378 
Expansion of Metals by Heat. 
[Dec. 
bore, 24 feet 11 inches long ; it rests horizontally upon a stout plank, supported 
on tripods. Into this pipe the brass rod is inserted, its two extremities pro- 
truding through corks fitted into the pipe, with a freedom of motion so as to pre- 
vent any flexure within the pipe ; indeed when one end of the rod was pressed in 
with the hand, the protrusion at the other extremity was found by trial to agree 
precisely in amount. 
C, D, and E, are three thermometers inserted through corks into the centre 
of the pipe ; they were previously compared together, and corrected by a standard 
thermometer. 
F is a pipe and cock for the admission of a stream of cold water from a large 
cistern. 
G in like manner admits a current of steam from the steam-pipe of a small 
engine provided with a boiler in an adjacent apartment. There is an ejection pipe 
at the other end of the tube near E at which the water and steam pass off into a 
cistern below, the table. 
The length of the rod, or rather the increase of its length, was measured by means 
of the contact of two small wires, M, interposed between the ends of the rod and 
the verniers of two sliding scales H and K, borrowed for the occasion from two of 
Wollaston’s thermometric barometers. This arrangement is shewn on a larger scale 
in fig. 3. A different form of wire N was necessary to measure the expansion of 
the leaden pipe by contact with it under the cork at B. 
The experiments were conducted in the following order : 
1. A stream of water was allowed to pass through the pipe for an hour, or until 
the temperature was perfectly equable throughout : the readings of the two 
micrometers were then taken, both for the brass rod and for the leaden pipe. 
2. Steam was next admitted, which quickly displaced the water and raised the 
temperature to the boiling point : the heat of the steam as it passed through the 
steam-pipe of the engine remained steadily at 218°, but fi'om the contraction of the 
stop-cock passage G, and the free escape of the steam at both ends of the leaden 
pipe, the temperature in the latter never rose above the boiling point : it was even 
remarked, that the thermometer at C generally stood a degree below the boiling 
point, while that at E, farthest from the source of heat, t stood as much above it ; 
the central thermometer at D remaining constantly at 212°. These two opposite 
effects are easily explained on the same principle : — the first arises from the expan- 
sion of the steam on entering the larger pipe after passing through the small con- 
necting pipe G ; the other is caused by the afflux and increased tension of t ie 
steam at B, necessary to drive it through the ejection pipe with the observed velocity. 
As soon as the temperatures were deemed steady, the micrometers were again 
read off ; after which the steam-pipe was closed, and the cold water again 
admitted. 
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It was generally observed, that the contractions measured fell somewhat shoi 
the expansions, unless a considerable time was allowed for the cooling down o 
apparatus, even after the thermometers had returned to the prior temperature. 
The verniers read off to the two hundredth part of an inch, and by estimation 
the 2000th, which would be sufficient to ensure accuracy to the sixth place of deci 
mals, were it not evident from the table of results that the errors of observation an 
those arising from other causes were considerably in excess of this limit. 
As it is usual to express the linear dilatation in the “ dimensions which a ar 
takes at 212°, whose length at 32° is 1,000000,” 1 have reduced all the experinien a 
results to these terms. ^j e( j 
In addition to the measures of the two metals already mentioned, I w as ena 
to try the expansion of wrought iron upon a half- inch wire-drawn lod o t\r 
