4. It is scarcely necessary to remark, that corrections should be applied to the damp bulb thermometer, the same as to the dry bulb, and both thermometers must be read to the nearest tenth of a degree. In case the dry bulb is read to only the nearest degree, it is not of much use to read off the damp bulb. The humidity of the atmosphere is then obtained by aid of tables. Those given by Blanford to accompany his "Indian Meteorologist's Vade-Mecum" seem the most convenient.
V.-Thermometer-Screen.
1. The correct determination of the temperature of the air is a most difficult problem. In the northern hemisphere, if we place a thermometer shaded by a screen within a certain distance to the south of a house, the result is generally that it reads too high, and on the other hand if placed to the north it will read too low. If we adopt an open screen, even if it protects the thermometers from the direct rays of the sun, other objects reflect heat towards them, and at times the rain will wet them and thus prevent their being properly registered. But if a closed screen is adopted, this if made of wood will absorb and retain a portion of the radiant heat. It will keep the thermometers cool in the morning and keep them reading too high at night. While a metal screen, when exposed to the rays of the sun during the day, may reach an abnormally high temperature.
2. It is recommended that in China a screen 2 feet long, 2 feet deep and 2 feet high be selected, the frame being made of wood, while the louvres (double) are made of zinc. The roof should be double. The inner roof should be made of zinc pierced with holes for ventilation. The screen should be painted white and repainted twice a year, it being of great importance that it is clean, so as to reflect light and heat.
The thermometers are fixed by screws on uprights. Their bulbs must be exactly four feet above the ground. The bulb of the minimum must be exactly on a level with the dry and damp bulb. The screen is placed on an open grass plot, and the grass should be kept trimmed short. If it is considered absolutely necessary, it may be furnished with a brass lock and key.
VI.-Rain-gauge.
1. With the rain-gauge we measure the depth of precipitation of dew, rain, snow, hail, &c. It consists of a funnel, whose circumference is exactly circular, and a receiving vessel. The diameter of the funnel is generally either 5 or 8 inches. If the funnel should meet with an accident, such as a blow or a squeeze, that puts it out of shape, it should be at once rejected. Copper rain-gauges are the best.
2. The rain is collected in a can through a spout, and is measured in inches, tenths and hundredths in a glass, which is graduated according to the diameter of the rain-gauge. The measuring glass is very narrow and magnifies the depth of rain, so that it can be correctly measured. The pouring from the receiving vessel into the measuring glass, requires some care, and should be done over a large dish, so as to catch any that may be spilled. The measuring glass of course cannot be used for a rain-gauge of different dimensions.
3. Judgment is required in selecting a proper site for a rain-gauge. It should be on the ground. It should not be placed on top of a house, as there are grave objections to roof rain-gauges. The gauge should preferably be situated on an open grass plot, as far as possible removed from buildings, trees and other objects, and care should be taken that no bushes or weeds are allowed to grow near it.
The funnel should be one foot above ground. Snow or hail are measured by thawing the quantity collected in the gauge.
The duration of precipitation in hours are to be registered according to the best ability of the observer.
VII. Wind.
1. A wind-vane consists of a balanced pointer, one end of which exposes a broad surface to the wind, while the other points towards the direction whence the wind is coming. To the rod on top of which the pointer revolves a cross is fixed, the arms of which indicate the four cardinal points, which must be adjusted by compass to true (not magnetic) north. The direction of the wind is easily estimated to within two points: N, NNE, NE, ENE, E, ESE, SE, SSE, S, SSW, SW, WSW, W, WNW, NW, NNW.
If the direction of the wind has changed much since last observation, it should be registered whether it has veered, i.e., changed with the sun, or backed, i.e., changed against the sun, or suddenly shifted.
The place selected should be as high and as exposed to the wind as possible, but the vane requires to be frequently oiled, and even then the direction of a light breeze is better ascertained by noting the direction from which smoke from a high chimney is carried, or by aid of a light piece of cambric fixed on a pole.
