screw. mechanical power increase the force of the prime mover? No; but it enables us to overcome a resistance which we should not be able to overcome without it. How many kinds of mechanical powers are there? There are three kinds with their varieties. What is the first? The lever. What are the varieties of the lever? The wheel and axis are nothing more than a universal lever. What is the second mechanical power? The inclined plane. What is the relation between the power and weight on the inclined plane? The power is to the weight as the height of the plane is to its length. What are the varieties of the inclined plane? The wedge and Give examples of the wedge. Scissors, knives and razors. What is the next mechanical power? The pulley. How do you estimate the power of the pulley? The power is to the weight as the space passed over by the weight is to the space passed over by the power What do you understand by a compound machine; Any mechanical combination of the simple mechanical powers. Give an example of a machine. The clock, or a steam-engine. What is the prime mover in a clock? The attraction of the earth for the large weight attached to a cord passing round a barrel. What is the prime mover in a steam-engine? The elastic force of steam. How is this elastic force increased? By raising the temperature of the steam. On the Centre of Gravity. What is the centre of gravity? It is the point round which all the parts of a body are balanced in all positions. What is the centre of gravity of a uniform straight bar, the thickness of which may be neglected? It is its middle point. How do you find the centre of gravity of a triangle? Draw a line from any angle to the middle of the opposite side, then of this line from the angle will 量 be the position of the centre of gravity. How do you find the centre of gravity of a circle or an ellipse? It is the centre of the figure. How do you find the centre of gravity of a ring? It is the centre of the whole figure. What is the centre of gravity for the cone and pyramid? The centre of gravity is in the axis at a distance of of the axis from the vertex. When a stone is thrown obliquely upwards, what does it describe? A parabola. What figures do the planets describe? Ellipses. What figures do the comets describe? Some of them ellipses round the sun. When a man in wrestling is likely to be thrown down, what does he enlarge? His base of sustentation by putting his feet as wide apart as possible. What is a straight line falling perpendicular to the ground from the centre of gravity called? The line of direction. When the centre of gravity is very high what happens? The body may be very easily overturned. When the mast and the sails and the yards of a vessel are very high, what will happen in a breeze? It will be easily upset. Where is the centre of gravity of a man? It is so situated that the line of direction falls between his feet: the same is the case in quadrupeds. On the Laws of Motion. What is the first law of motion? If a body is set in motion it will continue to move uniformly in a straight line until it is acted upon by some external force. What is the second law of motion? Motion, or change of motion is proportional to the force impressed, and takes place in the direction in which that force acts. If a force acts upon a body at rest, what will it do? It will set it in motion. What is the third law of motion? When a body is set in motion by pressure, the moving force is proportional to the pressure, that is, action and reaction are always equal and contrary. Why does a heavy body generally fall more quickly than a lighter body? The reason why a heavy body, as a leaden bullet, generally falls more quickly than a lighter body, as a feather, is that the surface of the bullet is much less in proportion to its weight than the surface of the feather. When will they fall equally? In the receiver of an airpump, from which the air is withdrawn, they are found to descend with the same rapidity. What is the name of the machine by which the third law of motion may be illustrated? Attwood's machine. On the Pendulum. What do you call a pendulum? A heavy body that can freely move round the point of suspension. What may be considered a simple pendulum? A heavy mass suspended by a very fine thread or hair. When a pendulum is raised from its perpendicular to an inclined position, and then let free again, what will take place? Because gravity attracts it downwards it will begin to swing in circular arcs. What do you call the angle by which the inclined position of the pendulum differs from the perpendicular direction? The angle of oscillation or elongation. What is the most remarkable property of the pendulum? It consists in the equal duration of its oscillations. Is the duration of the oscillations of a pendulum influenced by the length of the arc which it describes? It is but very little influenced by the magnitude of the arc. On what does the time which is needed for one oscillation depend? It depends upon the length of the pendulum. What law is there with regard to this dependency? The lengths of any two pendulums are in proportion to the squares of the times needed for one of their respective oscillations. What are the uses of the pendulum? 1. In exhibiting the direction of gra vity. 2. It shows that great masses influence the attraction of gravity. 3. The pendulum prores that all bodies are equally attracted by gravity. 4. The pendulum shows that the attraction of gravity is less near the equator than near the poles. 5. The pendulum is the best regulator of a clock, because the duration of its oscillations are as nearly as possible equal to one another. IMPONDERABLE BODIES. What do you understand by the philosophy of im-` ponderable agents? It is that branch of natural philosophy which treats of heat, light, and electricity. What are the effects of heat on a body? Heat expands a body in all its dimensions. Do all bodies expand uniformly by equal degrees of heat? No, some expand more and some less. What are the bodies which expand most uniformly? All the gases, mercury and alcohol. What fluid would you use for measuring the degrees of heat? Æther, air, alcohol, and mercury. What is the thermometer? An instrument which measures the expansion of the liquid contained in it, and consequently the degree of heat of a fluid in which it is immersed. What do you understand by the freezing point? The point at which water freezes, or ice thaws. What do you understand by the boiling point? The point at the scale to which the liquid rises in the tube, when the bulb of the thermometer is immersed in boiling water. Is this point always the same? No, it is higher when the atmosphere is heavier, and lower when its weight diminishes. Into how many equal parts is the space between the freezing and boiling points divided? 180° in Fahrenheit's thermometer and into 100 in the Centigrade. What do you understand by the zero or 0 in Fahrenheit's scale? It is the degree of cold produced by the mixing of salt and snow. How many degrees is this below the freezing point? 32. What number is placed at the boiling point of Fahrenheit's thermometer? 212. What numbers are placed at the freezing and boiling points of the Centigrade scale? O at the freezing point and 100 at the boiling point. If you observe in a French newspaper that the cold at Paris is so intense that the Centigrade thermometer falls to 10° below zero, how many degrees is that in our thermometer? As 100: 180 :: 10: 18 or 5 : 9 :: 10 : 18; hence it is 18° below the point at Fahrenheit's thermometer or 32° — 18° — 14°. If the Cen tigrade thermometer stand at 15°, what is that in Fahrenheit's? As 5 : 9 :: 15 : 27; hence 32° + 27° = 59°. Would you venture to put your finger in boiling water? Yes, if I were at the top of a very high mountain, or in a balloon. If water be converted into steam how many times its bulk will it be expanded? About 1800 times. What is the elastic force of the steam of boiling water equal to? The pressure of the atmosphere or fourteen pounds on every square inch. What is the principle on which the steam engine acts? The pressure of steam alternately on the opposite sides of a piston, which works steam tight in a cylinder. What is the difference between a high and a low pressure engine? In a high pressure engine the elasticity of the steam is much greater than the pressure of the atmosphere, whereas in · a low pressure engine it is seldom much higher, LIGHT. In what way do all philosophers account for the phænomena of light? According to Sir Isaac Newton, light consists of small particles shot off at the rate of 192,000 |