Fig. 40.
Fig. 40.
What is the cup on which the spoon rests called?
It is called aFulcrum.
What is a fulcrum?
A fulcrum is any solid body against which a lever rests.
When the spoon is called a lever, what are the parts on each side of the fulcrum called?
They are called thearmsof the lever.
What is necessary, so that a lever may balance?
The arms of the lever must be of equal weight, or they will not balance.
If the bowl of the spoon be filled with water, what will result?
The bowl will sink, and the handle of the spoon will rise.
What will cause the bowl of the spoon to sink?
The weight of the water in it will cause it to sink.
How can the bowl of the spoon be prevented from sinking?
By applying a force to the handle of the spoon, equal to the weight of the water put in the bowl.
What is the force applied to the handle called?
It is called thePower.
What three things are always necessary when the lever is used?
The power, thefulcrum, and theweight, are always necessary when the lever is used.
Which of these three things are movable?
The power and the weight are movable.
Which one does not move?
The fulcrum does not move.
How many kinds of levers are there?
There are three kinds of levers.
What is the first kind of lever?
It is where the fulcrum is between the power and the weight, as is shown inFig. 41.
Fig. 41.
Fig. 41.
When the fulcrum is under the middle of the lever, how can the lever be kept balanced?
The lever will remain balanced, so long as the power and the weight used are equal to each other.
When the lever is moved, so that the fulcrum is nearer the weight, which arm of the lever is the heavier?
The arm towards the power, because it is the longer arm.
Will the power applied to the long arm of the lever be as heavy as the weight?
No; the nearer the fulcrum is to the weight, the less will be the power needed to balance that weight.
How will it be when the fulcrum is nearer to the power than it is to the weight?
Then the power must be heavier than the weight.
When the long arm of the lever is twice the length of the short arm, what will be the difference in the weights?
One pound applied to the long arm will then balance two pounds applied to the short arm.
Upon what does the power of a lever depend?
It depends upon how much nearer the fulcrum is placed to one end of the lever, than to the other end.
Of what use is the lever?
It is very useful in raising heavy bodies.
Fig. 42.
Fig. 42.
What is the weight to be lifted inFig. 42?
The stone is the weight to be lifted.
Where is the fulcrum placed?
The fulcrum is placed near the stone.
Where is the power applied?
The power is applied by the hand to the long arm of the lever.
How may a see-saw be made?
A see-saw may be made by placing a plank or a board across a rail in a fence, so that it will balance.
Is the see-saw, as shown inFig. 43, an example of the lever?
Yes; the see-saw is an example of the lever.
Fig. 43.
Fig. 43.
What are the parts of the plank on each side of the fence-rail called?
They are called the arms of the lever.
What is the fulcrum in the see-saw?
The fence-rail on which it rests, is the fulcrum.
What is the power in the see-saw?
The power is the boy on one end of the lever.
What is the weight in the see-saw?
The weight is the boy on the other end of the lever.
How do children play at see-saw?
By pushing against the earth with their feet, the arms of the lever are alternately forced to rise and fall in the air, thus making a see-saw.
Where must the fulcrum be when the two boys are of the same weight?
The fulcrum must be under the middle of the lever.
Where must the fulcrum be when one boy is heavier than the other?
The fulcrum must then be nearest to the heavier boy, so that they may balance.
Does it require much force to play at see-saw?
No; when the see-saw is balanced, a very little force will cause it to move up and down.
Of what use is a pair of scales?
A pair of scales is very useful in weighing different substances.
Fig. 44.
Fig. 44.
Are the scales, as shown inFig. 44, an example of the lever?
The scales are an example of the lever.
Where is the fulcrum of the scales?
The fulcrum is the point on which the lever rests.
What is the weight?
The weight is the lead put in one scale.
What is the power?
The power is the substance put in the other scale, so as to balance the weight.
When the lead put in one scale weighs exactly a pound, how much sugar must be put in the other scale to balance the lead?
Exactly one pound of sugar must be put in to balance the lead.
For what is a balance mostly used?
A balance is mostly used for weighing heavy bodies.
Is the balance a lever?
Yes; the balance is a lever.
Which is the fulcrum in the balance?
The fulcrum is the hook that holds the balance up.
Fig. 45.
Fig. 45.
What is the weight?
