Copyright
Richard Green Parker.

A school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of online

. (page 35 of 38)
Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 35 of 38)
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Buoyancy? upward when immersed by force equal to
the difference between the weight of the body and the
weight of an equal bulk of water ; this force is called the
buoyancy of the body.
What is the 1436. The specific gravity of such a body

^{%ofK$t is found by dividing the weight f the body

solids ? . by its weight added to its buoyancy.

The buoyancy is found by determining how much
weight is required to sink the body. (See page 127.)

Define 1437. Hydraulics treats of the laws gov-

Hydraulws. erning liquids in motion, and of the useful
application of these laws in the employment of water as a
motive power, the water supply and drainage of cities, and
the improvement of rivers and harbors.

How is water em- !438. Water is employed as a motive
ployed for power* power k y utilizing the energy of its fall
as it descends the rivers on its way to the sea.



MECHANICS.



419



1439. The water power of running streams is made useful by
selecting a place on the stream where the fall is sufficiently rapid,
and, if necessary, building a dam to secure a vertical fall. From
the store of water thus held back in the stream, an artificial channel
or flume conducts the water to the water-wheel.



Mention the '
different forms
of Water-wheels.



When is an
Overshot Wheel
used f




1440. Water-wheels are of four different
kinds: Overshot, Undershot, Breast and
Turbine. (See page 82.)

1441. The Overshot Wheel is employed
when the stream is small and the fall high.
The flume is continued Fig. 215.

out over the wheel, as represented in
the figure, and the buckets are filled
in succession.

The wheel being overloaded on one
side, turns with a force proportioned
to this extra weight. The whole force
of the falling water can never be util-
ized by this means, even if all the water of the stream falls
into the buckets, for, as may be seen from the figure, a por-
tion of the water falls from the bucket again before it has
reached its lowest point.

Fig. 216.

1442. The Breast
Wheel is adapted to
a larger supply of
water and a lower
fall. The water is re-
ceived at about half
the height of the
wheel in buckets or
upon floats. If the
water comes upon the
buckets above the
centre, it is called a high-breast wheel ; if below, a loiv-
18




420



MECHANICS.



breast wheel. The loss of power occurs here as in the
Overshot Wheel neither of them affording more than
75 per cent, of the real power of the stream.

The Undershot Wheel is a ruder



What is said of

the Undershot kind of motor ; it is furnished with floats,

Wheel? n . . .. ,

or flat projecting surfaces only, against



Fig. 217.




which the running water
impinges, and, by aid of
the momentum acquired in
its previous descent, affords
power to turn the wheel.
Only 20 per cent, of the
power of the water is ren-
dered effective by this kind
of wheel.

What is said of 1444. The Turbine Wheel is by far the
the Turbine? best of all the hydraulic motors. It 13

readily made to utilize
from 80 to 90 per cent, of
the power of the stream.

1445. It is placed hori-
zontally, and is entirely
immersed in the water.



Fig. 218.




Fig. 218 represents a plan
of the wheel. Fig. 219
is an elevation or verti-
cal section of the same
wheel.

The flume conducts
Describe Figs, the water vertically upon the wheel ; the size
218 and 219. O f |h e fl ume \ s equal to the size of the cen-
tral portion of the wheel marked a a a in Fig. 218 and



MECHANICS.



421



G G in Fig. 219. The buckets are shown at vvv. The
spiral lines marked a a a represent curved partitions or
guide-curves fixed in the flume, and form no part of the




wheel ; they serve to give such direction to the descending
current as to enable it to act to the best advantage on the
buckets.

1446. It will be seen by examining Fig. 219 that the water de-
scending the flume D G is prevented from passing directly down-
ward by the lower plate HUH, and has no other outlet than the
space between P and H, where the buckets are placed. The shaft
E E turns with the wheel, and affords the means of communication
with such machinery as the wheel is designed to carry.

The above figures represent the Fourneyron Turbine, so called
from the inventor.

1447. There is another variety known as Centrevent Turbines,
from the fact that the water flows in at the circumference, and
escapes at the centre. Fig. 220 represents one of this class. The
water is conducted from the vertical flume C by a horizontal con-
duit at the bottom, entirely around the wheel. The only escape



422



MECHANICS.



for the water is between the buckets and cut at the central open-
ings marked / at the top and bottom of the wheel.

