UNIVERSITY OF CALIFORNIA.
THEIR THEOEY AND DESIGN
THEIR THEORY AND DESIGN
DE B. PAKSONS, B.S., M.E.
Member American Society Mechanical Engineers ; Member American
Society Civil Engineers ; Member Soc. Naval Arch, and Marine
Engineers ; and Professor of Steam Engineering,
Rensselaer Polytechnic Institute.
LONGMANS, GREEN,. AND CO
91 AND 93 FIFTH AVENUE, NEW YORK
LONDON, BOMBAY, AND CALCUTTA
LONGMANS, GREEN AND CO.
LONGMANS, GREEN AND CO.
All rights reserved.
First Edition, December, 190:>. Second Edition, January, 1905.
Third Edition. Revised, October, 1907.
ROBERT DR^yMONO COMPANS", PRINTERS. NEW YORK.
IS DEDICATED TO
IN presenting this book to the Engineering Profession and to
fellow students in practical science, the Author desires to state that
no claim is made for originality. In fact it would be nearly impos-
sible, if not quite so, to write a work on this subject which could
be considered original.
These pages comprise, in book form, a series of lectures delivered
to the Senior Class of the Rensselaer Polytechnic Institute, Troy,
New York, rewritten and divided into chapters. The only original-
ity claimed for the work is the effort to cover such points as in
practical office work may be found to be perplexing.
No one should attempt to design a steam-boiler until he has had
some experience in, or personal acquaintance with, boiler-shop prac-
tice. There are many things in the actual putting together of the
parts of a boiler which cannot be clearly described, and for just such
things even a short shop experience would be most valuable.
The Author acknowledges obligations for the free use which he
has made of literature on the subject; and, while many references
are mentioned by name, he now expresses his thanks to those to
whom special reference has not been made.
H. DE B. PARSONS.
NOTE ON THE SECOND EDITION
IN preparing this book for the second edition, the Author has
made some changes to which he desires to call special attention.
The frontispiece which has been added illustrates the dissipa-
tion of the heat produced by the combustion of a fuel on a grate.
The upper part of the cut shows a boiler; engine, condenser, hot-
well, feed-pump, and economizer, all connected by piping. The
lower part shows the heat flowing in streams, drawn to scale,
under an assumed set of conditions. Where the heat is rejected
from such a cycle, so as not to be recoverable, the streams appear
as if running off into space. In the cut, the heat at the grate
corresponds to the total heat of combustion of one pound of an
assumed coal. By a combination of an engine and a boiler, it is
not possible to transform all of this heat into useful work, as is
proven by the study of thermodynamics. From any boiler or
engine trial, when sufficient data have been obtained, a similar
diagram can be constructed to illustrate the conditions so found.
Fig. 8 illustrates one of the latest arrangements of a locomotive
fire-box for burning liquid fuel. The arrangement is designed to
provide a thorough mixture of the oil and air, and allow time for
the combustion to take place before the products are drawn into
the small tubes and chilled below the temperature of ignition.
In the text, under Liquid Fuels, some changes have been made
and attention called to the valuable report of the Liquid Fuel
Board of the United States Navy Department, 1904.
Fig. 20 illustrates one of the most approved English methods
for the brick setting of a Lancashire boiler.
The general plan of the book remains unchanged.
H. DE B. PARSONS.
PHYSICAL PROPERTIES 1
Solid Bodies. Fluid Bodies. Liquid Bodies. Gaseous Bodies.
Perfect Gas. Laws of Gases. Heat. Conduction. Convection.
Radiation. Mechanical Equivalent of Heat. Absolute Zero.
Specific Heat. Latent Heat. Total Heat of Evaporation. Weight
of Water. Boiling. Relative and Specific Volumes of Steam.
Factor of Evaporation.
General Conditions. Definition. Smoke. Coal-gas. Marsh
Gas. Olefiant Gas. Air. Temperatures of Ignition. Laws of
Avogadro. Requirements for Perfect Combustion. Products of
Combustion. Composition of Gases from Combustion. Refuse.
