But spirit thermometers, especially in cold weather, are liable to have not only their column broken, but to have a part of the spirit evaporated and deposited at the top of the tube. Spirit thermometers should therefore be examined every day to ascertain that there is not a drop of spirit anywhere detached from the column. The drop is sometimes hidden by the brass band that supports the upper part of the tube. The defect is cured either by holding the thermometer vertically in the hand and shaking it, or by grasping it in the right hand by the upper end, holding the bulb downwards, and swinging it quickly round, when the centrifugal force will cause the spirit to leave the top, or by cooling the bulb and heating the upper part of the tube, when the drop is distilled over. The defect is likely to occur again if the thermometer, after being repaired, is not left standing vertically, bulb downwards, for several hours.
If the black colour of the graduations is washed out, it may be restored by rubbing lamp-black into them. The tube is easily cleaned by rubbing the finger along it. The graduations may also be blackened by rubbing a lead pencil along the tube, but the graphite is soon rubbed out again, especially in wet weather.
IV.-Psychrometer
1. With the psychrometer we measure the humidity of the atmosphere. This is expressed by stating the quantity of vapour present in the air as a percentage of the greatest amount of water vapour that the air could contain at the existing temperature. The quantity of water vapour that can exist in a certain space, is sensibly the same, whether any other gas is present or not. This quantity increases with the temperature. The temperature of air containing water vapour occasionally falls below the lowest temperature at which it can contain the amount of vapour present. The excess of vapour is then condensed and deposited as dew. Hence that lowest temperature is termed "the dew-point.” The barometrical pressure exercised by the vapour present in the atmosphere is termed "the elastic force of aqueous vapour."
The psychrometer consists of two thermometers. One of them, the dry bulb, indicates the temperature of the air and has been described in the preceding paragraph. The other, the damp bulb, is covered by a damp piece of fine muslin, and indicates a lower temperature. It is well known that cold is produced by evaporation, and the quicker the evaporation the greater the cold. This is why wet clothes feel cold to the touch. Now therefore the difference between the dry bulb and the damp bulb thermometer at a certain temperature will be greater the less water vapour the air contains, because the cloth dries at a more rapid rate.
2. The damp bulb thermometer should be covered with only one thickness of the finest muslin or cambric. Care must be taken, that the whole of the bulb is covered, and that the cloth fits it tightly. Close above the bulb it should be tied with fine sewing thread and a few threads of lamp-wick passed round the tube at the same place touching the top of the cloth. The wick should be three or four inches long, and should be immersed into and remain in a small glass bottle with narrow neck filled with water and placed to one side (that farthest from the dry bulb) of the damp bulb and one inch below it.
3. The damp bulb thermometer requires more attention than the other instruments. In the hands of ignorant or careless observers its indications are only misleading. The water in the bottle should be clean rain water, distilled water, or at any rate well boiled and carefully decanted spring water. But spring water is seldom pure enough even when boiled. It leaves a crust on the bulb, which has to be removed with nitric acid or spirits of salt, because the indications of a thermometer with an encrusted bulb are untrustworthy.
The observer should look at the damp bulb thermometer some time before it is to be read, and satisfy himself that it is damp, but he must take care not to touch the bulb with his finger, or grease will be deposited upon the cloth, which will then not soak up the water. It may be touched with a small camel hair brush. It must not be too wet, or it will indicate the temperature of the water, which is the same as that of the air.
The cloth is changed once a week and the wick washed in pure rain water. It should be entered in the register that the cloth has been changed. A new piece of muslin that does not properly absorb the water should be rejected and another piece put on.
In case of frost the damp bulb should be covered with a rather thick film of ice. It should be repeatedly dipped in water half an hour before it is to be read, so as to secure its being covered with ice at the time of reading.
If the damp bulb is not in good order at the proper time, the observer should read the dry bulb alone, and when afterwards it has been put right, he may enter in the register in the damp bulb column the dry bulb and the damp bulb read together. The difference between those readings subtracted from the reading of the dry bulb at the time the instruments ought to have been read off, will give a very near approximation to what the damp bulb would have then indicated.
A good plan is to wet the damp bulb some time before observing, and to note the lowest point to which it descends while drying. It is this lowest reading that is to be adopted in determining the humidity.