The weight is the large body hanging from the short arm of the lever.
What is the power?
The power is the smaller body hanging from the long arm of the lever.
InFig. 45, the distance from the fulcrum to the power is ten times the distance from the fulcrum to the weight; now, if the small body weighs one pound, how much must the large body weigh, so as to balance it?
The large body must weigh ten pounds.
When the small body weighs ten pounds, how much must the large body weigh, so as to balance it?
The large body must weigh ten times as much, or one hundred pounds.
If the smaller body be moved half-way to the fulcrum, how much weight will then balance it?
It will then take only half the weight, or fifty pounds to balance it.
What other familiar example of this kind of lever may be mentioned?
The pump-handle is a lever of this kind.
Fig. 46.
Fig. 46.
What is the power?
The power is the force applied to the pump-handle.
What is the fulcrum?
The fulcrum is the screw that fastens the handle to the pump.
What is the weight?
The weight is the water raised by the bucket.
Are scissors an example of the lever?
Yes; scissors are a double lever.
What is the fulcrum in the scissors?
The fulcrum is the rivet which holds the two levers together.
What is the power?
The power is the force applied by the fingers.
What is the weight?
The weight is the paper, or other substance, to be cut by the scissors.
Can scissors be made very strong?
Yes; some scissors are made so strong that they are used for cutting iron, tin, and zinc.
What other useful instruments are made upon the same principle as the scissors?
Such instruments as nippers, pincers, tweezers, and blacksmith’s tongs.
What is the second kind of lever?
It is where the weight is between the power and the fulcrum, as is shown inFig. 47.
Fig. 47.
Fig. 47.
Of what use is this kind of lever?
It is very useful in lifting heavy weights a short distance from the earth.
What example is there of this kind of lever?
The door on its hinges is a lever of this kind.
What is the fulcrum?
The hinges are the fulcrum.
What is the weight?
The door is the weight.
What is the power?
The force that opens or shuts the door is the power.
What other examples of this kind of lever have we?
We have an example in the lid of the tea-kettle, and in the cover of the ink-stand, when they are fastened on by hinges.
What is the third kind of lever?
It is where the power is between the fulcrum and the weight, as is shown inFig. 48.
Fig. 48.
Fig. 48.
What examples of this kind of lever have we in common use?
The shovel, the spade, and the pitch-fork, are levers of this kind.
What is the fulcrum?
One hand is the fulcrum.
What is the power?
The other hand is the power.
What is the weight?
The earth raised on the shovel is the weight.
What other familiar example have we of this kind of lever?
The fishing-rod is an example of this kind. One hand is the fulcrum, the other hand is the power, and the line, with the fish, is the weight. This is made plain byFig. 49, representing a lad just drawing a fish out of the water.
Fig. 49.
Fig. 49.
There are several other examples of this kind of lever; will you name some of them?
Yes; brooms, hoes, rakes, and axes are all levers of this kind.
What is the first kind of lever mentioned?
It is where the fulcrum is between the power and the weight, as inFig. 50.
Fig. 50.
Fig. 50.
What is the second kind of lever mentioned?
It is where the weight is between the power and the fulcrum, as inFig. 51.
Fig. 51.
Fig. 51.
What is the third kind of lever mentioned?
It is where the power is between the fulcrum and the weight, as inFig. 52.
Fig. 52.
Fig. 52.
What other way have we for raising weights?
We have thewheel and axle, as is shown inFig. 53.
Fig. 53.
Fig. 53.
What is the wheel and axle?
The wheel and axle is only another form of the lever.
Where is the power applied?
The power is applied to the rope which passes around the wheel.
Where is the weight?
The weight is the bucket attached to the rope which passes around the axle.
Where is the fulcrum?
The fulcrum is the frame on which the ends of the axle rest.
Is the power applied to the wheel equal to the weight to be raised?
No; the power is always less than the weight to be raised.
Upon what do the power and the weight depend?
They depend upon the size of the wheel and the axle.
When the diameter of the wheel is ten times the diameter of the axle, how much will one pound raise?
One pound applied to the wheel, will then raise ten pounds on the axle.
What is a windlass?
Awindlassis another form of the wheel and axle, as is shown inFig. 54.
Fig. 54.
Fig. 54.
What is used in a windlass in place of the wheel?
