Sidebilder
PDF
ePub
[merged small][graphic][subsumed][subsumed][merged small][subsumed][merged small][subsumed][merged small][merged small][merged small]

The THERMOMETER is an instrument founded on the principle that most bodies, fluids especially, expand by heat and contract by cold. It is used for the purpose of measuring the amount of heat in the atmosphere or other substances brought into contact with or in the vicinity of it. In construction it differs from the barometer in having the tube closed at each end. The mercury contained in the tube moves in a vacuum caused by the expulsion of the air by boiling the mercury and then closing the top of the tube by means of the blow-pipe. There are three descriptions of thermometers in common use, all constructed on the same principle, but differing in the divisions or graduations of their respective scales. Fahrenheit's thermometer is generally used in England and America.

Upon Fahrenheit's thermometer, melting ice is marked 32°, and boiling water 212°, the interval being divided into 180 equal parts. The same graduation is extended downward from 32° to zero (0°), and may be continued below zero as far as desired. Degrees below zero are distinguished by the minus (-) sign.

For the purpose of measuring the temperature of air on board ship, the thermometer should be exposed in the open air, where the circulation is unobstructed; it should be always in the shade, removed at least a foot from the bulkhead or other material from or near which it is hung, protected

against the heat reflected from the neighboring objects, and kept sheltered from the rain and spray.

If the thermometer should happen to become moistened by rain or spray, the bulb should be carefully dried about five minutes before reading and recording the observations. A wooden frame of open lattice-work will be found to be a good covering for a thermometer, provided it is so constructed as to admit a perfectly free circulation of air about the instrument.

The thermometer is an invaluable instrument to the careful navigator in making observations of temperature, simultaneously, of the air and the surface of the ocean. The difference in the temperature of the air and the surface of the sea gives warning to the seaman of his approach to icebergs, banks, shoals, or land, and, being on soundings, by being lower than where there are none of these obstructions to navigation; and by showing a higher temperature of the surface-water upon entering the Gulf Stream, the Brazil Current, the equatorial currents of the Atlantic and Pacific oceans, the Japan Current, etc., and a lower temperature on leaving them, which are now more or less accurately laid down on our charts.

In approaching land at night, or in navigating in the vicinity of shoals, observations for temperature of the air and water should be frequently made, and at regular intervals of time, with the greatest care and precision, and the differences and changes observed and noted.

The HYGROMETER is a most valuable and useful instrument to the seaman, especially as an adjunct to the barometer and thermometer in foretelling weather.

Any instrument (and there are several instruments of the kind) adapted to measure the amount of moisture in the air is called a hygrometer (or psychrometer, "cold-measuring moisture measurer"), but the one best suited to use on board ship is the wet-bulb thermometer.

The psychrometric hygrometer consists of two good equal thermometers, mounted on the same frame of wood, the bulb of one thermometer being naked, while the bulb of the other is wrapped in some thin, absorbent covering, such as a little muslin bag, with a kind of wick reaching from it into a small cistern of water.

The evaporation of the water produces cold, and this (the wet bulb) thermometer habitually (with very rare exceptions) stands lower than a drybulb thermometer similarly exposed. This depression, strictly, measures only the evaporating power of the air, yet, as the latter depends upon the amount of moisture present in the air, the depression of the wet-bulb thermometer measures the humidity of the air.

[ocr errors][ocr errors]

When the temperature is in the neighborhood of the freezing-point, the observation of the psychrometer requires very peculiar care.

During fog the wet-bulb thermometer may sometimes be higher than the dry bulb; then the air is over-saturated, and contains, besides the vapor at its maximum of tension, water suspended in a disseminated liquid state. This is, however, not a frequent occurrence.

If the temperature of the air (i. e., the dry bulb) should have descended below 32°, it will often happen that the wet-bulb thermometer will for a time read higher than the dry bulb. Such observations must not be recorded; but when the water surrounding the wet bulb has begun to freeze, the proper readings will take place.

If the water in the muslin covering the wet bulb be frozen, the readings will be perfectly correct.

If the muslin he found dry, it should be wetted with a brush or small sponge, and then be left a little while for the water in the muslin to be frozen; and when satisfied that such is the case the observer may proceed to take the readings in the usual way. Unless this caution is attended to, the wet bulb will read as high or higher than the dry bulb. When the weather is frosty, the muslin should be thoroughly wetted some time (say an hour) before the usual and chief hour of observation.

If the temperature should have ascended above 32° (in frosty weather), immerse the wet bulb thermometer in warm water for a minute or so, that any ice remaining on the muslin may be melted. Unless this be attended to, the wet bulb thermometer will continue to read 32°, so long as any ice remains in contact with it.

The muslin and wick should so act as, by capillarity, to keep the wet bulb always wet, but not actually dripping, so that rapid evaporation may be constantly going on.

*

If the air is very still, it is well to increase the evaporation by setting the air in motion by a fan. If the wind is too strong, the instrument should be protected by the movable blind. The reading must be made rapidly, and, as much as possible, at a distance, and without opening the window; for the proximity of the observer, either by the heat radiating from his body or by his breath, as well as the temperature and hygrometrical state of the air issuing from the chamber, which is always different from that of the external air, especially in winter, would infallibly act upon the instruments, and would falsify the observation.

*The little muslin bag and its cotton wick should be changed about once a month.

INDICATIONS OF THE BAROMETER, THERMOMETER, AND HYGROMETER,

AS AIDS IN FORETELLING WEATHER.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

* Weather is the condition of the air at any time as regards heat, moisture, wind, rain, cloud, and electricity; and a change of weather implies a change in one or more of these atmospheric elements.

Difficult as it is to foretell weather accurately, much useful foresight may be acquired by combining the indications of instruments (such as the barometer, thermometer, and hygrometer), with atmospheric appearances.

* Commodore Thornton A. Jenkins, U.S.N.

1. The barometer shows the pressure of the air.

2. The thermometer (in the shade) shows heat and cold, or the temperature of the air.

3. The hygrometer shows the degree of moisture of the dampness of the air.

4. It should always be remembered that changes in weather almost always give signs of their coming, for the instruments are affected before the wind actually begins to blow or the rain to fall; thus the instruments may be said to enable us to "feel the pulse" of the atmosphere. It must not be forgotten that the length of time which passes between the first appearance of a change of weather and its actual setting in is not always the same. It is much greater when a south-west wind is going to succeed a north-east wind, than when the opposite change is going to take place. We shall see further on why this is the case, and also how the appearance of the sky will aid us in forming an opinion as to probable weather.

5. The general principles on which the following explanation is founded have been laid down by Professor Dové, and may be briefly stated thus: *

a. If the barometer is about at its average height, viz.: 30 inches, at the sea level, and is steady or rising while the weather gets colder and the air becomes drier, northwesterly, northerly, northeasterly winds, or less wind, less rain or snow may generally be expected.

b. On the contrary, if the barometer falls while the weather gets warmer and the air becomes damper, wind and rain may be looked for from the south-east, south, or south-west.

c. If these rules are broken in any way, there is reason to fear.

*"EXPLANATORY CARD.

"(In south latitude read south for north.)

66 WEATHER GLASS.

“The barometer rises for northerly wind (including from northwest, by the north to the eastward) for dry or less wet weather; for less wind, or for more than one of these changes, except on a few occasions when rain, hail, or snow comes from the northward with strong wind.

"The barometer falls for southerly wind (including from south-east, by the south to the westward,) for wet weather; for stronger wind, or for more than one of these changes, except on a few occasions when moderate wind with rain (or snow) comes from the northward.

"For change of wind toward northerly directions—a thermometer falls. "For change of wind toward southerly directions-a thermometer rises.

« ForrigeFortsett »