CHAPTER XV.

DESCRIPTION OF THE METEOROLOGICAL INSTRUMENTS, AND A FEW REMARKS ON METEOROLOGY. ·

As meteorological observations are now so commonly made, and as in the army instruments are provided at many foreign stations, it is desirable to give a few plain instructions on the use of these instruments.' For the convenience of beginners, a few observations on Meteorology are also added.

The following is the official circular issued by the Army Medical Department :

Official Instructions for Reading the Meteorological Instruments.

The observer should make himself thoroughly acquainted with the scale of every instrument, especially with that of the barometer and its attached vernier, and by frequent comparisons ascertain that he and his deputy read the instruments alike, and record the observations accurately.

All observations must be recorded exactly as read. The corrections are to be made only at the end of each month on the "means" of the "sums.”

Barometrical observations must be recorded to the third decimal place; thermometrical to the first decimal. When the readings are exactly to the inch or degree, the places for the decimals must be filled up with ciphers.

The observations should be made as quickly as possible, consistent with perfect accuracy, and the observer must avoid breathing on the instruments, particularly the dry and wet bulb, and maximum thermometers.

Barometer Readings.-Note the temperature of attached thermometer in degrees only; by means of the thumb-screw at the bottom adjust the mercury in the cistern to its proper level, the point of the ivory cone, which should just touch the mercury without breaking the surface; then bring the zero line of the vernier to the level of the apex of column of the mercury, and read off in the manner described at pages 15 and 16 of Sir H. James's Book of Instructions.?

Thermometer Readings.-The scales are divided to degrees only, but these are so open that the readings can be determined to the tenth of a degree. Practice and attention will insure accuracy.

Maximum Thermometer in Shade.-The maximum thermometer must be hung at such a distance (2 or 3 inches) from the water-vessel of the wet-bulb thermometer, that its readings may not be affected by evaporation.

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In hanging the maximum, care must be taken that the end of the tube is slightly inclined downward, which will have the effect of assisting in preventing the return of any portion of the column of mercury into the bulb on a decrease of temperature. read the instrument, gently elevate the end furthest from the bulb to an angle of about 45°, in which position of the instrument note the reading. To re-set the thermometer, a gentle shake or swing, or a tap on the wooden frame of the instrument. will cause the excess of mercury to return to the bulb, and it is again ready for use.

Maximum in Sun's Rays, or the Vacuum Solar Radiation Thermometer.—Being constructed on the same principle as the last-mentioned instrument, it must be read in a similar position. After completing the reading, by giving the instrument a slight

For these are now substituted Instructions in the Use of Meteorological Instruments, by R. H. Scott, M.A., F.R.S., 1877. The Barometer corrections are explained at pp. 30, 31 of that work.

SECTION I

THERMOMETERS FOR TAKING THE TEMPERATURE OF THE AIR.

Maximum Thermometers.

Two maximum thermometers are issued-one to observe the greatest heat in the sun, the other in the shade.

The Sun Maximum or Solar Radiation Thermometer is formed by a glass case (from which the air is removed), containing a mercurial thermo

shake, with the bulb still inclined downward, the excess of mercury will return to the bulb, and the thermometer be ready for the next observation.

Minimum Thermometer in Shade.-The minimum thermometer must be so hung that the bulb may be about one inch lower than the other extremity of the instrument, because in this position the index is less likely to be affected by a rise in temperature. The extremity of the index furthest from the bulb shows the lowest degree to which the spirit has fallen since the last observation. The reading on the scale corresponding to this is the temperature to be recorded. Then by elevating the bulb, the index will float toward the end of the spirit. When it has nearly arrived at that point, the instrument is re-set.

Minimum on Grass, Terrestrial Radiation Thermometer is constructed like the last, and the directions above given are also applicable to it.

After reading and re-setting the self-registering thermometers, compare them with the dry-bulb thermometer in order to ascertain that their readings are nearly the same. Dry and Wet-Bulb Thermometers. -Bring the eye on a level with the top of the mercury in the tube of the dry-bulb thermometer, and take the reading, then complete the observation by noting in like manner the reading of the wet-bulb thermometer.

The temperature of the air is given by the former, that of evaporation by the latter. From these data the hygrometrical results are to be calculated by Glaisher's Tables, 3d

edition.

Rain-Gauge and Measure.-Pour the contents of the gauge into any convenient vessel with a lip, and from this into the glass measure, which has been graduated especially for the gauge, and is only to be used in measuring its contents. It is graduated to the hundredths of an inch.

