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regard to safety being had), as in all cases the less the weight of the boat itself, of a given size, the greater cargo it would carry that a boat which would sink in one fluid, would float merrily in another which was heavier, &c.; for instance, a load which would sink in fresh, would float in salt water, and be buoyant in mercury. The teacher would naturally point out that the same boat would carry a heavier cargo on salt water than on fresh. What would it be on oil, milk, mercury, &c.? The number of things which the principles connected with floating bodies may be called upon to illustrate is very great. It may be well also to point out that a floating body is stable, when the line joining the centre of gravity of the body and that of displaced fluid is vertical.

Having made them understand what is meant by the term specific gravity, and that by taking the weight of a certain volume of water as a standard, we calculate the weight of other bodies, it will be well to have a table of the specific gravities of substances in common use, metals, woods, &c., suspended on a card in the schoolroom; and to show them by experiment how these results are arrived at. It is quite a mistake to think that boys about twelve or thirteen years of age cannot be made to understand them, and not only thatthey will take a great interest in them.

A short list is added, merely for the purpose of working an example or two from it. Taking water as 1.

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A simple inspection of this table may be made a useful lesson, by pointing out to them the comparative weight of those substances they are continually handling, the difference among them being much greater than they are in the habit of thinking it-that those substances the specific gravity of which is less than 1 will float. In this way the comparing one thing with another makes them think. Also why distilled water is a standard-that water varies in weight with the substance it

holds in solution-that its boiling point varies with these substances.

Assuming the weight of a cubic foot of distilled water, and at the temperature of 63° Fahrenheit, to be 1000 ozs., (why distilled water, and why a fixed temperature?) let them show that the weight of a cubic inch =

divisor is 1728.

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1728'

and why the

When we speak of the specific gravity of lead being 11-352, and of iron 7.788, we mean that the weight of any given volume of lead or iron will be so many times that weight of the same volume of water, and knowing the one, the other is easily calculated.

Thus a cubic foot of water weighs 1000 ozs., therefore a cubic foot of lead weighs 1000 ozs. × 11·352 = 11352 ozs., of iron 1000 ozs. × 7·788, or 7788 ozs., of an inch in the

same way.

The specific gravity of dry oak is 925, of fir 550, of elm 600, therefore any given volume of these woods would float, being lighter than the same volume of water. A cubic foot of dry oak would be 1000 ozs. x 925, or 925 ozs.; of fir 1000 ozs. 550, or 550 ozs., a little more than half the weight of oak.

As applied to these substances, a good deal depends on their state of dryness, sap in them, &c.

The following questions of a practical kind may suggest

others :.

What is the weight of a block of marble, granite, &c., of regular figure (or any other which they can measure), base of it fifteen feet six inches by five feet two inches, and four feet high?

A given number of feet of oak, elm, ash, &c.? A given mass of metal, what would be its weight? The weight of metals is exactly known from measurement, supposing them to be pure.

In this way they will easily see what horse-power, or manpower-moving power-it will take to move given masses of these materials; and would, if called upon to put it into practice, contrive accordingly-strengthening their machinery, &c. adapting it to the work required to be done.

From this also may be shown the reason why heavy bodies appear so much lighter when moved in a fluid like water-the heavier the fluid the easier they move-as when they raise a bucketful of water from a well; its increased heaviness the moment it gets to the surface of the water-given size of the bucket how much increased in weight?-would it be heavier if raised out of the water into a vacuum, and how much?moving masses of stone, as granite, under water-floating beams of timber, &c. Having given the volume and the specific gravity of the fluid in which they are moving, to calculate what they lose in weight.

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Suspend a cubic foot of lead by a chain from one end of a balance: what weight would balance it at the other end, or over a single pulley? A weight equal to itself.-Now let it fall into a vessel of water: will it take the same weight to balance it as before? No, sir; a weight less than itself, by the weight of a cubic foot of water.-What does a cubic foot of water weigh? 1000 ozs.-Well, I don't recollect the weight of a cubic foot of lead, but what is its specific gravity?-look at your table, 11.352; therefore the weight of the lead in air as 11352, and deducting 1000 ozs., the weight of a cubic foot of water, which is the weight lost by the lead, gives 10352, the weight necessary to balance the lead when in water.

