Table II exhibits the correction for reducing to mean sea level, which is added to the reading, but it is convenient to construct a table for use at each station, which can be done as soon as the height of the cistern of the barometer is accurately known.
The temperature of the air indicated by the thermometer placed outside the house, the external thermometer, must be adopted in using this table. Under no circumstances would it be legitimate to adopt the temperature indicated by the attached thermometer.
8. But even when barometers are properly read off and corrected by aid of these tables, their readings will be generally found to differ more or less. It is therefore useless to register barometers whose index-corrections are not known. The index-correction to be applied to a standard barometer is generally constant at all pressures, but will in case of a Marine barometer be found variable along the scale.
The Hongkong Observatory undertakes to furnish free of cost corrections of barometers (including their attached thermometers) sent there by observers who regularly send their registers to the Government Astronomer.
9. A barometer should always be packed and carried cistern upwards, and the vernier should be brought to the bottom of the scale. It should be placed in a double case and surrounded with some soft and elastic matter, such as shavings, tow or paper cuttings. The inner case should be also surrounded with some such matter. The lids of the cases must be screwed, not nailed down, and marked: "Glass with care, to be kept flat or this end upwards." If a standard barometer is found to have leaked a little, it does not make any difference, but any leakage whatever in case of a Marine barometer introduces an error, in consequence of which a new index correction is required.
Before packing a Fortin's barometer the cistern screw is turned, till only a small vacant space is left in the tube to allow the mercury to expand without bursting it.
A wooden back is supplied with a standard barometer which is fixed to the wall at a height to suit the observer. It is better to hang a barometer a little too low than too high. The barometer is hung on a bracket, and it then assumes the vertical position. There are three screws at the bottom to fix it, but these should be occasionally unscrewed and refixed, to make sure that the barometer is still hanging truly vertical.
The Mariné barometer is fixed on an arm that is either screwed to the wall or slips into a bracket to be screwed to the wall.
III.-Thermometer.
1. With the thermometer we measure the temperature of the air or other media. It consists of a glass reservoir (the bulb), which is joined to a long glass tube of very small bore (the stem). The reservoir is filled with mercury, alcohol, or some other fluid, which also extends into the hollow stem. When the fluid expands with rising temperature, it is very perceptible in the tube, owing to its small bore. The greater the bulb and the finer the bore, the more sensitive is the thermometer other circumstances equal. The sensibility depends also to a great extent upon the area of the surface of the bulb, for if this is large, the fluid more quickly assumes the temperature of the surrounding medium. Thermometers with spherical bulbs are therefore often more sluggish than those with long cylindrical bulbs.
The stem is divided into degrees. All thermometers used by meteorological observers have the divisions etched on the stem.
There are two divisions fixed with reference to natural phenomena, viz., that at which ice melts and water freezes i.e., 32°.0 Fahrenheit, and that at which water boils at the average atmospheric pressure i.e., 212°.0 Fahrenheit. The scale between those two points is divided into 180 degrees.
In Celsius's thermometer the freezing point is marked 0° and boiling point 100°. In Reaumur's thermometer the freezing point is marked also 0° but boiling point 80°. It is easy to convert thermometer readings expressed in either way to Fahrenheit, but they are not used by meteorological observers in Great Britain and the Colonies.
2. The temperature is registered in degrees and tenths. It is easy to do this, but certain precautions must be taken. The thermometer must be read quickly, and the observer must not be too near, or it will rise owing to the heat radiated from him or from his lamp at night. The eye must be exactly on a level with the top of the column, if it is vertical, or in the vertical plane of the top of the column, if it is horizontal.
The top of the convexity is read in case of a mercurial thermometer, the lowest part of the concavity of the spirit column in case of a spirit thermometer.
3. The maximum thermometer registers the highest temperature attained since it was last set. Two forms of maximum thermometers are used by meteorological observers. In Phillip's maximum a small air-bubble is introduced into the column at from one to two inches from the top. When the mercury contracts with falling temperature the portion beyond the air-bubble is left behind, and records the highest temperature. In Negretti and Zambra's maximum the bore is bent and contracted just above the bulb, which causes the column to break at this point when the mercury begins to contract. It therefore registers the maximum temperature.