Anemometers.-The dials are read from left to right. The first on the left records hundreds of miles, the second tens, the third miles, the fourth tenths of a mile, and the fifth hundredths of a mile.

The reading of the anemometer is obtained by deducting from the amount registered by the dials the total sum registered at the period of the preceding observation. The difference between those (subject to a small correction) indicates the velocity or horizontal movement of the air in miles during the interval, and must be entered in the return. When the instrument is first set up, the reading on the dials must be noted, in order that it may be deducted from the total registered by the dials at the end of the first period of observation.

In making observations on the presence of ozone, a box has been found to be unnecessary, equally satisfactory results having been obtained by fixing the paper immediately under the penthouse of the stand, which shelters it sufficiently from a strong light, while it secures proper exposure.

The minimum thermometers are liable to get out of order-first, by carriage, when the index may be wholly or partly driven out of the spirit, or a portion of spirit may become detached from the main column; and, secondly, by slow evaporation of the spirit, which rising in the tube, condenses at the upper end. The first-mentioned errors are corrected by taking the thermometer in the hand, with its bulb downward, and giving it a swing up and down. The second is remedied by the inclined position of the instrument, which allows the condensed spirit to trickle back to the main column.? N. B.-On no account whatever is artificial heat to be applied to a spirit thermometer. In re-setting the minimum, the index should never be brought quite to the end of the column of spirit.

1 A 6th edition is now published.

2 It is generally necessary to swing the instrument to get back the broken portion of the column.

VOL II.-7

meter with a blackened bulb. The case shelters from currents of air; the black bulb absorbs the sun's rays. The tube of the thermometer is slightly bent near the bulb, and a piece of porcelain is inserted which narrows the tube. The effect of this is to make the thermometer self-registering, as, after the mercury has expanded to its fullest extent, instead of retiring into the bulb on cooling, it is stopped by the porcelain, and the mercury breaks between the porcelain and the bulb. The instrument is placed at a height of four feet from the ground on wooden supports, and in any place where the sun's rays can freely fall on it.

The Shade Maximum is a mercurial thermometer, not inclosed in a case, but mounted on a frame. Its construction and manner of reading are otherwise similar to those of the sun thermometer.

It is placed in the shade four feet above the ground, and sufficiently far from any walls to be unaffected by radiation. It should be freely exposed to the air, but perfectly protected from the sun's rays.

Minimum Thermometers.'

Two minimum thermometers are supplied.

The Shade Minimum is an alcoholic thermometer with a small index in the alcohol. It is set by allowing the index to slide nearly to the end of the spirit; as the spirit contracts during cold, it carries the index down; when it expands again it cannot move the index, but leaves it at the degree of greatest cold. The end of the index farthest from the bulb is the point to read.

This thermometer is placed in the shade four feet above ground, under the same conditions as the shade maximum.

The Grass Minimum or Terrestrial Radiation Thermometer is a thermometer of the same kind, but protected by a glass shield. It is placed almost close to the ground on grass, suspended on little tripods of wood, but it should not touch the ground; it is intended to indicate the amount of cooling produced by radiation from the ground. If snow lies on the ground the bulb should be placed in the snow. Scott recommends a black board on which to lay the thermometer where no grass can be obtained.*

Common Thermometer.

The dry bulb of the "wet and dry bulb thermometer" is read as a common thermometer.

Reading of the Thermometers.

All these thermometers can be read to tenths of a degree. The maximum and minimum thermometers are read once a day, usually at 9 A.M.; the former marks the highest point reached on the previous afternoon, and must be so entered on the return; the latter, the lowest point reached on the same morning. For the army returns the common thermometer is read twice a day, at 9 A.M. and 3 P.M.

1 Great difficulty is found with spirit thermometers on account of their being so much less sensitive than mercurial. To remedy this the bulb is sometimes made forkshaped, or otherwise modified so as to expose as large a surface as possible.

* Instructions, etc. Scott adds: "Under any circumstances, a board gives a better measure of terrestrial radiation than grass."

It is desirable that these thermometers should be read both morning and evening. In winter the maximum sometimes occurs in the early morning and the minimum in the afternoon. In winter the range depends more on the direction of the wind

Range of the Temperature.-The maximum and minimum in shade give most important climatic indications; the difference between them on the same day constitutes the range of the diurnal fluctuation. The range is expressed in several ways.

The extreme daily range in the month or year is the difference between the maximum and minimum thermometer on any one day.

The extreme monthly or annual range is the difference between the greatest and least height in the month or year.