Suppose a cubic foot of lead resting on a pile under water, what force must be exerted to pull it off, supposing no resistance from friction on the pile? About ths of its own weight.

From this to explain how it is that the sand, stone, shingle, &c., are so easily tossed about on the sea-shore-how the human body floats, &c.

Questions: A vessel full of mercury, the bottom of which is nine inches by 4.56, and the height ten inches, what is its weight?

Suppose a cistern, twelve feet long, five feet wide, and four feet six inches high, made of lead a quarter of an inch thick, what would be its weight?

What is the weight of a cylinder of iron thirty inches in diameter and six feet high? Of a block of granite in the form of a circle, four feet six inches in diameter and twenty inches thick?

A statue of marble is placed in a vessel full of quicksilver,

and causes six cubic feet to run over, what is its weight? Would it sink? Would a statue of cast-iron sink?

Why is the line of the angler more likely to break after the fish is out of water than when it is in it?

Do you see any connexion between the weight of a given mass of matter and the altitude of the barometer? and how might a dealer in any bulky commodity profit by observing that connexion?

The specific gravity of ice is to that of water as 8 to 9, and a field of ice of uniform thickness, has 10 feet above water, how many feet below it?

A cubic foot of a metal weighs 1000 lbs. when weighed in air; the weight of a cubic inch of air being about th part of a cubic inch of water at a temperature of 63°, what would be the weight of the body in vacuo? also if weighed in water -and if in air of half the density,-work out the arithmetical results.

Making them reduce the fluid measures into cubic inches, feet, &c., is a good exercise.

How many cubic inches in a pint? 34.659.

in a quart?
in a gallon? &c.

Then of course they easily calculate the weight of any of these measures filled with a fluid, the specific gravity of which is given.

In aeriform bodies, common atmospheric air is taken as a standard instead of water, the weight of which is about one eight-hundredth part of the former; therefore, as a cubic foot of water weighs 1000 ozs., the weight of a foot of air will be 1000 or 1.25 oz.; ten feet will be 12.5 ozs., 100 feet 125 ozs., &c.; then having the specific gravities of other gaseous substances, some of which are heavier, some lighter than the atmosphere, they may be made to calculate the weights of given volumes.

800

The principle of the thermometer should be explainedhow it is made-how graduated-and how the freezing and boiling points are determined-why the tube is of a narrow bore, &c.

In the Boy's school here there is a barometer and a thermometer, which they are in the habit of observing; registering the height when they go in, and noticing the course of

its rise from increased temperature; this is registered three times a day, and a thermometer kept in the open air-the height of the barometer-the taking a weekly and a monthly average forms an exercise of their arithmetic.

Attention might be called as to how such averages of the thermometer are affected by swampy and marshy grounds of great extent-improved drainage*-how this is likely to affect the temperature of a district, so much so, even as to advance the period of harvest-how the height of the thermometer may be affected by particular aspects-whether the line of country slopes towards the north or south, or is a level plain, &c.

The subject of heat is one of great interest, and one on which the teacher may bring to bear a variety of experiments not attended with much expense, and having this additional 'recommendation that they have an intimate relation with many of the comforts and conveniences of life.

Heat is present everywhere and in every kind of matter: we cannot measure its quantity, but we can measure the quantity in one body relatively to that of another.

The general effect of heat upon matter is to expand it, that is, an increase of heat in the same body produces an increase of volume in some proportion to the increased temperature.

This increase of volume for a given increase of temperature varies in different kinds of matter; air and gases expand most, fluids next, and then solids.

Instances of each have been mentioned-as a full kettle swelling and flowing over just before it boils-a round piece of iron fitting exactly into a ring when cold, when heated is too large.

Then, again, heated bodies impart heat to everything around them until all have acquired the same temperature; as the heater of a box-iron, for ironing linen, when put into the fire becomes red-hot like the cinder-when taken out it is put into the box, communicates heat to it, and so to the linen; and, when used for a certain time, becomes of the same temperature with the things around it.

We call things which we touch hot or cold, according as they are hotter or colder than the human body, but in this the sense

* I was told by an experienced farmer in the county of Cambridge, that he believed the average period of harvest in that county was earlier by ten days, within the memory of man, owing to improved drainage.

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