Loss of Unburned Coal in Ash-pit. Quantity of Air Required.
Methods of Firing. Thickness of Fire. Heat of Combustion.
Heating Power of a Fuel.
Coal. Classification. Anthracite. Semi-anthracite. Semi-
bituminous. Dry Bituminous. Bituminous Caking. Long-flam-
ing Bituminous. Lignite. Size of Coal. Culm. Weight of CoaL
Peat or Turf. Wood. Coke and Charcoal. Miscellaneous Fuels.
Sawdust. Straw. Bagasse. Protection from Weather. Chem-
ical Composition of Coals. Liquid Fuels. Gaseous Fuels.
FURNACE TEMPERATURE AND EFFICIENCY OF BOILER 56
The Temperature. Color Test. Rankine's Method for Calculating.
Dissipation of Heat Generated. Percentage of Heat Utilized. Re-
suits. Evaporation per Pound of Fuel and of Combustible. Practi-
BOILERS AND STEAM GENERATORS 62
General Conditions. Classification. Horse-power. Centennial
Standard. Am. Soc. M. E. Standard. Heating Surface. Ratio of
Heating to Grate Surface. Evaportion per Square Foot of Heating
Surface. Design. Description of Certain Boilers. Water-tube
Boilers. Proportioning a Boiler to Perform a Given Duty. Steam
Space. Priming. Water Surface.
CHIMNEY DRAFT 125
Problem of Gravitation. Theory of Peclet as Expressed by
Rankine. Natural Draft. Rate of Combustion. Author's Experi-
ence. Area and Height of Chimney.
Cast Iron. Wrought Iron. Rivet Iron. Charcoal Iron for
Boiler-tubes. Wrought Steel. U. S. Naval Requirements for
Boiler Steel. Steel Rivets. Steel for Boiler Braces. Mild Steel
Affected by Temperature. Cast Steel. Copper. Brass. Bronze.
BOILER DETAILS 154
The Shell. Strength of Shell, Longitudinally and Transversely.
Factor of Safety. Rules for Thickness of Shell. Limits of Thick-
ness. Arrangements of Plates. The Ends. Rules for Thickness
of Heads. Flat Surfaces. Rules for Flat Surfaces. Flues.
Strengthening Rings. Corrugated and Ribbed Flues. Rules for
Flues and Liners. Tubes. Rules for Thickness. Stays. Rules
for Stays. Girders. Combustion-chambers. Riveting. Welding.
Setting. Bridge-wall. Split Bridge.
BOILER FITTINGS 231
Mountings and Gaskets. Steam-dome. Steam-drum. Steam-
superheater. Steam-chimney. Steam-pipe. Stop-valve. Dry
Pipe. Boiler-feed. Injectors and Pumps. Feed- water Heaters,
Purifiers, and Economizers. Filters. Mud-drums. Blow-off, Bot-
torn Blow and Surface Blow. Safety-valve. Fusible Plug. Steam-
gauge. Water-gauge. Try-cocks. Water-alarm. Manhole and
Handhole. Grates, Stationary and Shaking. Down-draft Grate.
Ash-pit. Fire-doors. Breeching. Uptake. Smoke-connection.
Draft-regulators. Steam-traps. Separators. Evaporators.
MECHANICAL STOKERS 295
Classes, Overfeed and Underfeed. Advantages. Disadvantages.
Results Obtained by Use.
ARTIFICIAL DRAFT 300
Advantages. Disadvantages. Classification. Selection De-
pends on Local Conditions. Boiler Must be Suited to Draft.
Vacuum and Plenum Systems Compared. Economy. Intensity.
Jet in the Stack. Jet under the Grate. Fans. Power Required.
Closed Ash-pit. Closed Fire-room. Induced Draft.
Scurf. Fur. Sludge. Scale. Conductivity. Solid Matter in
Water. Analysis of Scales. Behavior of Lime and Magnesium Salts.
