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But for the purposes of

the same effects. , 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 differ ent 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 two hundred 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 expan sion and contraction, and capable of exhibiting a more extensive scale of heat.

ahrenheit's thermometer is chiefly used in reat Britain, and Reaumur's on the Con

nent.

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

Father. It is: place your thumb on e bulb of the thermometer.

Emma. The quicksilver gradually rises. Father. And it will continue to rise Il the mercury and your thumb are of qual heat. Now you have taken away our hand, you perceive the mercury is alling as fast as it rose.

Charles Will it come down to the same oint at which it stood before Emma touchd it?

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Father. It will, unless, in this short pace of time, there has been any change n the surrounding air. Thus the thernometer indicates the temperature of the air, or, in fact, of any body with which it s 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,* and you will find that the mercury rises to 212°. Afterwards you may, when it is cool, place it in ice, in its melting state, and it will fall to 32°.

Emma. Why are these particular num bers pitched on ?

Father. You will not perhaps be satis fied if I tell you, that the only reason why 212 was fixed on to mark the heat of boiling water, and thirty-two that to show the freezing point, was, because it so pleased M. Fahrenheit: this, however, was the case.

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

the sudden heat.

Charles. I can easily conceive that at he same degree of cold, water will always begin to freeze; but surely there are diferent degrees of heat in boiling water, and herefore it should seem strange to have only one number for it.

Father. In an open vessel, boiling water s always of the same heat, that is, provided the density of the atmosphere be the same; and though you increase your fire in a tenfold proportion, yet the water will never be a single degree hotter; for the superabundant heat, communicated to the water, flies off in the form of steam or vapour.

Emma.

steam.

But suppose you confine the

Father. Before I should attempt this I must be provided with a strong vessel, or, as you have seen under the article of the steam-engine, it would certainly burst. But in a vessel proper for the purpose, water has been made so hot as to melt solid lead. Charles. Will you explain the construction of the thermometer?

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Father. A B (Plate v11. Fig. 29.) represents a glass tube, the end a is blown into a bulb, and this, with a part of the tube, are filled with mercury. In good thermometers, the upper part of the tube approaches to a perfect vacuum, and of course the end B is hermetically sealed. If the tube be now placed in pounded ice, the mercury will sink to a certain point x, which must be marked on the tube, and on the scale opposite to this point 32 must be placed, which is called the freezing point. Then let it be immersed in boiling water, the mercury will rise, and, after a few minutes become stationary. Against that point make another mark, and write on the scale 212 for the heat of boiling water. Between these points let the scale be divided into 180 equal parts.

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Emma. Why 180 parts?

Father. Because you begin from 32, and you subtract that number from 212, the remainder will be 180. Also below 32, and above 212, set off more

divisions on the

scale, equal to the others. The scale is fi

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