Fig. 220.




1448. Turbine Wheels are always made of iron, and are so accu-
rately made that no sensible leakage occurs between the fixed and
movable portions (as at G, Fig. 219).

1449. Turbines are much smaller than other water-wheels de-
signed for the same streams. It is not uncommon to replace an
overshot wheel twenty feet in diameter and six feet wide by a tur-
bine only three feet in diameter and six inches deep, and obtain
greater efficiency by the change.

What is the 1450. It should be carefully remembered

U yidded^by e a that no Und of hydraulic motor can afford
stream? any more power than is due to the weight of

the water yielded ly the stream descending through the
height of the fall.

How is the power of a 1451. The theoretical horse-power of
stream calculated? a stream is found by a survey which



MECHANICS.



Fig. 221.



measures the amount of water flowing by a particular
point in one minute, and the amount of fall that can be
made available.

The number of pounds per minute multiplied by the number of
feet fall, and the product divided by 33,000, determines the entire
horse-power developed by the stream ; the percentage of this
amount utilized depends upon the kind of wheel used.

Describe 1452. Barker's Mill, represented in Fig.

Barker's Mitt. ^21, is frequently employed as a motor. If

water be supplied so fast
as to keep the central
tube DC filled, there will
result a hydrostatic pres-
sure within the curved
arms at the bottom.
This pressure is in all
directions ; the arms be-
ing opened so as to dis-
charge the water in one
direction from each arm,
the pressure is relieved
on that side, but remain-
ing on the opposite, it
gives motion to the arms.

1453. It is popularly be-
lieved that Barker's Mill
runs by aid of a push of the
outflowing jet against the
air ; but the fact that it runs
better under an exhausted
receiver than in the open
air, serves to dispel this
impression! Small Barker's
Mills are made in order to
show this experiment. (See
Fig. 98)

are cities sup- 1454. The water supply of cities is ac-
uiih water? complished by providing a reservoir of




424



MECHANICS.



water higher than the dwellings, and from this reservoir
distributing the water through the streets in iron pipes.

In some cities, as Boston and New York, the water comes
from natural sources several miles off, but high enough to
answer the purpose of distribution. In Brooklyn, Philadel-
phia, and Chicago the water is pumped up into reservoirs
which have the required height.

1455. The distribution-pipes are laid about the streets at a suffi-
cient depth to provide against freezing. The sizes for different dis-
tricts are fixed in accordance with the known principles of Hydrau-
lics. The water can, of course, rise no higher in pipes in the houses
than the surface of the water in the reservoir. It very rarely rises
as high, in consequence of the constant flow of water out of the
pipes at lower levels.

The friction of the sides of the pipe and the loss of force by
change of direction at the bends, are serious impediments to the
flow of water through pipes.

Fig. 222.




1456. Fig. 222 represents a reservoir and a foun-
Fig. 222. tain supplied by it. If a pipe were extended high
enough, from the centre of the pond, in place of the foun-



MECHANICS.



425



tain, the water would slowly rise to the height of the water
in the reservoir. The fountain jet will not rise so high,
both on account of the friction of the pipe and resistance
of the air. The farther the fountain is from the reservoir,
the less will the height of the jet be, because the friction is
proportioned to the length of the pipe.

1457. When a liquid is allowed to flow through an ori-
fice in the side or bottom of a vessel, it is found by experi-
ment that the shape of the outlet influences very largely
the rapidity of the discharge.

1458. Fig. 223 represents different forms of orifices, and
aids to explain the difference in their action.

Fig. 223.





tl!



I '//




Explain I n the first, it will be seen that the currents (rc-
Fig. 223. presented by broken lines), coming from the oppo-
site sides of the outlet, oppose each other, so that the velo-
city is somewhat checked ; the result is a contraction of the
outflowing stream to a size considerably less than the ori-
fice. The contraction is found to occur at a distance from
the orifice varying from half its diameter to the whole
diameter.

In the second form of orifice the currents do not oppose
each other, and a better flow of liquid is the result ; the
stream is the whole size of the orifice, and flows with great
emoothness.






426 MECHANICS.

The third is the most unfavorable form for a free dis-
charge, as currents are formed in the vessel having a direc-
tion nearly opposite to the discharge.

