, or 
other persons of unquestionable understand- 
ing and honesty, commissioned for that pur- 
pose, should take care that able workmen 
be employed, and that they execute their 
several parts in a proper manner. 
Sluices ai’e made different ways, accord- 
ing to the uses they are intended for : when 
they serve for navigation, they are shut 
with two gates, presenting an angle towards 
the stream ; but when nsade near the sea, 
there are two pair of gates, one to keep 
the water out, and the other to keep it in, 
as occasion requires: the pair of gates next 
the sea present an angle that way, and the 
other pair the contrary way; the space.in- 
closed by these gates is called a chamber. 
AVhen sluices are designed to detain the 
water in some parts of the ditch of a for- 
tress, they are made with shutters to slide 
up and down in grooves; and when they 
are made to cause an inimdation, they are 
then shut by means of square timbers let 
down into cullises, so as to lie close and 
firm. Particular care must be taken, in 
the building of a sluice, to lay the founda- 
tion in the securest manner possible; to 
lay tlie timber-grates and floors in such a 
manner, that the water cannot penetrate 
through any part, otherwise it will under- 
mine tlie work ; and, lastly, to make the 
grates of a proper strength, in order to sup- 
port the pressure of the water ; and yet to 
use no more timber than is necessary. 
SMACK, a small vessel with but one 
mast. Sometimes they are employed as 
tenders on a man of war, and are used for 
fishing upon the coast, &c. 
SMALT, a kind of glass of a dark-blue 
colour, which, when levigated, appears of a 
most beautiful colour ; and if it could be 
made sufficiently fine, would be an excel- 
lent succedaneum for ultramarine, as not 
only resisting all kinds of weather, but even 
tlie most violent fires. It is prepared by 
melting one part of oxide of cobalt with 
two of flint-powder, and one of potass. At 
the bottoms of the crucibles in which the 
smalt is manufactured, we generally find a 
regulus of a whitish colour inclining to red, • 
and extremely brittle. This is melted 
afresh, and, when cold, separates into two 
parts ; that at the bottom is the cobaltic 
regulus, which is employed to make more 
of the smalt ; the other is bismuth. 
SMARAGDITE, in mineralogy. This 
