Graduation of the Manometer.—The graduation of this instrument consists in dividing the tube where the air is to be compressed, into a given number of parts of equal capacity; but as, in general, such tubes are employed as are nearly capillary and very regular, the operation is reduced to a linear division, where every degree occupies an equal space.

Graduation of Thermometers. Construction of Standard Thermometers.—Having constructed your instrument with a very regular tube, or one which has been divided into parts of equal capacity, and having filled it with the proper liquid, according to the instructions given in a preceding section, the graduation is to be effected as follows. Procure very pure ice, break it into small pieces, and fill a vessel with it. When the ice begins to melt, plunge the thermometer into the middle of it, in such a manner that, without touching the sides of the vessel, the whole thermometer, or at least that part of it which contains the liquid, may be covered with ice. Allow the instrument to remain in this state until, in spite of the gradual melting of the ice, the surface of the column of liquid remains at a fixed point, and neither falls nor rises. Mark this point very carefully on the stalk of the thermometer, either with a thread or a little drop of sealing-wax, or with the trace of a diamond or a flint. This is the freezing point, the zero of the centigrade scale, the thirty-second degree of Fahrenheit’s scale.

As for the second fixed point, it is marked during an experiment with boiling water, performed as follows:—You employ a vessel of tin plate sufficiently high to enclose the whole thermometer; you pour into this vessel distilled water, till it is about an inch deep, and then you heat it. The vessel is surmounted by a cover pierced with two holes, one of which is intended to receive the stalk of the thermometer, the other to allow the steam to escape. When, on continuing the ebullition, you observe that the mercury ceases to rise in the tube, you mark the point at which it has stopped, just as you marked the first point. The last mark indicates the boiling point; the one hundredth degree of the centigrade scale, the two hundred and twelfth degree of Fahrenheit’s scale. You transfer to paper the distance which is found between the first point and the second point determined, and you divide this distance into one hundred equal parts, or degrees, for the centigrade thermometer, into eighty parts for the thermometer of Réaumur, and into one hundred and eighty for that of Fahrenheit. If the tube of the instrument is very regular in the bore, the degrees should be equal in length; if, on the contrary, you have been obliged to divide it into parts of equal capacity, you find how many of these parts or little spaces it is necessary to take to constitute one of the above degrees. You find this by dividing their whole number by 100, or 80, or 180, according to the degrees of the scale which you intend to make use of. Thus, if you find between the two points fixed by melting ice and boiling water, three hundred divisions of equal capacity, it is necessary to include three of these divisions in every degree of the centigrade scale.

The vessel employed to take the boiling point must be of metal, and its surface should be perfectly clean and well polished, and have no rough points. If sand, or other matters, were permitted to repose on the vessel, and to form asperities, the water would enter into ebullition at an inferior temperature.

This operation should, moreover, be performed under an atmospherical pressure, which is indicated by the barometer when the mercury stands at twenty-nine inches and a half. But as this pressure is different according to the elevation of the place of operation, and, indeed, suffers continual variations even in the same place, it follows that the temperature of boiling water is subject to continual changes, and that, in the graduation of the thermometer, it is indispensably necessary to take notice of the height of the barometer at the very moment that the point denoting the degree of boiling-water is fixed upon. You succeed in making the necessary corrections by the help of the following table, which is founded on the experiments of Sir G. Shuckburg and of the Committee of the Royal Society.

[See the Table on the opposite page.]

Common Thermometers.—Having, by the method which we have just described, obtained a Standard Thermometer, you may procure with facility as many ordinary thermometers as you desire. It is proper to employ the most regular tubes which you can obtain, and when the instruments are ready to be graduated, you must bring them into comparison with your standard thermometer. You place them together into a liquid of which you gradually raise the temperature, and you mark several points on the scale of the new thermometer, the intervals between which are subsequently divided into as many degrees as are marked on the scale of the standard thermometer. Thus, for example, you mark the 10° and 15°, and afterwards divide the interval into five equal parts. This gives you the length of a degree on the stalk of the new instrument. The more you multiply these fixed points, the more you insure the precision of the thermometer. When you have taken a certain number of points, you measure the remainder with the compasses.

Height of the Barometer in Inches.Correction in
1000ths of the
interval between
the freezing
and boiling points
of Water.
When the boiling
point is found
by immersing
the Instrument
in Steam.		When the boiling
point is found
by immersing
the Instrument
in Water.
...30.6010Lower.
...30.509
30.7130.418
30.5030.297
30.4830.186
30.3730.075
30.2530.954
30.1430.843
30.0330.732
29.9130.611
29.8030.500
29.6929.391Higher.
29.5829.282
29.4729.173
29.3629.064
29.2528.955
29.1428.846
29.0328.737
28.9228.628
28.8128.519
28.7010
The boiling point to be
marked so much higher or
lower than the stand of
the mercury during the
experiment.