But spirit thermometers, especially in cold weather, are liable to have not only their column broken, but to have a part of the spirit evaporated and deposited at the top of the tube. Spirit thermometers should therefore be examined every day to ascertain that there is not a drop of spirit anywhere detached from the column. The drop is sometimes hidden by the brass band that supports the upper part of the tube. The defect is cured either by holding the thermometer vertically in the hand and shaking it, or by grasping it in the right hand by the upper end, holding the bulb downwards, and swinging it quickly round, when the centrifugal force will cause the spirit to leave the top, or by cooling the bulb and heating the upper part of the tube, when the drop is distilled over. The defect is likely to occur again if the thermometer, after being repaired, is not left standing vertically, bulb down- wards, for several hours.
If the black colour of the graduations is washed out, it may be restored by rubbing lamp-black into them. The tube is easily cleaned by rubbing the finger along it. The graduations may also be blackened by rubbing a lead pencil along the tube, but the graphite is soon rubbed out again, especially in wet weather.
IV.-Psychrometer.
1. With the psychrometer we measure the humidity of the atmosphere. This is expressed by stating the quantity of vapour present in the air as a percentage of the greatest amount of water vapour that the air could contain at the existing temperature. The quantity of water vapour that can exist in a certain space, is sensibly the same, whether any other gas is present or not. This quantity increases with the temperature. The temperature of air containing water vapour occasionally falls below the lowest temperature at which it can contain the amount of vapour present. The excess of vapour is then condensed and deposited as dew. Hence that lowest temperature is termed "the dew-point.” The barometrical pressure exercised by the vapour present in the atmosphere is termed "the elastic force of aqueous vapour."
The psychrometer consists of two thermometers. One of them, the dry bulb, indicates the temperature of the air and has been described in the preceeding paragraph. The other, the damp bulb, is covered by a damp piece of fine muslin, and indicates a lower temperature. It is well known that cold is produced by evaporation, and the quicker the evaporation the greater the cold. This is why wet clothes feel cold to the touch. Now therefore the difference between the dry bulb and the damp bulb thermometer at a certain temperature will be greater the less water vapour the air contains, because the cloth dries at a more rapid rate.
2. The damp bulb thermometer should be covered with only one thickness of the finest muslin or cambrio. Care must be taken, that the whole of the bulb is covered, and that the cloth fits it tightly. Close above the bulb it should be tied with fine sewing thread and a few threads of lamp-wick passed round the tube at the same place touching the top of the cloth. The wick should be three or four inches long, and should be immersed into and remain in a small glass bottle with narrow neck filled with water and placed to one side (that farthest from the dry bulb) of the damp bulb and one inch below it.
3. The damp bulb thermometer requires more attention than the other instruments. In the hands of ignorant or careless observers its indications are only misleading. The water in the bottle should be clean rain water, distilled water, or at any rate well boiled and carefully decanted spring water. But spring water is seldom pure enough even when boiled. It leaves a crust on the bulb, which has to be removed with nitric acid or spirits of salt, because the indications of a thermometer with an encrusted bulb are untrustworthy.
The observer should look at the damp bulb thermometer some time before it is to be read, and satisfy himself that it is damp, but he must take care not to touch the bulb with his finger, or grease will be deposited upon the cloth, which will then not soak up the water. It may be touched with a small camel hair brush. It must not be too wet, or it will indicate the temperature of the water, which is the same as that of the air.
The cloth is changed once a week and the wick washed in pure rain water. It should be entered in the register that the cloth has been changed. A new piece of muslin that does not properly absorb the water should be rejected and another piece put on.
In case of frost the damp bulb should be covered with a rather thick film of ice. It should be repeatedly dipped in water half an hour before it is to be read, so as to secure its being covered with ice at the time of reading.
If the damp bulb is not in good order at the proper time, the observer should read the dry bulb alone, and when afterwards it has been put right, he may enter in the register in the damp bulb column the dry bulb and the damp bulb read together. The difference between those readings subtracted from the reading of the dry bulb at the time the instruments ought to have been read off, will give a very near approximation to what the damp bulb would have then indicated.
A good plan is to wet the damp bulb some time before observing, and to note the lowest point to which it descends while drying. It is this lowest reading that is to be adopted in determining the humidity.
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