descriptive of this instrument, and which include a just compliment to the memory of Torricelli and Boyle, both of whom are celebrated for their discoveries in this part of science :

You charm'd, indulgent SYLPHS! their learned toil,

And crown'd with fame your TORRICELL and BOYLE;

Taught with sweet smiles, responsive to their

pray❜r,

The spring and pressure of the viewless air; -How up exhausted tubes bright currents flow Of liquid silver from the lake below;

Weigh the long column of th' incumbent skies, And with the changeful moment fall and rise. BOTANICAL GARDEN,

CONVERSATION XX.

Of the Barometer, and its Application to the measuring of Altitudes.

CHARLES. In those lines you gave us to learn, Dr. Darwin says, "Weigh the long column of the incumbent skies:" is the height of the atmosphere known?

Father. If the fluid air were similar to water, that is, every where of the same density, nothing would be easier than to calculate its height. When the barometer stands at 30 inches, the specific gravity of the atmosphere is 800 times less than

that of water*; but mercury is about 14 times heavier than water, consequently the specific gravity of mercury is to that of air as 800 multiplied by 14 is to 1; or mercury is 11,200 times heavier than air. In the case before us, a column of mercury, 30 inches long, balances the whole weight of the atmosphere; therefore, if the air were equally dense at all heights to the top, its height must be 11,200 times 30 inches; that is, the column of air must be as much longer than that of the mercury, as the former is lighter than the latter. Do you understand me?

Charles. I think I do: 11,200 multiplied by 30 give 336,000 inches, which are equal to 5 miles nearly.

* See Conversation VI, of this volume,

Father. That would be the height of the atmosphere if it were equally dense in all parts: but it is found that the air, by its elastic quality, expands and contracts, and that at 3 miles above the surface of the earth it is twice as rare as it is at the surface; that at 7 miles it is 4 times rarer; at 10 miles it is 8 times rarer; at 14 miles it is 16 times rarer; and so on, according to the following

Now, if you were disposed to carry on the addition on one side, and the multiplication on the other, you would find that, at 500 miles above the surface of the earth, a single cubical inch of such air as we breathe, would be so much rarefied as to fill a hollow sphere, equal in diameter to the vast orbit of the planet Saturn.

Emma. Is it inferred from this that the atmosphere does not reach to any very great height?

Father. Certainly; for you have seen that a quart of air at the earth's surface weighs but about 14 or 15 grains; and by carrying on the above table a few steps, you would perceive, that the same quantity, only 49 miles high, would weigh less than the 16 thousandth part of 14 grains, consequently at that height its density