The discharge is also influenced by nearness to the side
of the vessel. In No. 4 it will be seen that the current
would be deflected by the momentum of particles from one
side.

ox,- /* .*... 1459. To calculate the velocity and hence
How is the quantity th tit of n id discharge d from any ori-

falcula^ 9 fice ' we employ the principle (explained on
page 129, note) that the velocity of the stream
is equal to that acquired by a body falling through a height equal
to the depth of the liquid. The velocity is therefore calculated by
multiplying the square root of the depth in feet by 8. The result
expresses the velocity in feet per second ; but by reason of the
various resistances, the velocity is generally only about 76 per cent,
of this calculated amount.

PNEUMATICS.

1460. Recent applications of the principles of Pneumatics
invest this branch of Philosophy with a new interest.

TP-T , , . The compressibility and perfect elasticity of

^reoTsmd'l the air all ^ of its e nom * cal use in th e pro-

i/ * " 9 pulsion of engines, wherever the means of con-

use in Mechanics? g ensation are g readilv obtained. The drilling

machinery of the Mont Cenis and Hoosac Tunnels were operated by
the expansive property of compressed air. The condensation was
effected by power obtained from water-wheels outside of the tunnel,
and conducted through tubes to the machines to be operated. The
work could not have been as efficiently done by any other known
source of power.

The air, after doing its work, served to ventilate the tunnel ; if
steam power had been employed, a large excess of the power neces-
sary to perform the work would have been required to afford the
proper ventilation to the workmen.

1461. Recent experiments have demonstrated the practicability
of conveying the compressed air fifteen or twenty miles through
pipes from the locality where the air is compressed to the engines
which it is to be employed to drive. The project of employing a
portion of the enormous store of waste power of Niagara to com-
press air to be used at Buffalo, twenty miles distant, has been seri-
ously entertained.

1462. Diving-bells of larger dimensions than ever before em-
ployed have played an important part in the work of preparing the
foundations for the East River Bridge, between New York and



MECHANICS. 427

Brooklyn. Instead of the ordinary diving-bell described on page
151, an enormous box, or ccvisson, was made, 164 feet long and 102
feet wide, and launched into the water open side downward. After
being floated to the proposed site of the bridge pier, it was gradu-
ally sunk to the bottom by the masonry laid on top. Ir.on tubes or
shafts had been previously prepared projecting through the top of
the caisson, which was several feet thick ; to provide for the en-
trance of the workmen and discharge of the material, air was forced
in by air-pumps (worked by steam on shore) to such a pressure that
the water was forced out, and the workmen stood on the bottom.
Seventy or eighty men at a time worked with ease in this caisson.
The material dug from the bottom allowed the caisson to sink by
degrees until a depth of forty-live feet from the surface of high
water was reached. The masonry upon the top was built fast
enough to keep the surface above water. When its final resting-
place was reached, the interior was filled with concrete, and the
caisson left to form the base of the granite pier.

Many interesting phenomenon were noticed belonging to the
denser atmosphere in the chamber. Sounds were heard more dis-
tinctly ; a painful pressure on the drum of the ear gradually passed
away ; breathing seemed at first slightly difficult ; candle flames
burned with a great deal of smoke ; a workman who had occasion
to go under water in a pit in the bottom of the caisson found he
could remain for an unusual length of time without the least in-
convenience.

At St. Louis, where an iron caisson was sunk to the depth of
110 feet, it was found necessary to change workmen at the greatest
depth at intervals of two hours, as longer stay induced temporary
paralysis. At this caisson the compressed air was applied to an-
other purpose. Tubes reaching from the sand and water at the
bottom were extended upwards to the outer air ; another set of
tubes conducted the compressed air just into the lower open ends
of the first-mentioned tubes ; the strong upward current of air drew
with it sand and water at a rate that precluded the necessity of any
other means of dredging, and all of the material for fifty feet of
doscent was removed by this means.

This dragging action of a current of air, when urged over liquids,
or even over bodies of air, has been lately utilized in several ways.

Currents in the ocean under the influence of strong winds are a
phenomena of the same kind. It seems to be an exhibition of the
property of adhesion.

1463. Heat is a motion of the minute par-
ticles or molecules of a body. All kinds of
bodies, whether animal, vegetable, or mineral, and in any
condition of matter solid, liquid, or gaseous possess this
motion in their molecules. When this motion is less than
usual in any particular body, we say the body is cold ; but
we know of no instance of the entire absence of heat. Hot
lb*



428 MECHANICS.

bodies are those whose molecules are vibrating with great
rapidity.

Define 1464. When a hot body is placed in con-

Conduction. ^ ac ^ w ith a colder one, the molecular motion
of the former is gradually imparted to the latter, begin-
ning at the point of contact, and gradually extending
throughout the mass. This communication of heat from
particle to particle is termed Conduction.

Explain 14G5. The rapidity of conduction is very differ-
Fig. 224. en ^ j n different solid bodies. This fact is ex-
hibited by the apparatus Pigi 224>
represented in Fig. '#24.
Rods of different sub-
stances are fitted in the
side of a water-tight ves-
sel ; to the end of each
rod is attached a marble,
held by a bit of common
wax. The box being filled with hot water, the different
conducting powers of the several rods is approximately
shown by the shortness of the time required in each case
to release the marble.

146C. Liquids and Gases are poor conductors of heat.
The molecular motion is not easily communicated from
one molecule to another on account of the ease with
which each moves over or away from the other.

How are liquids 1467. Heating is generally brought

and gases heated? about j n t ] iege two c l asses O f bodies by

contact with heated solid bodies.

Explain 1468. A vessel of water is made hot throughout,
Fig. 225. jf ] iea t ^g applied at the bottom of the vessel, as
represented in Fig. 225. The heat at the bottom of the




MECHANICS.



429



Fig. 225.




vessel extending to the nearest molecules of liquid, they,
by reason of the vibrations imparted to them, are pushed
a little further apart, so that
this portion of the liquid is
made lighter than the sur-
rounding portions, and rises
to the top ; the particles tak-
ing the place thus vacated
are in turn heated, and con-
tinuous currents are thereby
established.

These currents are made
visible in a class-room experi-
ment by adding a little fine
sulphur to the water.
What is the pro- This process of heating by circulation is
cess called ? called convection.

How is heat 1469. Heat is transmitted to great dis-

transmitted ^ tances and in all directions from hot bodies
by the vibrations of the ether which is sup-
posed to fill all space ; this kind of transmission of heat is
called radiation, and the vibrations pass through space
with the velocity of 186,000 miles per second.

Define Latent 1470. When a body receives heat from any

Heat, also source whatever, two effects are immediately
Sensible Heat. , , , . , , . , .

produced; besides being raised in tempera-
ture, it is expanded in volume. That portion of the heat
which is concerned in expanding the body is said to be
latent, as it cannot be detected or measured at once by the
usual methods. That portion of the heat which raises the
temperature is said to be sensible.

1471. The amount of heat necessary to raise the same



430 MECHANICS.

amount of different substances to the same temperature
varies considerably. It requires thirty times as much heat
to raise a certain weight of water one degree in tempera-
ture as is required to raise the same weight of mercury
through the same temperature.

What is a 1472. A heat unit is the amount necessary

Heat Unit f ra i se O ne pound of water one degree
Fahrenheit.

Define *1473. The heat necessary to raise one

Specific Heat. p 0un d O f an y substance one degree in tem-
perature is (when measured in heat units) called its spe-
cific heat.

The specific heat of water is therefore 1 ; of mercury,
^'o ; of iron, \ ; of copper, about T x r ; and of nearly all
known substances, whether solid, liquid, or gaseous, the
specific heat is less than that of water.

1474. In all practical applications of heat the laws of latent heat
are of the highest importance. Illustration of some of the leading
facts is afforded in the following account of the behavior of water
when heated from below the freezing to above the boiling point.

Place a pound of ice in a suitable vessel for the application of
heat. Upon heating it will be found that the melting begins ex-
actly at 32' Fahrenheit, and that the water will remain at that tem-
perature until all the ice has disappeared. This shows that heat
must be expended to convert a pound of ice at 32 to a pound of
water at the same temperature. By careful experiment it has been
found that this expenditure of heat is enough to raise 143 pounds
of water 1. The latent heat of water is therefore said to be 143
Fahrenheit.

If now the pound of water be heated to the boiling point (212
Fahrenheit), the escaping steam will be found to be of the same
temperature ; and although a long-continued application of the heat
be necessary to boil the water away, no rise of temperature above
312 J will be found in either water or steam. Heat has been neces-
sary again to effect the change of state, and in this change 967 heat
units have been employed. In other words, it requires 180 units of
heat to raise 1 lb. of water from 32 fo 212. and it requires 5-V times
as much heat to simply convert the same water into steam, without
raising its temperature above 212".

