CONVERSATION XIII.

Of the Methods of finding the Specific Gravity of Bodies.

CHARLES. I have endeavoured to find out the specific gravity of this piece of beech-wood; but, as it will not sink in the water, I know not how to do it.

Father. It is true, that we have hitherto only given rules for the finding of the specific gravity of bodies that are heavier than water; a little consideration, however, will show you how to obtain the specific gravity of the beech. Can you con trive means to sink the beech in the water

Charles. Yes; if I join a piece of lead, or other metal, to the wood, it will sink.

Father. The beech weighs 660 grains; I will annex to it an ounce, or 480 grains of tin, which in water loses of its weight 51 grains. In air the weight of the wood and metal taken together is 1140 grains: but in water they weigh but 138 grains; 138 taken from 1140 leave 1002, the difference between the weights in air and in

water.

Charles. I now see the mode of finding what I want. The whole mass loses 1002 grains by immersion, and the tin by itself lost in water 51 grains; therefore the wood lost 951 grains of its weight by im mersion and 660 grains, the weight of the beech in air, divided by 951, which it may be said to lose by immersion, leaves in decimals for a quotient 694.

Father. Then making water the standard equal to 1, the beech is 694, or nearly ths of 1: that is, a cubic foot of water is to a cubic foot of beech as 1000 to 694,

for the one weighs 1000 ounces, and the other 694 ounces.

Emma. It seems odd how a piece of wood that weighs about 660 grains in air, should lose of its weight 951 grains.

Father. You must, in this case, consider the weight necessary to make it sink in water, which must be added to the weight of the wood.

I will now endeavour to make the subject easier by a different method.

This small piece of elm a, I will place between the tongs (Plate III. Fig. 22.) that are nicely balanced on the beam, (Fig. 20.) The elm weighs 36 grains. To detain it under water, I must hang 24 grains to the end of the lever on which the tongs are fixed: then, by the Rule of Three, I say, as the specific gravity of the elm is to the specific gravity of water, so is 36, the weight of the elm, to 60, the weight of the elm and the additional weight required to sink it in water, or as 60: 36: S. G. W. S. G. E.

Emma. You have not obtained the specific gravity of the elm, but a proportion only.

Charles. But three terms are given, because the water is always considered as unity or 1, therefore the specific gravity of the elm is

36X1

=.6.

60

Emma. I do not yet comprehend the reason of the proportion assumed.

Father. It is very simple. The elm is lighter than the water, but by hanging weights to the side of the balance, to which it is attached in order to detain it just under water, I make the whole exactly equal to the specific gravity of the water; by this means it is evident, that the comparative gravity of the elm is to that of the

water as 36 to 60.

Try this piece of cork in the same man

ner.

Emma. It weighs

an ounce, or 240

grains, in air; and to detain the cork and

tongs just under water, I am obliged to hang 2 ounces, or 960 grains, of lead on the lever therefore the specific gravity of

:

the cork is to that of the water as 240 is to 1200; and 240 divided by 1200 gives the decimal .2.

Father. Then the specific gravity of water is 5 times greater than that of cork.

Charles. We have accordingly obtained the specific gravities of water, beech, elm, and cork, which are as 1, .7 nearly .6 and .2.

Father. You now understand the methods of obtaining the specific gravity of all solids, whether lighter or heavier than water. In making experiments upon light and porous woods, the operations must be performed as quickly as possible, to prevent the water from getting into the pores.

Charles. And you have likewise shown us a method of getting the specific gravity of fluids, by weighing certain quantities of each.

Father. I have still a better method: