In fig. 2 the pointer gives the scale reading 29.85. The first long line coincides with a scale division. Its value 0.01 must be added to 29.850, and 29.860 is therefore the reading.
If none of the vernier lines coincides with a scale line, but there is one a little above it and the next a little below it, 0.001 is to be added to that which would give the lower reading. Thus the reading in fig. 1 is more correctly 29.885.
On board ship and in lighthouses, as well as for telegrams, it is sufficient to read the hundredths of an inch, and then it is necessary only to note on the vernier the long line nearest to that which coincides. In the second example above, 29.86 would be adopted as the reading.
3. Mercury expands considerably with an increase of temperature. A column of mercury of a certain height therefore indicates a lower atmospheric pressure if the temperature at the time of observation is high, than would be indicated by the same height of the barometer if the temperature were lower. It therefore becomes necessary to reduce barometer readings to a certain standard temperature, for which 32° Fahrenheit is generally chosen. This is done by aid of Table I. A thermometer is attached to the barometer, which should be registered immediately before the pointers on the barometer are adjusted. The barometer should be placed in the shade in a room, but not near a fireplace.
4. Standard barometers are constructed of two forms, either with an adjustable rod, in which case the ivory point is raised or lowered until it just touches the surface of the mercury in the cistern, or with a fixed rod (Fortin's barometer), in which case the cistern consists of a chamois leather bag, which is raised by aid of a screw acting against a piece of wood, that supports the mercury bag. Some time before the reading is to be taken, the mercury should be lowered beneath the ivory point, so that it is always raised when pointing the barometer.
It is of importance that the barometer be placed in a favourable position with regard to reflected light. When the index is pointed, it should, together with its reflected image, present the appearance of a double cone. After reading a Fortin's barometer the observer should satisfy himself, that the index is still properly pointed. In a surface of dusty and oxidised mercury it is difficult to perceive the reflected image, but except the observer happens to be accustomed to the manipulation of physical instruments, he had better not undertake to clean it.*
5. In the Marine barometer (Fitzroy's barometer) the cistern is closed, and the rod cannot be adjusted, but this is rendered superfluous, as the divisions on the scale have been reduced in proportion, so as to effect the necessary correction.
It is hung in gimbals so as to remain always vertical. If a barometer does not hang truly vertical, the readings are always in excess. The Marine barometer, while being pointed, must be free to keep the vertical position. After it is pointed, while the careful observer assures himself that the pointer on the vernier is a tangent to the surface of the mercury the barometer should not even be touched with the hand.
To obviate the incessant "pumping" on board ship, i.e., the rising and falling of the mercury, when the vessel pitches or rolls, the middle of the tube is contracted, which greatly lessens the motion of the mercury. The same may happen to a barometer on shore, when the wind is high, if it is not fixed to a firm wall. The mean between the highest and the lowest points of the oscillation should be entered.†
Owing to its simple construction and the facility with which it is read, the Marine barometer is highly recommended for use in lighthouses and in harbours. The attached thermometer is of course registered immediately before the barometer is pointed.
6. Table 1 exhibits the correction to be applied to barometers with brass scales, to reduce the readings to 32° Fahrenheit. The vertical columns exhibit the correction corresponding to the nearest third of an inch of the reading of the barometer for each half degree indicated by the attached thermometer. The correction is obtained by passing the eye down the vertical column till the figure opposite the nearest half degree of the thermometer is reached. The tables extend from 29° to 102°. The correction is given in thousandths of an inch, and should be subtracted from the reading. Below 29° the correction is additive, but is not here exhibited.
7. When a barometer is read at various heights above the ground, the column sinks as the height increases. In order to compare readings of barometers in different places, they must all be reduced to mean sea level. It is therefore necessary to know the height above mean sea level at which the cistern of the barometer is placed.
* Some common barometers have a closed cistern, so that it is impossible to adjust the rod. The divisions are fixed on the wooden side of the barometer, so that the temperature correction cannot be applied, the tables being computed on the supposition that the rod is of brass. The cistern is frequently too small to hold the mercury when the barometer falls low, so that it cannot fall under a certain height. It is evidently useless to read off such a barometer. It can be of no use.
+ If the tube is very much contracted, which becomes necessary for use on men of war, where heavy firing is going on, sluggishness is caused thereby. To examine the Marine barometer for this defect, read it off, fix the vernier at one inch and a half above the reading, slant it, so that the mercury rises above the vernier, allow it to resume the vertical position, and note the instant, when the mercury in its fall passes the vernier, then lower the vernier one inch (when it will be still half an inch higher than the original reading) and note the instant the mercury passes it again. If the interval (the time it takes the mercury to fall one inch) is less than three minutes, it is fit for use on shore.