It should be remembered that the latent heat becomes sensible
when steam is condensed to water, or water converted to ice.



MECHANICS. 431

prn, . . ,, 1475. Heat is employed in various ways to pro-

\r a ! / duce motive power chiefly, however, in the steam-
Mecnanicat engilie> The relation between heat and work has
J^qmrnient been carefullv determined, and is found to be as
follows : " The heat required to raise one pound of
water one degree is equivalent to a force necessary to raise 772
pounds one foot high ;" or, more briefly, " one heat unit is equiva-
lent to 772 units of work."

By reason of various losses in our heat motors, we do not re-
alize more than -^ of this amount even in our best steam-engines.

OPTICS.

What is 1476. Light is the result of vibrations in the
Light? ether which fills all space. These vibrations are
of different degrees of rapidity ; the slowest, which affect
our senses, we recognize as heat, as already explained in a
previous section ; those which are capable of producing
vision vary also in their rate of vibration ; the slowest that
can affect the eye being those that produce a dull red, and
the most rapid a violet color. These waves all move for-
ward at a rate of 186,000 miles in a second. Even the
longest of these waves (the red) are so minute that 39,000
are included in a single inch, while of the violet 57,500 are
contained in the same length.

When the waves are high, the light is said to be intense.

1477. A Eay is only the direction along
Define a Ray. . . ..

which the wave is moving.

Describe the dif- 1478. Light and heat waves are essen-
{ouTd^andl^ht tiall F different in their character from sound
waxes. waves. The undulations which produce

sound are only backward and forward motions in the air
or other medium which transmit them. The light and
heat waves are vibrations on all sides of the line along
which they are propagated.

1479. The number of these vibrations that enter the eye
in a second is found by multiplying the number of inches



432



MECHANICS.



in 186,000 miles by the number of vibrations in a single
inch ; these for extreme violet are 660 millions of millions.*

How many 1480. Lenses are transparent bodies with

forms of lenses curved surfaces. They are generally made
are there? , n , .~ , ,, -.

of glass, and are classified generally under

gix varieties.

Fig. 226.

M IT



The double convex (M), the plano-convex (N), the men-
iscus (0), the double concave (P), the plano-concave (Q),
and the concavo-convex (E).

1481. Lenses thickest in the middle magnify; those
thickest at the edges diminish. This property leads to
their being classified under two classes ; the first three in
the above list being convex, or magnifying glasses, and the
last three concave, or diminishing glasses.

Explain. 1482. The action of a double convex lens upon
Fig, 227. parallel rays of light is represented in Fig. 227.

Fig. 227.




All the rays that strike the glass obliquely are bent from

* For the laws of Reflexion of Light, see 816-838.

The principles of Kefraction, as defined in 842, are best understood hy con-
sidering the properties of lenses.



MECHANICS. 433

their course both on entering and leaving the lens. The
ray X keeps its direction, as it is perpendicular to both
surfaces. All the rays meet at a common point, F, called
the focus.

1483. Practically, this result is rarely realized. When the sur-
faces are spherical, the rays that fall upon the lens nearest the edge
meet at a focus nearer the lens than rays that are nearer the centre.
The highest degree of skill is required in the optician to bring the
glass to such a shape that all the rays having the same direction
shall meet at the same point.

Explain 1484. Fig. 228 shows how the magnifying effect
Fig. 228. O f a double convex lens is produced. The insect

Fig. 228.




A B is seen by the light reflected from it, which passes
through the lens. The ray from A to (7, instead of passing
directly on, is bent in accordance with the laws of refrac-
tion to D ; at this point it is again bent downward, so that
the eye which receives it sees this portion of the insect in
the direction D a. In like manner, the ray of light from
B so reaches the eye us to appear to come from #. Other
rays from all parts of the insect show the corresponding
points to the eye, so that the complete insect appears to
extend from a to I.

Explain 1485. Fig. 229 shows how the rays from any
Fig. 229. "bright object, when allowed to pass through the
lens and fall upon a flat surface properly placed, will form



434 MECHANICS.

an inverted image of the object. Eays from the point of



Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 35 of 38)