A crank or handle is used in place of the wheel.
Does the crank take the place of the wheel?
Yes; when the crank is turned, it forms a circle the same as the outside of the wheel.
To what use is the windlass often applied?
The windlass is often used to draw water from wells.
What is a pulley?
Apulleyis a wheel with a groove or hollow cut in its edge.
What is the groove for?
The groove is cut so that a rope may move over the wheel without slipping off.
Fig. 55.
Fig. 55.
InFig. 55, where is the power applied?
The power is applied to one end of the rope by the hand.
Where is the weight?
The weight is attached to the other end of the rope.
When the power and the weight are equal, what do they do?
They balance each other, and neither of them will move.
When the power is greater than the weight, what takes place?
The weight is then lifted up.
Of what use is the single pulley?
The single pulley is used for hoisting flags to the top of high poles, for raising sails to the topmastsof vessels, and for raising weights to the upper stories of mills and factories.
How many pulleys are needed, when a horse is to raise the weight?
When a horse is to raise the weight, two pulleys are needed, as may be seen inFig. 56.
Fig. 56.
Fig. 56.
To what use do farmers apply the pulley?
They arrange the pulleys so as to lift the hay into their barns, by means of a horse; thus saving themselves much hard labor.
When one end of a plank is raised up, while the other end remains on the floor, is its surface level?
No; its surface is not level.
What may the surface of the plank be called?
It may be called an inclined surface, or aninclined plane.
What is an inclined plane?
An inclined plane is a surface that inclines upwards or downwards.
Of what use is the inclined plane?
It aids very much in raising weights.
Are all planes inclined alike?
No; some planes are much steeper than others.
Fig. 57.
Fig. 57.
InFig. 57, where is the power?
The power is at P.
Where is the weight?
The weight is at W.
Upon what does the power and the weight depend?
They depend upon the height and the length of the inclined plane.
When an inclined plane is ten feet long and only one foot high, how much will one pound at P balance?
One pound at P will balance ten pounds at W.
When an inclined plane ten feet long is two feet high, how much will one pound at P balance?
One pound at P will then balance only five pounds at W; because the plane is five times as long as it is high.
Where else may the power be exerted?
The power may be exerted behind the weight, as inFig. 58.
Fig. 58.
Fig. 58.
Will any more power be needed behind the weight than before it?
No; the power required is the same in both cases.
Would it require as much power to roll barrels into a wagon as to lift them in?
No; they can be rolled into a wagon much more easily than lifted in.
Can they be taken out of the wagon any more easily on an inclined plane than without one?
Yes; they can be taken out more easily on an inclined plane.
Why are drays, used in cities, made in the form of the inclined plane?
So that barrels and hogsheads may the more easily be rolled on them.
Are our stairs in the form of the inclined plane?
Yes; they are inclined planes with steps cut in them to make the ascent easy.
Why are house-roofs made in the form of the inclined plane?
So that the water falling on them may run off readily.
What familiar example of the inclined plane may be mentioned?
The hills, all over the country, are so many inclined planes.
Of what use are the hills?
The hills carry off the waste water from the earth, so that it is kept dry enough for plants to grow.
What other example of the inclined plane may be mentioned?
The beds of the creeks and rivers are so many inclined planes down which their waters flow.
What is a wedge?
Awedgeis a double inclined plane, as is represented inFig. 59.
Fig. 59.
Fig. 59.
Of what use is the wedge?
The wedge is used in splitting logs of wood and blocks of stone.
For what other purposes is the wedge used?
The wedge is used to fasten handles in hammers, hatchets, axes, and hoes.
How is power applied to the wedge?
Power is applied to the wedge by means of sudden blows from a hammer.
What prevents the wedge from flying back after the blow?
The friction between the wedge and the other body, keeps it from flying back.
What instruments are made in the form of a wedge?
Knife-blades, hatchets, axes, razors, and cutting instruments of nearly all kinds, are in the form of the wedge.
What is a screw?
Ascrewis another form of the inclined plane.
Where is the inclined plane on a screw?
It begins at the bottom of the screw, and winds around and around the screw until it reaches the top, as is shown inFig. 60.
Fig. 60.
Fig. 60.
What is this inclined plane called?
It is called the thread of the screw.
How do the threads affect the screw?