4. It is scarcely necessary to remark, that corrections should be applied to the damp bulb thermometer, the same as to the dry bulb, and both thermometers must be read to the nearest tenth of a degree. In case the dry bulb is read to only the nearest degree, it is not of much use to read off the damp bulb. The humidity of the atmosphere is then obtained by aid of tables. Those given by Blanford to accompany his "Indian Meteorologist's Vade-Mecum" seem the most convenient.
V.-Thermometer-Screen.
1. The correct determination of the temperature of the air is a most difficult problem. In the northern hemisphere, if we place a thermometer shaded by a screen within a certain distance to the south of a house, the result is generally that it reads too high, and on the other hand if placed to the north it will read too low. If we adopt an open screen, even if it protects the thermometers from the direct rays of the sun, other objects reflect heat towards them, and at times the rain will wet them and thus prevent their being properly registered. But if a closed screen is adopted, this if made of wood will absorb and retain a portion of the radiant heat. It will keep the thermometers cool in the morning and keep them reading too high at night. While a metal screen, when exposed to the rays of the sun during the day, may reach an abnormally high temperature.
2. It is recommended that in China a screen 2 feet long, 2 feet deep and 2 feet high be selected, the frame being made of wood, while the louvres (double) are made of zinc. The roof should be double. The inner roof should be made of zinc pierced with holes for ventilation. The screen should be painted white and repainted twice a year, it being of great importance that it is clean, so as to reflect light and heat.
The thermometers are fixed by screws on uprights. Their bulbs must be exactly four feet above the ground. The bulb of the minimum must be exactly on a level with the dry and damp bulb. The screen is placed on an open grass plot, and the grass should be kept trimmed short. If it is considered absolutely necessary, it may be furnished with a brass lock and key.
VI.-Rain-gauge.
1. With the rain-gauge we measure the depth of precipitation of dew, rain, snow, hail, &c. It consists of a funnel, whose circumference is exactly circular, and a receiving vessel. The diameter of the funnel is generally either 5 or 8 inches. If the funnel should meet with an accident, such as a blow or a squeeze, that puts it out of shape, it should be at once rejected.-Copper rain-gauges are the best.
2. The rain is collected in a can through a spout, and is measured in inches, tenths and hundredths in a glass, which is graduated according to the diameter of the rain-gauge. The measuring glass is very narrow and magnifies the depth of rain, so that it can be correctly measured. The pouring from the receiving vessel into the measuring glass, requires some care, and should be done over a large dish, so as to catch any that may be spilled. The measuring glass of course cannot be used for a rain-gauge of different dimensions.
3. Judgment is required in selecting a proper site for a rain-gauge. It should be on the ground. It should not be placed on top of a house, as there are grave objections to roof rain-gauges. The gauge should preferably be situated on an open grass plot, as far as possible removed from buildings, trees and other objects, and care should be taken that no bushes or weeds are allowed to grow near it.
The funnel should be one foot above ground. Snow or hail are measured by thawing the quantity collected in the gauge.
The duration of precipitation in hours are to be registered according to the best ability of the observer.
VII. Wind.
1. A wind-vane consists of a balanced pointer, one end of which exposes a broad surface to the wind, while the other points towards the direction whence the wind is coming. To the rod on top of which the pointer revolves a cross is fixed, the arms of which indicate the four cardinal points, which must be adjusted by compass to true (not magnetic) north. The direction of the wind is easily estimated to within two points: N, NNE, NE, ENE, E, ESE, SE, SSE, S, ssw, sw, WSW, W, WNW, NW, NNW.
If the direction of the wind has changed much since last observation, it should be registered whether it has veered, ie., changed with the sun, or backed, i.e., changed against the sun, or suddenly shifted.
The place selected 'should be as high and as exposed to the wind as possible, but the vane requires to be frequently oiled, and even then the direction of a light breeze is better ascertained by noting the direction from which smoke from a high chimney is carried, or by aid of a light piece of cambric fixed on a pole.
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