}
In fig. 2 the pointer gives the scale reading 29.85. The first long line coincides with a scale division. Its value 0.01 must be added to 29.850, and 29.860 is therefore the reading.
If none of the vernier lines coincides with a scale line, but there is one a little above it and the next a little below it, 0.001 is to be added to that which would give the lower reading. Thus the reading in fig. 1 is more correctly 29.885.
On board ship and in lighthouses, as well as for telegrams, it is sufficient to read the hundredths of an inch, and then it is necessary only to note on the vernier the long line nearest to that which coincides. In the second example above, 29.86 would be adopted as the reading.
3. Mercury expands considerably with an increase of temperature. A column of mercury of a certain height therefore indicates a lower atmospheric pressure if the temperature at the time of observation is high, than would be indicated by the same height of the barometer if the temperature were lower. It therefore becomes necessary to reduce barometer readings to a certain standard temperature, for which 32° Fahrenheit is generally chosen. This is done by aid of Table I. A thermometer is attached to the barometer, which should be registered immediately before the pointers on the barometer are adjusted. The barometer should be placed in the shade in a room, but not near a fireplace.
4. Standard barometers are constructed of two forms, either with an adjustable rod, in which case the ivory point is raised or lowered until it just touches the surface of the mercury in the cistern, -or with a fixed rod (Fortin's barometer), in which case the cistern consists of a chamois leather bag, which is raised by aid of a screw acting against a piece of wood, that supports the mercury bag. Some time before the reading is to be taken, the mercury should be lowered beneath the ivory point, so that it is always raised when pointing the barometer.
It is of importance that the barometer be placed in a favourable position with regard to reflected light. When the index is pointed, it should, together with its reflected image, present the appearance of a double cone. After reading a Fortin's barometer the observer should satisfy himself, that the index is still properly pointed. In a surface of dusty and oxidised mercury it is difficult to perceive the reflected image, but except the observer happens to be accus- tomed to the manipulation of physical instruments, he had better not undertake to clean it.*
5. In the Marine barometer (Fitzroy's barometer) the cistern is closed, and the rod cannot be adjusted, but this is rendered superfluous, as the divisions on the scale have been reduced in proportion, so as to effect the necessary correction.
It is hung in gimbals so as to remain always vertical. If a barometer does not hang truly vertical, the readings are always in excess. The Marine barometer, while being pointed, must be free to keep the vertical position. After it is pointed, while the careful observer assures himself that the pointer on the vernier is a tangent to the surface of the mercury the barometer should not even be touched with the hand.
To obviate the incessant "pumping" on board ship, ie. the rising and falling of the mercury, when the vessel pitches or rolls, the middle of the tube is contracted, which greatly lessens the motion of the mercury. The same may happen to a barometer on shore, when the wind is high, if it is not fixed to a firm wall. The mean between the highest and the lowest points of the oscillation should be entered.†
Owing to its simple construction and the facility with which it is read, the Marine barometer is highly recommended for use in lighthouses and in harbours.--The attached thermometer is of course registered immediately before the barometer is pointed.
6. Table 1 exhibits the correction to be applied to barometers with brass-scales, to reduce the readings to 32° Fahrenheit. The vertical columns exhibit the correction corresponding to the nearest third of an inch of the reading of the barometer for each half degree indicated by the attached thermometer. The correction is obtained by passing the eye down the vertical column till the figure opposite the nearest half degree of the thermometer is reached. The tables extend from 29° to 102°. The correction is given in thousandths of an inch, and should be subtracted from the reading. Below 29° the correction is additive, but is not here exhibited.
7. When a barometer is read at various heights above the ground, the column sinks as the height increases. In order to compare readings of barometers in different places, they must all be reduced to mean sea level. It is therefore necessary to know the height above mean sea level at which the cistern of the barometer is placed.
* Some common barometers have a closed cistern, so that it is impossible to adjust the rod. The divisions are fixed on the wooden side of the barometer, so that the temperature correction cannot be applied, the tables being computed on the supposition that the rod is of brass. The cistern is frequently too small to hold the mercury when the barometer falls low, so that it cannot fall under a certain height. It is evidently useless to read off such a barometer. It can be of no use.
+ If the tube is very much contracted, which becomes necessary for use on men of war, where heavy firing is going on, sluggishness is caused thereby. To examine the Marine barometer for this defect, read it off, fix the vernier at one inch and a half above the reading, slant it, so that the mercury rises above the vernier, allow it to resume the vertical position, and note the instant, when the mercury in its fall passes the vernier, then lower the vernier one inch (when it will be still half an inch higher than the original reading) and note the instant the mercury passes it again. If the interval (the time it takes the mercury to fall one inch) is less than three minutes, it is fit for use on shore.
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