by an arrangement of wheels, the number of miles traversed by the wind can be approximately ascertained.

Osler's anemometer is a large and very beautiful instrument. It registers simultaneously on a piece of paper fitted on a drum, which is turned by clock-work, direction, velocity, and pressure.

Other anemometers, Lind's, Whewell's, etc., need not be described.

The average velocity of wind in this country near the surface of the earth is from six to eight miles per hour; its range is from zero to 60 or even 70 miles per hour, but this last is very rare; it is seldom more, even in heavy winds, than 35 to 45 miles per hour. In the hurricanes of the Indian and China seas it is said to reach 100 to 110 miles per hour.

Force. The force of the wind is reckoned as equal to so many pounds or parts of a pound on a square foot of surface. Osler's anemometer, as just stated, registers the force as well as the velocity and direction, but Robinson's (used in the army) marks only the velocity; the force must then be calculated. The rule for the calculation of the force from the velocity is as follows:

Ascertain the mean velocity per hour by observing the velocity for a minute, and multiplying by 60; then square the hourly velocity and multiply by .005. The result is the pressure in pounds or parts of a pound per square foot.

The formula is, if V velocity per hour,

=

V2 x .005 P.

If the force be given, the velocity may be found:

√200 PV.

When no anemometer is in use, the Beaufort scale may be employed, 0 = calm, about 3 miles an hour, and 12 = hurricane, 90 miles and over.

SECTION VII.

CLOUDS (PLATE IX.).

The nomenclature proposed by Howard' is now almost universally adopted.

There are three principal forms and four modifications.

Principal Forms.

Cirrus. Thin filaments, which by association form a brush, or woolly hair, or a slender network. They are very high in the atmosphere, probably more than ten miles, but the exact height is unknown. It has even been questioned whether they are composed of water; if so, it must be frozen. In this climate they come from the northwest.

Cumulus.-Hemispherical or conical heaps like mountains rising from a horizontal base; cumuli are often compared to balls of cotton. Stratus.-A widely extended, continuous horizontal sheet, often forming at sunset.

Modifications.

Cirro-cumulus.-Small rounded, well-defined masses, in close, horizontal arrangement; when the sky is covered with such clouds it is said to be fleecy.

1 Climate of London.

Cirro-stratus.-Horizontal strata or masses, more compact than the cirri; at the zenith they seem composed of a number of thin clouds; at the horizon they look like a long narrow band.

Cumulo-stratus.-Stratus blended with the cumulus.

Cumulo-cirro-stratus, Nimbus, or Rain-cloud.-A horizontal sheet above which the cirrus spreads, while the cumulus enters it laterally or from below.

Of the above forms Nos. 1, 2, and 3 of the plate (copied by permission from Mr. Scott's "Instructions") are "upper" clouds; the others are "lower" clouds. To those described is added the form shown in No. 5, viz., Rollcumulus, which consists of portions of cumulus rolled into a cylindrical shape, and either separate or packed together, as shown in the plate. Alongside the names in the plates are contractions, which ought to be used in description.

Estimation of Amount of Cloud.-This is done by a system of numbers: 0 expresses a cloudless sky, 10 a perfectly clouded sky, the intermediate numbers various degrees of cloudiness. To get these numbers, look midway between the horizon and zenith, and then turn slowly round, and judge as well as can be done of the relative amount of clear and clouded sky. This is to be entered without reference to the thickness of the cloud.

SECTION VIII.

OZONE.1

Papers covered with a composition of iodide of potassium and starch, and exposed to the air, are supposed to indicate the amount of ozone present in the atmosphere. Schönbein, the discoverer of ozone, originally prepared such papers, and gave a scale by which the depth of blue tint was estimated. Subsequently similar but more sensitive papers were prepared by Dr. Moffat, and Mr. Lowe afterward improved on Moffat's papers, and also prepared some ozone powders.

The papers are exposed for a definite time to the air, if possible with the exclusion of light, and the alteration of color is compared with a scale.

Schönbein's proportions are-1 part of pure iodide of potassium, 10 parts starch, and 200 parts of water. Lowe's proportion is 1 part of iodide to 5 of starch; Moffat's proportion is 1 to 24. The starch should be dissolved in cold water, and filtered so that a clear solution is obtained; the iodide is dissolved in another portion of water, and is gradually added. Both must be perfectly pure; the best arrowroot should be used for starch.

The paper, prepared by being cut into slips (so as to dry quicker and to avoid loss of the powder in cutting) and soaked in distilled water, is placed in the mixed iodide and starch for four or five hours, then removed with a pair of pincers, and slowly dried in a cool dark place, in a horizontal position. The last point is important, as otherwise a large amount of the iodide drains down to one end of the paper, and it is not equally diffused. The papers when used should hang loose in a place protected from the sun and rain; a box is unnecessary; they should not be touched with the fingers more than can be helped when they are adjusted.

1 For a full account of the tests of ozone, see Dr. Fox's work on Ozone and Antozone, 1873, already referred to. After discussing all the tests, he gives the preference to the iodine plan. He has not found Schönbein's thallium method satisfactory.

When Schönbein's papers are used they are moistened with water after exposure, but before the tint is taken. Moffat's papers are prepared somewhat similarly to Schönbein's, but do not require moistening with water. The estimation of ozone is still in a very unsatisfactory state, and this arises from two circumstances.

1. The fact that other substances besides ozone act on the iodide of potassium, especially nitrous acid, which is formed in some quantity during electrical storms. Cloez has shown that air taken about one metre above the ground often contains nitrous acid in sufficient quantity to redden litmus. Starch and iodide paper is colored when air contains.00005 of its volume of nitrous acid.

2. The fact that the papers can scarcely be put under the same conditions from day to day; light, wind, humidity, and temperature (by expelling the free iodine) all affect the reaction.

Chemical objections have also been made.' Supposing that iodine is set free by ozone, a portion of it is at once changed by additional ozone into iodozone, which is extremely volatile at ordinary temperatures, and is also changed by contact with water into free iodine and iodic acid. Hence a portion of the iodine originally set free never acts on the starch, being either volatilized or oxidized. Again, the iodine and caustic potash set free by the ozone combine in part again, and form iodate and iodide of potassium (4th of the former and ths of the latter), and in this way the blue color of iodide of starch first produced may be removed. The ozone may possibly, and probably, act on and oxidize the starch itself, and hence another error.

The conclusion arrived at by the Vienna congress was the following: "The existing methods of determining the amount of ozone in the atmosphere are insufficient, and the congress therefore recommends investigations for the discovery of better methods."

SECTION IX.

ELECTRICITY.

The instruments used by meteorologists are simple electroscopes, with two gold-leaf pieces which diverge when excited, or dry galvanic piles acting on gold-leaf plates or an index attached to a Leyden jar (Thomson's electrometer). For further details, see Scott's "Instructions," op. cit.

SECTION X.

THERMOMETER STAND.

A stand is issued by the War Office, and provided at every station. Or it would be very easy to make a stand by two or three strata of boards, placed about 6 inches apart, so as to form a kind of sloping roof over the thermometers, which are suspended on a vertical board.

The dry and wet bulb thermometers are placed in the centre; the maximum on the right side, and the minimum on the left. The wood

1 Beiträge zur Ozonometrie, von Dr. v. Maach; Archiv für Wiss. Heilk., Band ii.,

p. 29.

should be cut away behind the bulbs of the maximum and minimum thermometers, so as to expose them freely to the air. and wet bulbs should also fall below the board.

The bulbs of the dry These stands are made

to rotate on the pole so as to turn the roof always to the sun.

A much better stand is Stevenson's screen, a square or oblong box, with double louvred sides and open below. This is raised upon legs, four feet from the ground, placed upon grass.'

SECTION XI.

WEATHER.

In registering the kind of weather it is well to adhere to the Beaufort notation and symbols, which are carefully explained in Scott's "Instructions." Columns are given in the return to be filled up in this way.

SECTION XII.

DISEASES AND VARIATIONS IN THE METEOROLOGICAL ELEMENTS.

The variation in the prevalence of different diseases at a particular place, in connection with the simultaneous variation of meteorological elements, is an old inquiry which has at present led to few results. The reason of this is that the meteorological elements are only a few out of a great many causes affecting the prevalence and severity of diseases. Consequently, in order to estimate the real value of changes of temperature, pressure, humidity, ozone, etc., the other causes of disease, or of variations in prevalence or intensity, must be recognized and eliminated from the inquiry. The best of the late observations are those by Guy, Ransome, Vernon, Moffat, Tripe, Scoresby-Jackson, and Ballard. tions have also been made by Fodor and others on the continent of Europe, and by various observers in America and elsewhere. But they must be much more extended and numerous before anything practical can be drawn from them.

'Scott's Instructions, Fig. 10, p. 41.

CHAPTER XVI.

INDIVIDUAL HYGIENIC MANAGEMENT.

THIS subject is an extremely large one, and the object of this book does not permit of its discussion. It would require a volume to itself. Only a few very general remarks can be made here. The application of general hygienic rules to a particular case constitutes individual management.

It is impossible to make general rules sufficiently elastic, and yet precise enough, to meet every possible case. It is sufficient if they contain principles and precepts which can be applied. While individual hygiene should be a matter of study to all of us, it is by no means desirable to pay a constant or minute attention to one's own health. Such care will defeat its object. We should only exercise that reasonable care, thought, and prudence which, in a matter of such moment, every one is bound to take.

Every man, for example, who considers the subject bona fide, is the best judge of the exact diet which suits him. If he understands the general principles of diet, and remembers the Hippocratic rule, that the amount of food and exercise must be balanced, and that evil results from excess of either, he is hardly likely to go wrong.

"Temperance and exercise," was the old rule laid down, even before Hippocrates,' as containing the essence of health; and if we translate temperance by "sufficient food for wants, but not for luxuries," we shall express the present doctrine.

The nutrition of the body is so affected by individual peculiarities, that there is a considerable variety in the kind of food taken by different persons. The old rule seems a good one, viz., while conforming to the general principles of diet, not to encourage too great an attention either to quantity or to quality, but avoiding what experience has shown to be manifestly bad, either generally or for the particular individual, to allow a considerable variety and change in amount from day to day, according to appetite. Proper and slow mastication of the food is necessary; and it is

2

It is quite plain from the context, that Hippocrates, by temperance, meant such an amount of food as would balance, and neither exceed nor fall short of the exercise. He had a clear conception of the development of mechanical force from, and its relation to, food. He lays down rules to show when the diet is in excess of exercise, or the exercise in excess of diet. In either case he traces disease.

2 Celsus carried the plan of variety so far as to recommend that men should sometimes eat and drink more than is proper, and should sometimes not exceed; and Lord Bacon has a remark which leads one to believe he held a similar opinion; but there can be no doubt of the incorrectness of this opinion. It has been truly said that the first general rule of Hippocrates, which prescribes continual moderation, is much truer, and the best writers on hygiene, ancient and modern, have decided against Celsus. Besides being erroneous, the rule of Celsus opens a door to intemperance, and, like a harmless sentence in Hippocrates, has been twisted to serve the argument of gourmands. Its influence is felt even at the present day. This much is certain, that prob