The mean monthly range is the daily ranges added and divided by the number of days in a month (or between the mean of all the maxima and the mean of all the minima).

The yearly mean range is the monthly ranges added and divided by 12. Mean Temperature. The mean temperature of the day is obtained in the following ways:—

(a) At Greenwich and other observatories, where by means of photography the height of the thermometer at every moment of the day is registered, the mean of the hourly readings is taken. This has been found to accord with the absolute mean (found by taking the mean of the whole curve) to within 1th of a degree.

(b) Approximately in several ways. Taking the mean of the shade maximum and minimum of the same day. In this country, during the cold months (December and January), the result is very close to the truth; but as the temperature increases, a greater and greater error is produced, until in July the mean monthly error is + 1.9° Fahr., and in some hot days is much greater. In the tropics, the mean of the maximum and minimum must give a result still further from the truth.

Monthly corrections can be applied to bring these means nearer the truth. Mr. Glaisher's corrections for this country are as follows:Subtract from the monthly mean of the maximum and minimum de

The result is the approximate mean temperature. But this is true only for this country.'

In a great number of places the mean temperature of the day and year, as stated in books, is derived solely from the mean of the maximum and minimum. According to Scott, the approximation to the true mean is very close in most parts of the world, especially if the observations be taken as near the end of the period as possible, near midnight, for instance, for the mean of the civil day of twenty-four hours.

The approximate mean temperature may also be obtained by taking observations at certain times during the day, and applying a correction.

than on the time of day (Scott). But uniformity of practice is the primary essential, and at stations where observations are made only once a day, viz., at 9 A.M., or even twice, unless the second reading is after 6 P.M., the above rule as to entry must be followed.

These numbers of Mr. Glaisher are likely to be modified very considerably; they are largely dependent on the pattern of the thermometer stand employed.

With a Stevenson screen the simple mean of the maximum and minimum is very near the truth.

Mr. Glaisher has given some very valuable tables of this kind,' which can be consulted.2

If the temperature be taken twice a day at homonymous hours, such as 9 A.M. and 9 P.M., the mean of these does not differ much from the true daily mean (Scott).

The nearest approach to the mean temperature of the day by a single observation is given at from 8 to 9 P.M.; the next is in the morning-about 8 o'clock in July and 10 in December and January.

The nearest approach to the mean annual temperature is given by the mean of the month of October. Observations made from a week before to a week after April 24th, and again in the corresponding weeks of October, give a certain approximation to the yearly mean temperature.

3

The changes in temperature of any place, during the day or year, are either periodic or non-periodic. The former are dependent on day and night, and on the seasons, i.e., on the position of the place with respect to the The periodic changes are sometimes termed fluctuations, and the differences between day and night temperatures, or the temperatures of the hottest and coldest months, are often called the amplitudes of the daily or yearly fluctuations.

sun.

Daily Periodic Changes.-On land the temperature of the air is usually at its lowest about 3 o'clock A.M., or just before sunrise, and at its maximum about 2 o'clock P.M.; it then falls nearly regularly to 3 o'clock A.M. At sea, the maximum is nearly an hour later.

The amount of diurnal periodic change is greater on land than on water; in the interior of continents than by the sea-side; in elevated districts than at sea-level. As far as land is concerned, it is least on the sea-coast of tropical islands, as at Kingston in Jamaica, Colombo in Ceylon, Singapore, etc.

Yearly Periodic Changes.-In the northern hemisphere, the coldest month is usually January; in some parts of Canada it is February. On the sea, the coldest month is later, viz., March. The hottest month is in most places July, in some few August; on the sea it is always August. The coldest days in this country are toward the 21st January; the hottest, about the 18th to the 21st July. At Toronto the hottest day is 37 days after the summer solstice; and the coldest, 55 days after the winter solstice.

It is thus seen that both for the diurnal and annual alterations of heat the greatest heat is not simultaneous with, but is after, the culmination of the sun; this is owing to the slow absorption of heat by the earth.

The amplitude of the yearly fluctuation is greater on land than sea, and

1 On the Corrections to be applied to Meteorological Observations for Diurnal Range, prepared by the Council of the British Meteorological Society, 1850. These corrections are applicable only to this country.

2 The following rules, which are applicable in all parts of the world, are given by Herschel :-4

If observations are taken three times daily—at 7 A.M., 2 P.M., and 9 P.M.,-hours which we may denote by t, t', and t"; then

If the hours are 8 A.M., 3 P.M., and 10 P.M., the formula is—