Scale Prevention. Blowing-off. Chemical Agents. Mechanical
Agents. Galvanic Agents. Surface-condensing. Heating and
Filtering. Internal Collecting Apparatus. Manual Labor.
CORROSION. GENERAL WEAR AND TEAR. EXPLOSIONS 319
Corrosion. Wasting. Pitting and Honey-combing. Grooving.
Influence of Air and Acidity. Galvanic Action. Zinc Plates. Ex-
ternal Corrosion. Dampness. Wear and Tear. Idle Boilers. Ex-
plosions. Stored Energy.
CHIMNEY DESIGN 327
Object. Selection of Height. Compare Cost of Stack with Me-
chanical Draft. Individual Short Stacks in Lieu of One Large
Stack. Self-supporting and Non-self-supporting Stacks. Wind-
pressure. Batter. Brick Stacks. Section. Lining. Top. Light-
ning. Ladder. Leakage. Steel Stacks.
SMOKE PREVENTION 336
Losses Due to Smoke. Public Nuisance. Smoke Ordinances.
Requirements to Prevent Smoke. Prof. Ringelmann's Smoke Scales.
Smokeless Fuels. Composition of Smoke. Mixing Coals. Air
Admissions. Hollow Bridge. Extracts from Report by Prof.
TESTING. BOILER COVERINGS. CARE OF BOILERS 345
Object of Testing New Boilers. Hydraulic Pressure. Methods
Adopted. Measuring for Changes of Form. Limit of Test Pressure.
Testing by Steam for Leaks. Boiler Trials. Directions for Calcu-
lating some Results. Boiler and Pipe Coverings. Heat Losses.
Savings. Care of Boilers.
SUPERHEATED STEAM . 361
LIST OP ILLUSTRATIONS
THERMAL-EFFICIENCY DIAGRAM Frontispiece
THE ABSOLUTE ZERO 5
COMBUSTION ON GRATE 18
BAGASSE FURNACE STILLMAN TYPE " 38
BAGASSE FURNACE 39
ROCKWELL FUEL-OIL BURNER, operated by steam reduced to between
40 and 80 pounds 44
LASSOE-LOVEKIN FUEL-OIL BURNER (Patented), operated by air at 1^
pounds pressure 44
STEAM SPRAY ATOMISER FOR FUEL-OIL HOLDEN SYSTEM 46
OIL-BURNING LOCOMOTIVE. Heintzelman and Camp arrangement 47
END OF A PLAIN CYLINDRICAL BOILER 71
HORIZONTAL RETURN-TUBULAR BOILER, with extended or half-arch front 72
HORIZONTAL RETURN-TUBULAR BOILER. Front and section 73
HORIZONTAL RETURN-TUBULAR BOILER, with flush or full front 74
HORIZONTAL RETURN-TUBULAR BOILER. Front and section 75
HORIZONTAL RETURN-TUBULAR BOILER, with a link suspension 76
HORIZONTAL RETURN-TUBULAR BOILER. Section 77
UPRIGHT OR VERTICAL BOILER 81
UPRIGHT OR VERTICAL BOILER, with submerged tube-sheet 82
MANNING VERTICAL BOILER 84
MANNING VERTICAL BOILER. Sections 85
FLUE AND RETURN-TUBULAR BOILER 86
FLUE AND RETURN-TUBULAR BOILER. Front view and section 87
FLUE AND RETURN-TUBULAR BOILER. Back view 88
CORNISH BOILER 89
CORNISH BOILER. Front view and section 90
LANCASHIRE BOILER 91
LANCASHIRE BOILER. Front view and section 92
GALLOWAY BOILER, with expansion rocker support 93
SECTIONS OF GALLOWAY BOILER 94
LANCASHIRE BOILER, with Galloway tubes 95
LANCASHIRE BOILER. Sections 96
SCOTCH BOILER, single-ended, with common combustion-chamber 97
xiv LIST OF ILLUSTRATIONS
SCOTCH BOILER. End view and section 98