Table II exhibits the correction for reducing to mean sea level, which is added to the reading, but it is convenient to construct a table for use at each station, which can be done as soon as the height of the cistern of the barometer is accurately known.
The temperature of the air indicated by the thermometer placed outside the house, the external thermometer, must be adopted in using this table. Under no circumstances would it be legitimate to adopt the temperature indicated by the attached thermometer.
8. But even when barometers are properly read off and corrected by aid of these tables, their readings will be generally found to differ more or less. It is therefore useless to register barometers whose index-corrections are not known. The index-correction to be applied to a standard barometer is generally constant at all pressures, but will in case of a Marine barometer be found variable along the scale.
The Hongkong Observatory undertakes to furnish free of cost corrections of barometers (including their attached thermometers) sent there by observers who regularly send their registers to the Government Astronomer.
9. A barometer should always be packed and carried cistern upwards, and the vernier should be brought to the bottom of the scale. It should be placed in a double case and surrounded with some soft and elastic matter, such as shavings, tow or paper cuttings. The inner case should be also surrounded with some such matter. The lids of the cases must be screwed, not nailed down, and marked: "Glass with care, to be kept flat or this end upwards." If a standard barometer is found to have leaked a little, it does not make any difference, but any leakage whatever in case of a Marine barometer introduces an error, in consequence of which a new index correction is required.
Before packing a Fortin's barometer the cistern screw is turned, till only a small vacant space is left in the tube to allow the mercury to expand without bursting it.
A wooden back is supplied with a standard barometer which is fixed to the wall at a height to suit the observer. It is better to hang a barometer a little too low than too high. The barometer is hung on a bracket, and it then assumes the vertical position. There are three screws at the bottom to fix it, but these should be occasionally unscrewed and refixed, to make sure that the barometer is still hanging truly vertical.
The Mariné barometer is fixed on an arm that is either screwed to the wall or slips into a bracket to be screwed to the wall.
III.-Thermometer.
1. With the thermometer we measure the temperature of the air or other media. It consists of a glass reservoir (the bulb), which is joined to a long glass tube of very small bore (the stem). The reservoir is filled with mercury, alcohol, or some other fluid, which also extends into the hollow stem. When the fluid expands with rising temperature, it is very perceptible in the tube, owing to its small bore. The greater the bulb and the finer the bore, the more sensitive is the thermometer other circumstances equal. The sensibility depends also to a great extent upon the area of the surface of the bulb, for if this is large, the fluid more quickly assumes the temperature of the surrounding medium. Thermometers with spherical bulbs are therefore often more sluggish than those with long cylindrical bulbs.
The stem is divided into degrees. All thermometers used by meteorological observers have the divisions etched on the stem.
There are two divisions fixed' with reference to natural phenomena, viz., that at which ice melts and water freezes i. e., 32°.0 Fahrenheit, and that at which water boils at the average atmospheric pressure i. e., 212°.0 Fahrenheit. The scale between those two points is divided into 180 degrees.
In Celsius's thermometer the freezing point is marked 0° and boiling point 100°. In Reaumur's thermometer the freezing point is marked also 0° but boiling point 80°. It is easy to convert thermometer readings expressed in either way to Fahrenheit, but they are not used by meteorological observers in Great Britain and the Colonies.
2. The temperature is registered in degrees and tenths. It is easy to do this, but certain precautions must be taken. The thermometer must be read quickly, and the observer must not be too near, or it will rise owing to the heat radiated from him or from his lamp at night. The eye must be exactly on a level with the top of the column, if it is vertical, or in the vertical plane of the top of the column, if it is horizontal.
The top of the convexity is read in case of a mercurial thermometer, the lowest part of the concavity of the spirit column in case of a spirit thermometer.
3. The maximum thermometer registers the highest temperature attained since it was last set. Two forms of maximum thermometers are used by meteorological observers. In Phillip's maximum a small air-bubble is introduced into the column at from one to two inches from the top. When the mercury contracts with falling temperature the portion beyond the air-bubble is left behind, and records the highest temperature. In Negretti and Zambra's maximum the bore is bent and contracted just above the bulb, which causes the column to break at this point when the mercury begins to contract. It therefore registers the maximum temperature.
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