« ΠροηγούμενηΣυνέχεια »
duced cold had been known in warm climates from an early period, but this had escaped notice in the more temperate ones, until after the invention of the thermometer, when it was soon perceived that on the bulb being wetted, the mercury immediately fell in the stem.
The following may be taken as a way of applying this knowledge to the teaching of children :
Sugar from the sugar cane. The juices are pressed out by passing the cane between heavy rollers ; this contains, besides sugar, a great deal of water — the water is driven off by boiling - will go away slowly by evaporation.
A current of air over anything that is wet takes the moisture up in vapour, as it passes over the surface; this changing the wet upon anything into vapour is called evaporation, and produces cold; dip your finger in water, when there is so little wind that you do not know from what quarter it comes, and you will find the finger colder on one side than the other; this is the side on which the wind blows; and it is colder because there is a greater evaporation on that side of the finger than the other. The sailor knows this, and when he is becalmed at sea, and does not know from what quarter the wind blows, he wets his finger in his mouth, and holds it up to the air, the cold side is the wind side.
After a shower of rain on your clothes, and whilst they are drying on your back, do you not feel much colder than you did before ? - this is the cold arising from the wet on your clothes becoming yapour -- and for this reason you should not sit in your wet clothes after you get home.
Why does your ink get thick by standing in the inkstand? This, after what you have heard, you can answer yourselves.
In cold weather, you will sometimes observe a quantity of water collected at the bottom of the panes of glass in a room -- you recollect warm air holds up more vapour than cold-the warm air in the room coming in contact with the glass, which is cold from being in contact with the cold air of the atmosphere, is immediately made cooler; this causes the vapour in it to condense on the surface of the glass become water — it then runs down, and collects in large drops on the wood. What becomes of it? Point out how it is perhaps first absorbed by the wood - is changed again into vapour-again inixes with the atmosphere-reappears in rain-fertilizes the fields, etc.
With the aid of a sectional model of the steam-engine, and knowing something of the elastic power of vapour that its force of elasticity increases in a much higher ratio than that of its temperature -- that when reduced below a certain temperature it is immediately condensed – the teacher would be able to explain many of the more important parts of the machine, showing bow steam may be adapted to the purposes of man as a moving power,
He would explain how the steam enters alternately below and above the piston rod, and is carried off — by its elasticity giving an up and down motion to the large beam which sets the machinery in motion — pointing out the parallel motion at the end of the beam, causing the piston rod always to move in the same vertical plane—the up and down motion of the beam causing two dead points, one at its highest, the other at the lowest point of its motion how the contrivance of a fly-wheel, by its momentum when once set in motion, carries the machinery over the dead points, etc.
Then again the importance of having a great quantity of fire surface in the boiler, in order to generate steam rapidly--the saving of fuel by this - the different kinds of boilers in order to effect it—the nature of safety-valves that a safety-valve is, in fact, a weak part of the boiler made to give way when the elastic force of the vapour, from increased temperature, becomes so great as to endanger its bursting --the valve opens (or ought to do), at a pressure much below that which would burst the material of which the boiler is made - gauges for measuring the pressure on every square inch of surface at which the engine is working
nature of an atmospheric safety-valve opening inwards, and why wanted, etc.; that if the steam inside the boiler is suddenly condensed, the boiler would have a tendency to collapse, and an atmospheric valve would guard against this.
Again, when the water in the boiler is very low, the fire
surface of the boiler above the water would become heated in a very high degree; danger from this, in an engine not stationary, as in a steam-boat, of the water, from the rolling motion of the boat, being thrown over the heated surface, and all converted into steam, and an explosion taking place
– not perhaps immediately, but after the heated surface was cooled down to a certain temperature.
The boiler of the locomotive steam-engine is of a tubular kind, in order to expose as much surface as possible to the fire; and in this engine, as there can be no fly-wheel to get over the dead points, there are in each machine two engines at work, the dead points of which are at right angles to each other, so that they never occur together.
The following from Herschel's “ Discourse on the Study of Natural Philosophy,” will give the reader some idea of these hidden powers of nature when called into action, and show him how much they are perhaps beyond anything he may have been in the habit of imagining them.