SCOTCH BOILER, single-ended, with separate combustion-chambers 99
SCOTCH BOILER. End view and section 100
DOUBLE-ENDED SCOTCH BOILER 101
ADMIRALTY OR GUNBOAT BOILER 102
ADMIRALTY OR GUNBOAT BOILER. Sections 103
MARINE BOILER. Front end and section 103
MARINE BOILER WITH STEAM-DRUM 104
SCOTCH BOILER WITH STEAM-DRUM 105
SCOTCH BOILER. End view and section 106
MARINE BOILER WITH STEAM-DRUM. 107
LOCOMOTIVE BOILER 108
LOCOMOTIVE BOILER. Sections 109
BABCOCK AND WILCOX BOILER 112
BABCOCK and WILCOX BOILER. Front view and section 114
STIRLING BOILER 115
ALMY BOILER 116
NICLAUSSE BOILER 117
BELLEVILLE BOILER 118
THORNYCROFT BOILER 119
THORNYCROFT BOILER. Front view and section 120
YARROW BOILER 121
STRENGTH OF SHELL TO RESIST BURSTING PRESSURE 155
FLUE STRENGTHENING BY ANGLE RING 172
FLUE STRENGTHENING BY TEE RING 173
FLUE STRENGTHENING BY TEE RING JOINT 173
FLUE STRENGTHENING BY FLANGING. (Two styles.) 173
FLUE STRENGTHENING BY THE BOWLING HOOP 174
FLUE STRENGTHENING BY THE ADAMSON RING 174
FLUE STRENGTHENING BY GALLOWAY TUBES. (Two views.) 174
Fox's CORRUGATED FURNACE FLUE 175
MORISON'S SUSPENSION FURNACE FLUE 175
PURVES' RIBBED FURNACE FLUE 176
PROSSER'S TUBE EXPANDER 190
DUDGEON'S TUBE EXPANDER 190
EFFECT OF EXPANDING THE TUBE ENDS 190
FERRULES FOR TUBE ENDS 191
RETARDER FOR TUBES 192
ACME REFRACTORY CLAY RETARDER FOR USE WITH FUEL OILS 193
SERVE TUBE 194
SCREW STAY, ends upset and riveted 195
SCREW STAY, ends upset and fitted with nuts 195
SCREW STAY, ends not upset, fitted with nuts and washers 195
STAY FITTED WITH FERRULE. 195
LARGE STAY END WITH NUT AND WASHER 196
LIST OF ILLUSTRATIONS XV
LARGE STAY END WITH DOUBLE NUTS 196
STAY END WITH BOLT IN DOUBLE SHEAR 197
NUT FOR STAYS, showing packing groove 197
STAY END WITH BOLT IN DOUBLE SHEAR. (Two views.) 197, 198
A METHOD OF FAILURE 198
GUSSET PLATE STAY 199
STAY END FITTED TO STIRRUP TO DISTRIBUTE THE SUPPORT 199
STAY END SPLIT TO FORM STIRRUP TO DISTRIBUTE THE SUPPORT 200
DIAGONAL STAY, with rivets through palm in tandem and in parallel. . . . 200
HUSTON FORM OF STAY WITHOUT WELD 201
GIRDER STAY FOR SUPPORTING CROWN-SHEET 201
GIRDER STAY FOR SUPPORTING CROWN-SHEET, showing three forms of
strengthening stays to shell 202
SINGLE RIVETED LAPPED JOINT 208
DOUBLE RIVETED LAPPED JOINT 209
SINGLE RIVETED LAPPED AND STRAPPED JOINT 210
SINGLE RIVETED BUTT JOINT, with single or double straps 211
TREBLE RIVETED BUTT JOINT, with straps of unequal widths 212
JOINT BETWEEN TUBE SHEET AND FURNACE FLUE, showing countersunk
rivet where exposed to the fire 213
GOOD AND BAD CALKING 215
EFFECT OF INDIRECT PULL ON A LAPPED JOINT 215
EFFECT OF INDIRECT PULL ON A SINGLE STRAPPED JOINT 215
CRACKS IN LAPPED JOINT DUE TO BENDING 216
RIVETS IN PUNCHED HOLES 217
BUTT-STRAP ON A STAYED SHEET 224
DOUBLE-RIVETED BUTT-STRAP OF UNEQUAL WIDTH ON A STAYED SHEET. 225
