112 Experiments on boiling Tar. [Frs. 
carried in air, or repelled the tar for a short time, as common 
liquids are repelled from surfaces covered with certain dusts. 
I was lately taken by a friend through the King’s Dockyard at 
Chatham, and fortunately saw the operation of tarring ropes going 
on. ‘The cauldron or cistern in which the tar is heated is, I think, 
about six feet long and four feet wide, and 24 feet deep, of an oval 
form. The rope-strands enter at one end, and pass out at the other 
end, having been in their passage pressed deep under the surface. 
The fire is underneath, and the whole appeared in a state of ebul- 
lition. 
I asked the men if they had ever seen any one dip his hand into 
tar in that state. One of them immediately drew up his coat sleeve, 
and dipped his hand and wrist in, bringing out fluid tar, and pour- 
ing it off from his hand as from a ladle. The tar remained in 
complete contact with his skin, and he wiped it off with tow. 
Satisfied that there was no deception, and seeing no room for the 
exercise of any dexterity by which the heat could be avoided, 1 
asked him whether there was any danger in my trying it myself. 
Being assured there was not, 1 dipped in the entire length of my 
fore finger, and moved it about a short time before the heat became 
inconvenient. I began to think that the tar was only kept to a thin 
liquid state by a heat much below the boiling point. I asked 
whether they knew what the heat was. They answered that their 
thermometer was broken, and a new one expected; but that their 
orders were to keep it between 210° and 220°, and as near as they 
could to the lower of the two points. 1 procured a bottle (a green 
wine-pint), and putting a small quantity of water in it, suspended 
it in the tar. I waited till I was tired of watching for the boiling of 
the water, but cannot say how long. On pouring some out, it was 
barely hot. I then concluded that the heat of the tar was much 
below that of boiling water, and that the graduation of their ther- 
mometer was different from that of Fahrenheit, and attributed the 
apparent ebullition to the escape of air from the heated rope- 
strands. 
A few days afterwards, however, I had an opportunity of seeing 
the operation again, and a new thermometer was obtained, gra- 
duated according to Fahrenheit, marked at 212° “ water boils,” 
and a space left to mark “ tar boils,” against the degree to be found 
on trial. The apparatus for the thermometer consisted of a copper 
tube, of about 2+ or 3 inches diameter, closed at bottom. It was 
fixed upright near the middle of the cauldron, plunged two feet 
into it, and projecting nearly two feet out of it. The thermometer 
was suspended low in the copper tube, and a thin cap was put on 
the upper end. Nearly a quarter of an hour passed before the 
thermometer ceased to rise (it having been frequently examined), 
and it then stood at 180°, and could not be raised higher. I saw 
the tar evidently boil round the edges of the cauldron. A phial of 
water which I had left for some time suspended in it seemed to boil ; 
but as the tar and scum had got into it, I could not easily ascertain 
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