“It is well known to modern engineers that there is virtue in a bushel of coals, properly consumed, to raise seventy millions of pounds weight a foot high. This is actually the average effect of an engine at this moment working in Cornwall. Let us pause a moment and consider what this is equivalent to in matters of practice.
“ The ascent of Mont Blanc from the valley of Chamouni is considered, and with justice, as the most toilsome feat that a strong man can execute in two days. The combustion of two pounds of coal would place him on the summit.
"The Menai Bridge consists of a mass of iron, not less than four millions of pounds in weight, suspended at a medium height of about 120 feet above the sea. The consumption of seven bushels of coals would suffice to raise it to the place where it hangs.”
It will, perhaps, be difficult to understand the following description of what may be called the mechanical effects of a jet of steam without having recourse to diagrams; but they are curious, and as the same thing may in some measure be tried by a current of air blown or sent rapidly through a hollow tube, this may suggest simple things of an inte- ' resting kind.
A jet of steam issuing outwards in any direction, but suppose vertically from an orifice, will ascend into the air, with greater or less force, according to its temperature and elasticity, and will by its momentum displace the air which it meets with in its upward course. The jet will be rendered visible by the steam being condensed, and the effect of this jet upon the flame of any burning substance — or any light substances brought near to the axis of it—by its attracting them (a current of air setting in on all sides towards the axis of the jet), is striking and worthy of attention.
Take a piece of tow, dipped in spirits of wine and placed at the end of a rod, set it on fire, and approach the flame near the axis of the steam jet; when held a little above the orifice from which the steam proceeds, the flame will be attracted in a slanting direction, and the angle which the flame makes with the axis of the jet increases as the distance from the orifice increases, up to a certain point, when it becomes a right angle; elevated above this, it again assumes the position it had below this point, until it is elevated beyond the influence of the jet, when it of course assumes a vertical position.
This is better shown by taking a circular piece of iron, with a handle attached, and wrapped round with tow : moisten it with spirits of wine and kindle it, then place the circle of flame across the axis of the jet —up to a certain point above the orifice, the flame will assume a conical appearance ; here it will set itself at right angles to the jet, and appear a flat disc of flame-above this point the flame will again become a conical surface, until being farther elevated, it gets beyond the influence of the jet, and assumes an undisturbed position.
Light bodies when placed in the jet, or heavy bodies within certain limits, when placed in it, will be supported, or a flat surface of any kind held in the hand at a certain distance from the orifice will be forced upwards; but brought close to the surface in which is the orifice of the jet, it will be held down with considerable force.
It is from these properties of a jet of steam, that it has been proposed to ventilate coal and other mines, by creating
strong current of air up one shaft, to be supplied by
a down current from another, which could be regulated at pleasure, and in such a way as to produce even a gentle breeze or a perfect hurricane in the mine.
The same principle may be shown by taking a hollow tube of glass, or of tin, having two arms at right angles to each other for the convenience of blowing through ; otherwise a straight tube would do as well, and one end terminating in a perforated pasteboard, or tin disc, of a few inches diameter, through the centre of which the tubular opening runs, then blowing violently through it, and placing another piece of pasteboard, or tin, over the opening from which the air proceeds, it will be found to be violently attracted — if the apparatus be turned down, wards, so that the current instead of ascending is blown towards the ground, the under surface will be lifted up. - If water is poured into a bent glass tube, open at both ends, and a current of air is blown violently across one end, . the water in it will be found to rise.
On the subject of light there are many simple things easy of explanation, connected with experiments of so simple a kind, that the teacher may with advantage turn them to account in his teaching.
That some bodies, such as the sun—the stars flame of all kinds — bodies heated to a red heat, are self-luminous, possessing in themselves the power of throwing off light; others again, not being themselves the source of light, reflect that which they receive from self-luminous bodies. The flame of a candle is seen by the light which proceeds directly from it; the things in the room are seen by the light thrown upon them from the candle, and reflected back to the eye.
Why does the light passing through a window light the whole room, and not appear a mere column or light, the base of which is equal to the size and figure of the window, and why any light on each side of this column ? Or, rather why is it not a set of separate columns, as many in number as the panes of glass, and having circular, or square bases, etc., according as the panes may be circles, squares, diamonds, etc., with dark spaces of the thickness of the bars of the window between each column of light; so that a