DESIGN FOR A TRIPLE-RIVETED BUTT-STRAP 226
STEAM-DRUM, SINGLE NOZZLE 233
STEAM-DRUM, DOUBLE NOZZLE 234
STEAM-DRUM, PIPE CONNECTION 234
STANDARD NOZZLES OF CAST-IRON OR CAST-STEEL 235
ANGLE BRACES TO SUPPORT STOP-VALVE 236
REINFORCING STEAM-PIPES 240
SLIP-JOINT WITH STUFFING-BOX FOR STEAM-PIPE 243
PLAIN FLANGE FOR COPPER PIPE 244
COLLAR FLANGE FOR COPPER PIPE 244
PLAIN FLANGE WITH SLEEVE FOR COPPER PIPE 245
COLLAR FLANGE WITH EDGE OF COPPER PIPE TURNED OVER 245
FLANGE FOR IRON OF STEEL PIPE 245
FLANGES WITH TONGUE AND GROOVE 246
FLANGES CUT AWAY TO FACILITATE CALKING EDGES OF PIPE 246
FLANGES WITH RECESS AND PROJECTION 247
FLANGES WITH FACES GROUND TO FIT 247
FEED-PIPE ENTRANCE WITH DISTRIBUTING END. . . . 253
xvi LIST OF ILLUSTRATIONS
FEED-PIPE ENTRANCE. (Two views.) 254
THE METROPOLITAN INJECTOR 255
THE GOUBERT FEED-WATER HEATER CLOSED TYPE 259
HOPPES' COMBINED FEED-WATER HEATER AND PURIFIER 261
COCHRAN FEED-WATER HEATER OPEN TYPE 262
GREEN ECONOMIZER 263
HOT-WELL FILTER-BOX 264
HOT-WELL FILTER-BOX DETAILS 265
EDMISTON TYPE OF FEED-WATER FILTER 266
RANKINE'S PATENT FEED-WATER FILTER 266
SURFACE BLOW-OFF 268
DEAD-WEIGHT SAFETY-VALVE, COBURN TYPE 269
VARIOUS FORMS OF FUSIBLE PLUGS 272
WATER-GAUGE AND TRY-COCK COLUMN 274
GLASS PROTECTOR GUARDS FOR GLASS TUBE OF WATER-COLUMN 275
COMBINED HIGH- AND LOW-WATER ALARM AND WATER-COLUMN 276
MANHOLE AND COVER FOR A FLAT SHEET 278
MANHOLE AND COVER FOR A CYLINDRICAL SHELL 278
SPLIT BRIDGE WITH PASSAGE TO ADMIT AIR. (Two views.) 279
CAST-IRON GRATE-BAR 280
GRATE BEARER FOR CORRUGATED FURNACE 282
PLAN OF HERRING-BONE GRATE-BAR 282
SECTION OF ASHCROFT GRATE-BARS 283
GRATE FOR BURNING SAWDUST OR TAN-BARK 284
DOWN-DRAFT GRATE 284
FIRE-DOOR FOR FURNACE FLUE 287
MORISON FURNACE-DOOR AND FURNACE-FRONT 288
KIELEY DISCHARGE TRAP 289
BUNDY RETURN TRAP 290
"POTTER" MESH SEPARATOR LONGITUDINAL SECTION 291
"POTTER " MESH SEPARATOR CROSS-SECTION 291
STRATTON STEAM-SEPARATOR 292
SALT-WATER EVAPORATOR 293
THE RONEY MECHANICAL STOKER 295
THE AMERICAN MECHANICAL STOKER 296
THE MURPHY MECHANICAL STOKER 297
THE MURPHY MECHANICAL STOKER. Section 298
BLOOMSBURG JET IN STACK 302
RING JET IN STACK 303
BEGGS' ARGAND STEAM-BLOWER 304
BEGGS' ARGAND STEAM-BLOWER. Section 305
BEGGS' ARGAND BLOWER, arranged for a furnace-flue 306
INDUCED DRAFT ELLIS AND EAVES' SYSTEM 308
BRICK STACK. . . 331
LlST OF ILLUSTRATIONS xvil
LADDER FOR BRICK STACK 332
BRICK STACK, cast-iron cap for 333
SELF-SUPPORTING STEEL STACK 334
PROF. RINGELMANN'S SMOKE-SCALES 339
SUPERHEATER ATTACHED TO A BABCOCK AND WILCOX BOILER 361
SUPERHEATER ATTACHED TO A BABCOCK AND WILCOX BOILER. Section. 362
FOSTER'S SUPERHEATER ATTACHED TO A FIRE-TUBE BOILER 362
FOSTER'S DETAIL OF RETURN HEADER 363
FOSTER'S SUPERHEATER DIRECT-FIRED TYPE 364
FOSTER'S SUPERHEATER SECTIONS . . .. . 365
