and you get the answer to the question in pounds avoirdupois."

The earth's surface contains about 200,000,000 square miles: and as every square mile contains 27,876,400 square feet there must be 5,575,280,000,000,000

square

feet in the earth's surface, which number multiplied by the pressure on each square foot gives the whole weight of the atmosphere.

Charles. This is truly enormous! Father. But the pressure being equal, in all possible directions, it has no effect in disturbing either the annual or diurnal motion of the earth.

CONVERSATION XXI.

Of the Thermometer.

FATHER. As the barometer is intended to measure the different degrees of density of the atmosphere, so the thermometer is designed to mark the changes in its temperature, with regard to heat and cold.

Emma. Is there any difference between the thermometer that is attached to the barometer, and that which hangs out of doors?

Father. No; they are both made by the same person, and are intended to show the same effects. But for

the purposes of accurate observation, it is usual to have two instruments, one attached to, or near the barometer, and the other out of doors, to which neither the direct nor reflected rays of the sun should ever come. Though my thermometers are both of the same construction, and such as are principally used in this country, yet there are others made of different materials and upon different principles.

Charles. Does not this thermometer consist of mercury enclosed in a glass tube which is fixed to a graduated frame?

Father. That is the construction of Fahrenheit's thermometer: but when these instruments were first invented about 200 years ago, air, water, spirits of wine, and then oil,

were made use of; but these have given way to quicksilver, which is considered as the best of all the fluids, being highly susceptible of expansion and contraction, and capable of exhibiting a more extensive scale of heat. Fahrenheit's thermometer is chiefly used in Great Britain, and Reaumur's on the Continent.

Emma. Is not this the principle of the thermometer, that the quicksilver expands by heat and contracts by cold?

1

Father. It is: place your thumb on the bulb of the thermometer. Emma. The quicksilver gradually

rises.

Father. And it will continue to rise till the mercury and your thumb are of equal heat. Now you have taken away your hand, you perceive.

the mercury is falling as fast as it

rose,

Charles. Will it come down to the same point at which it stood before Emma touched it?

Father. It will, unless, in this short space of time, there has beep any change in the surrounding air. Thus the thermometer indicates the temperature of the air, or, in fact, of any body with which it is in contact. Just now it was in contact with your thumb, and it rose in the space of a minute or two from 56° to 62°; had you held it longer on it the mercury would have risen still higher. It is now falling. Plunge it into boiling water *

This should be done very gradually, by holding it some time in the steam, to prevent its breaking by the sudden heat.