When the threads are far apart, the inclined plane is steep; but it becomes less steep the nearer the threads are together.
When will the screw exert the most power?
The screw will exert the most power, when its threads are nearest together.
Of what use are screws, such as is shown inFig. 60?
Screws are used to fasten hinges and bolts on doors and shutters, and to fasten pieces of wood together in the construction of houses, boats, wagons, and other things.
For what other purposes is the screw used?
The screw is used where much power is needed, as in the vise; where pressure is desired, as in copying letters, and in coining money; and, also, for squeezing the juice from apples, grapes, and sugar-cane.
Fig. 61.
Fig. 61.
In what does a screw like that inFig. 61work?
It works in a nut which has threads cut on the inside, so that the threads of the screw move in them.
Where is the power applied?
The power is applied to the lever.
Can heavy bodies be raised by means of this screw?
Yes; loaded cars, and even heavy buildings, may be raised by means of a screw of this kind.
Sewing machine.
Decoration.
Illustrated Capital W
When a cat’s back is rubbed in the dark, what does it give off?
It gives off small sparks.
What are these small sparks called?
They are called electric sparks, orelectricity.
If a horse be rubbed in the winter when it is dark, what will be given off from him?
Electricity will be given off from the horse.
When sealing-wax is rubbed with a silk handkerchief, or a piece of flannel, what is produced?
Electricity is produced in the sealing-wax.
What will this electricity in the sealing-wax do?
It will attract bits of paper, straw, feathers, and hairs, when it is held near them.
What other substance will act the same as the sealing-wax?
A dry glass tube will act in the same manner when rubbed.
Where is electricity found?
Electricity is found in every body, and in every thing.
How is electricity developed?
Electricity is developed by motion, by friction, and by almost every change that takes place.
Is electricity found in the air?
Yes; the air is almost constantly electrified.
With what is electricity often accompanied?
Electricity is often accompanied by heat and light.
How many kinds of electricity are there?
There are two kinds of electricity.
What are they called?
They are calledpositiveandnegativeelectricity.
What do these two kinds of electricity always do?
They always attract each other.
When one body contains positive and the other body negative electricity, what will the two bodies do?
They will rush together.
When both bodies are positive or both are negative, what will they do?
They will repel each other.
When a body permits electricity to pass freely through it, what is it called?
It is called agood conductorof electricity.
What substances are good conductors of electricity?
Metals, charcoal, water, animals, vegetables, flame, and smoke.
When a body does not permit the electricity to pass through it, what is it called?
It is called anon-conductorof electricity.
What substances are non-conductors of electricity?
Gutta-percha, sulphur, glass, silk, wool, hair, feathers, cotton, and paper.
What is lightning?
Lightning is electricity passing from one place to another.
When does lightning pass from one cloud to another cloud?
Whenever one cloud becomes filled with more electricity than another, the cloud having the most electricity gives off a portion to the cloud having the least electricity.
When does lightning pass from the earth to a cloud?
Whenever the earth contains more electricity than the cloud does.
When does lightning pass from a cloud to the earth?
Whenever the cloud contains more electricity than the earth does.
How near are the clouds when the lightning comes to the earth?
The clouds are seldom half a mile away when the lightning comes from them to the earth.
What noise is made by the lightning?
The lightning passes so quickly through the air, that it makes a noise called thunder.
Do we always hear the thunder when there is lightning?
No; the lightning is sometimes so far away that the thunder cannot be heard.
What bodies attract the lightning to the earth?
Tall trees, steeples, high houses, and mountains.
Does the lightning do any harm?
Yes; it does much harm by destroying trees, houses, animals, and, indeed, almost everything in its course.
What does lightning sometimes do with buildings?
It sets the buildings on fire so that they are burned.
How are buildings protected from injury by lightning?
They are protected by means of the lightning-rod, which was invented by Dr. Franklin.
Of what is the lightning-rod made?
The lightning-rod is made of some metal, usually of iron or copper, which is a good conductor of electricity.
What does the lightning-rod do when the building is struck?
The lightning-rod conducts the electricity to the earth, so that it does not injure the building.
Must the lightning-rod be higher than the building?
Yes; because the lightning is apt to strike the highest object.
Of what should the point of the rod be made?
The point should be made of silver or platinum, so that it will always remain bright.
Should the rod extend deep into the ground?
The rod should extend four or five feet beneath the surface of the earth.
Why should ashes or charcoal be put around the bottom of the rod?
Because they are good conductors and will help to conduct the lightning into the earth.
How large should the lightning-rod be?
If made of iron, the lightning-rod should be about three-fourths of an inch in diameter.
Why is glass put around the rod where it is joined to the building?
The glass is a non-conductor, and prevents the lightning from leaving the rod.
Why is it not safe to stand near a fire during a thunder-storm?
Because the fire is a good conductor, and sometimes draws the electricity into the room.
What part of the room is the safest place?
The middle of a room is thought to be the safest place.
Is a feather-bed a safe place?
Yes; because the feathers are a non-conductor, and will not attract the lightning.
Is dry air a non-conductor of electricity?
Yes; dry air is a non-conductor of electricity.
When are thunder-storms most frequent?
Thunder-storms are more frequent in summer than in winter, and more so in the afternoon than in the morning.
By what are thunder-storms usually attended?
They are usually attended by some change in the direction of the wind.
Where do thunder-storms generally prevail?
They generally prevail in the lower regions of the air, near the earth.
What is the rapidity of lightning?
The rapidity of lightning is probably not less than 250,000 miles in a second, and the flash does not exceed the millionth part of a second in duration.
Of what does magnetism treat?
Magnetismtreats of the property of magnets.
What is a magnet?
Amagnetis a piece of iron ore having the power of attracting iron and steel.
How many kinds of magnets are there?
There are two kinds of magnets: natural magnets, and artificial magnets.
What other name is given to a natural magnet?
It is called a loadstone.
Where are natural magnets or loadstones found?
They are dug out of the earth, and are found most abundantly in Norway and Sweden.
What is an artificial magnet?
An artificial magnet is one made by magnetizing a bar of steel.
How may a bar of steel be magnetized?
A bar of steel, when rubbed on a natural magnet, becomes magnetized.
When the bar of steel is bent in the shape of a horse-shoe, what is it called?
It is then called a horse-shoe magnet, from its shape.
Where does a magnetic bar always point, when it is made to move easily on a pivot?
One end of it always points to the north, and the other end to the south.
What are the ends of the magnet called?
The north end is called the positive, and the south end is called the negative.
What is there curious about the magnet?
The positive end of one magnet always attracts thenegative end of every other magnet; but if the two positive or the two negative ends are brought together, they will repel each other.
Why are artificial magnets made of steel?
Because steel will remain magnetized, but iron soon loses the magnetic properties.
What is a compass?
Acompassis an instrument containing a magnetic needle, so balanced as to move easily on a pivot.
In what direction does this needle always point?
This needle always points toward the north.
By whom is the compass used?
It is used by travellers to guide them on their journey; by surveyors to mark out the boundary lines between different properties; and by seamen to point out their way across the sea from one place to another.
Before the Mariner’s Compass was invented, seamen were afraid to venture out of the sight of land, for fear of being lost on the ocean; for they then had no means of telling where they were, or in what direction to guide their vessels, so as to reach the land again.
Are magnetism and electricity much alike?
Yes; they are so nearly alike that many persons think they are identical.
What is an electro-magnet?
Anelectro-magnetis a magnet obtained by means of electricity.
Of what is the magnet made?
The magnet is made of soft iron, around which arewound a great many coils of wire, and these are covered with silk.
How is the soft iron made into a magnet?
The iron is magnetized by the passage of electricity through the wire.
Of what use is this magnet?
It is used in the electro-magnetic telegraph.
When and where was the first line of telegraph built?
The first line of telegraph was built in 1844, from Washington to Baltimore, by Professor Morse.
What is the principle of the telegraph?
At one end of the telegraph wire is an electrical battery, and at the other end is an electro-magnet. When a message is to be sent, the electricity is transmitted through the wire to the soft iron of the electro-magnet, which causes the iron to move, and this motion is conveyed to the machinery, which is needed to print the message in letters or signals, so as to be understood.
Does electricity move rapidly?
Yes; its velocity is such as to move around the earth in a quarter of a second.
What is the electrical battery called?
The electrical battery is called akey.
What is the electro-magnet called?
The electro-magnet is called areceiver.
For what are the key and receiver used?
The key is used for sending messages, and the receiver for receiving them.