708. 
renewed, and the lixiviations continued. 
The number of thefe lixiviations, as well 
as the mode of making them, may be 
waried at pleafure: the eifential point is 
“to repeat them fo as to give the liquor 
a fuficient degree of concentration, 
which may be determined by. the hy- 
drometer, and proportioned to the quick- 
nefs required in the operation, and to 
the thickne!s of the hides and fkins to 
be -tanned, all which experience will 
foon teach. As all kinds of tan are not 
egually good, it will fometimes happen 
that fix or more filtrations will be necef 
fary, to obtain a lixivium of fix or eight 
degrees; in this cafe, the number of 
digeftors may be increafed, and the fame 
method purfucd as above, and when a 
weaker /vivium is wanted, three-or four. 
filtrations will be fufficient. 
‘The perfon who direéts thefe lixivi- 
ations fhould be provided with the folu- 
tion of glue and fulphat of iron already 
defcribed, in order ‘to afcertain the qua- 
lities of the different Aaivia as well as 
with. an hydrometer, or areometer, pro- 
_perly graduated, to determine their de- 
gree of concéntration or {pecific gra- 
Sy ity? 
Befides the very great favings in point 
of time and labour, the leather tanned 
according to the above method being 
more completely faturated, will be found 
to weigh heavier, to wear better, and to 
be lefts fufceptible of moifture than the 
leather tanned in the ufual way. The 
thickeft hides may be tanned in about 
fourteen days, and a boar’s fhicld has 
been completely tanned in about, three 
weeks, that, according to the common 
method, would require fix or feven 
years. ‘The faving, moreover, in other 
Felpects, ‘is at leaft 120 per cent. 
The other animal and vegetable fub- 
ftances already mentioned, by being 
fieeped for a certain time in a weaker 
or ftronger tanning lixivium, will acquire 
firength and incorruptibility. Cords, 
ropes, and cables made of hemp or fpar- 
tery, impregnated with this. principle, 
willfupport much greater weights with- 
out breaking, will be lefs liable to be 
worn’ out by friétion, will run more 
finoothly on’ pullies, &c. ‘This liquor 
in fhort will be found fo advantageous, 
particularly in the rigging of veffels, as 
to render the ufe otf tar in many cafes 
unneceffary, Even meat may be pre- 
~terved by it without falt. ee 
Inutanp NavicaTion. 
On the 1oth of Auguft, the {pecifica- 
tion of a patent was enrolled, at the 
 Montury Mac. No, IX, 
New Patents of Mr. Potts and Mr. Pratt. 
28 
Petty Bag Office, by Mr. THomas 
Ports, of Sanétuary, in the parifh of 
Penrice, in the county of Glamorgan, 
for his invention of a machine for the 
moving of veffels, boats and barges on 
canals, and other ftill waters. ‘“[he ma- 
chine confifts of a vertical oar, which is 
made to a€tat the ftern of the veilel 
that is required to be moved forward. 
The oar is put in motion by means of a 
pole,‘to one end of which it is faftened, 
which pole is fufpended above the deck 
by a rope or chain, on which it fwings. 
‘The pole is kept fieady and in its proper 
direétion, by lateral beams that project 
as far over the ftern of the veffel. as 
the oar Js intended to move backward 
and forward, "Fhe vertical oar; or 
power, is readily pufhed backward, and 
drawn forward by means of the {wing- 
ing pole, on elevating the end of the pole 
when the oar is to be pufhed back.and on 
pulling the end down when the oar Is to 
be drawn’ forwards. In thefe alternate 
movements confift the practice of the 
invention, ‘Fhe broad furface of the 
Oar, varying in its fize, according to the . 
depth of water and weight of the veiiel, 
is not expreffed in the {pecification. 
ComposiTion MILLsToNEs. 
Mr. Major Pratt, lime-burner, 
of Running Waters, in the parith of 
Pittington Halgarth, in the county of 
Durham, obtained a patent, on the rrth 
of March, which was enrolled on, the 
6th of September, for a method of manu- 
faéturing a compofition ftone, that will 
anfwer the purpofe of grinding every 
{fpecies of corn, and all the other pur- 
-pofes to which foreign and other mill- 
ffones are, or may be applied. The 
principle of his invention confifts in a 
due mixture of filideous and argillaceous 
earths, under certain circumftances, and 
converting the fame into fone by the 
application of heat. To produce the 
femi-vitrification neceffary to the hard- 
nefs of the ftone, an addition is made to 
the mixture of about one feventh of 
calcareous earth, for which he found lime 
to anfwer well; but various other fub- 
ftances he conceives may accomplifh the 
fame end, fuch as gypfum, alkaline falts, 
coal, iron, &c. The “heat requifite 
fhould vary according to circumftances, 
but the degree found to anfwer, is the 
fame as that ufed in the calcination of 
lime, fome of the compofition, having 
been prepared in a lime-kiln, during 
the ufual procefs of burning lime. 
Mr. Pratt conceives his compofi- 
tion, by being burnt in moulds of any 
4z —\ particulag 
J 