Solid Bodies. Fluid Bodies. Liquid Bodies. Gaseous Bodies. Perfect
Gas. Laws of Gases. Heat. Conduction. Convection. Radiation. Me-
chanical Equivalent of Heat. Absolute Zero. Specific Heat. Latent Heat.
Total Heat of Evaporation. Weight of Water. Boiling. Relative and
Specific Volumes of Steam. Factor of Evaporation.
THERE are two principal states in which all bodies are found,
namely, "Solids" and " Fluids." Fluids may again be divided
into " Liquids" and " Gases."
Solid Bodies may be defined as those which will resist a longi-
tudinal pressure, no matter how small that pressure may be, with-
out being supported by a lateral pressure.
Fluid Bodies may be defined as those which will not resist such
a longitudinal pressure.
Liquid Bodies may be defined as those which will only partly
fill a closed vessel, while the rest of the vessel may be either empty
or contain some other fluid.
Gaseous Bodies may be defined as those which will expand and
completely fill a closed vessel, no matter how small a portion may
be introduced. Gases are thus distinguished by their power of
A Perfect Gas may be described as one which obeys exactly the
laws of Mariotte and of Gay-Lussac. Such a perfect gas is now
known to be ideal, and the so-called permanent gases only approx-
imate in their action to these laws in accordance with their degree
Laws of Gases. First Law (Mariotte or Boyle): "At constant
temperature, the volume of a portion of gas varies inversely as the
pressure." That is, pv = constant.
Second Law (Gay-Lussac, Charles, or Dalton): "At constant
pressure, the volume of a portion of gas varies directly as the abso-
lute temperature." That is, v = const ant X r -
Heat. There are many words, such as "hot," "warm," "tepid,"
"cool," "cold," which are used to denote different sensations that
indicate a corresponding condition of the object with respect to
the heat which it is said to contain. These conditions or series of
states are called "Temperatures," and from the facts as found in
nature it must be admitted that there exists an infinite number
of these intermediate states or temperatures.
The temperature of a body, therefore, indicates how hot the
substance is. These temperatures are accompanied in each body
by certain conditions as to the relations between density and elas-
ticity. In general, the hotter the body, the less is its elasticity of
figure and the greater is its elasticity of volume.
Heat may be considered as a "mode of motion," and is gener-
ally recognized to be a vibratory motion of the particles composing
Heat is transferable from one body to another, that is, one body
can heat another by becoming less hot itself. This transfer of heat
between two bodies tends to bring them to a state called "uniform"
or "equal" temperature. At uniform temperature this transfer
of heat ceases.
Heat is transferred from a warmer body to a colder body by
one of three processes, namely, "Conduction," "Convection" and
Conduction is the transference of heat between two contiguous
portions of matter at different temperatures. Convection is the
distribution of heat by a movement of a portion of a fluid within
its own mass. Such a movement is called a convection current.
Radiation is the transference of heat from one body to another at
a distance, through an intervening transparent medium.
Heat is one of the forms of energy, since it may be transformed
into mechanical work.
As the condition of heat is a condition of energy, and is capable
of effecting changes, it may be indirectly measured, so as to be
PHYSICAL PROPERTIES 3
expressed as a quantity by means of one or more of the directly
measurable effects which it produces.
When the condition of heat is thus expressed as a quantity, it
is subject, like all other forms of energy, to a law of conservation.
Since the properties of all substances vary with their tempera-
tures, it has become customary to make use of two of these varia-
tions to indicate particular temperatures as points of reference.
These two variations were selected because they were abrupt and
well defined, and are :
First. The temperature at which ice melts under one atmos-
phere of pressure, equivalent to 14.7 pounds per square inch or u
barometric height of 29.95 inches. As this temperature varies but
slightly with changes of pressure, it can be easily reproduced under
Second. The temperature of steam generated from water when
boiled under one atmosphere of pressure.
Occasional use is made of other changes of state, which take
place at temperatures more or less well defined, such as the melting-
points of certain metals and alloys.
Ordinary temperatures are recorded from the reading of a mer-
curial thermometer.* For higher temperatures use is made of the
air-thermometer or some form of pyrometer.
Heat and work are mutually convertible in a fixed ratio, known
as the " Mechanical Equivalent of Heat." The relationship exist-
ing between heat and work, was demonstrated by various experi-
ments, the most noted being those of Benjamin Thompson, better
known as Count Rumford (1753-1814), Sir Humphry Davy (1778-
1829), Sadi Carnot (1796-1832) and Henry A. Rowland (1848-
1901). In 1842, Dr. Mayer, of Heilbronn, is said to have first intro-
duced the expression " Mechanical Equivalent of Heat," and in
the year following Dr. Joule, of Manchester, measured this equiva-
lent. The value placed by Joule was 772 foot-pounds of work as
equivalent to one British thermal unit. This result is still in use,
* In order accurately to read a mercurial thermometer, when the scale
is not on the same plane with the column of mercury, it will be found con-
venient to hold a small looking-glass behind the column of mercury; then,
when the eye is so reflected that the centre of the pupil is coincident with
the top of the mercury, the eye will be at right angles to the mercury and
also to the scale.
although later experiments show that 778 foot-pounds is nearer
the true figure. This latter result will be used in this work except
where otherwise stated.
The British thermal unit is the quantity of heat required to
raise one pound of pure water at its maximum density one degree
Fahrenheit. The temperature of water at maximum density is
very nearly 39.2 F.
The Absolute Zero. In order to simplify calculations with re-
spect to the action of perfect gases, all the formulae are based on
a scale of absolute temperatures. These absolute temperatures
express the heat of a body on a scale beginning at a point known
as the "absolute zero."
The absolute zero is a theoretical point on this temperature
scale that is fixed by assuming that the law of gases as deter-
mined by experiment remains constant throughout the whole
range of temperatures.
It may be said to be the temperature point corresponcjing to
the disappearance of gaseous elasticity, or the point at which the
expression pv for a perfect gas becomes zero.
When a portion of dry air is heated from the freezing-point of
water (32 F.) to the boiling-point (212 F.), that is, its tempera-
ture has been raised through 180 F., it will expand to 1.365 of its
original volume. Therefore, when the gas has been heated through
493.2, it will expand to twice its original volume. If, therefore,
the same law holds true for cooling, and the temperature be lowered
493.2 from the freezing-point (32), the volume of the gas will be
reduced to zero. But the law of expansion and contraction of the
so-called permanent gases varies appreciably, so that there is a
slight difference in the position of the absolute zero according to
the gas under experiment.