In addition to the two gases, oxygen and nitrogen, the atmosphere contains other constituents which are constartly present in it, but in varying, and always very minute, proportions. The most important of these are carbonic acid and aqueous vapour.

The first-named gas is notoriously deleterious to animal health. It is the product of the complete oxidation of carbon, as effected in combustion, and therefore in respiration, which is a slow combustion, and it is absorbed by plants under the influence of sunlight.

Its normal amount may be taken as 0.03 per cent. An average of several specimens of London air, taken in November 1869, gave Dr. Smith 0.04394 per cent., while examples taken in the tunnels of the Metropolitan Railway, in the same year, gave 0.1452. Dr. Smith, again, says: "We all avoid an atmosphere containing 0.1 of carbonic acid in crowded rooms; and the experience of civilised men is that it is not only odious but unwholesome. When people speak of good ventilation in dwelling-houses they mean, without knowing it, air with less than 0.07 of carbonic acid.'

The proportions of oxygen and carbonic acid, however, are not subjects of ordinary meteorological inquiry; but it is very different with the aqueous vapour, which plays a most important part in all meteorological changes, and the determination and consideration of which will form the subject of special chapters.

As to ozone, which is known to exist in the atmosphere and which is only oxygen in what chemists call an allotropic condition, its determination is deemed by

1 Air and Rain, p. 56.

some authorities to be of very high importance in connection with sanitary meteorology; but the assertion may safely be made that as yet no thoroughly unquestionable mode has been proposed for its detection and quantitative determination. To this subject, however, we shall return subsequently (Chapter X. p. 196).

17

CHAPTER III.

TEMPERATURE.

THE determination of the temperature of the air is without doubt, the most important of meteorological observations, for as Professor Angus Smith has pointed out, in his work already quoted, Heat is a more pressing want than even pure air.' If, however, the observation is important, and is apparently very easily made, it is yet almost the most uncertain of all, if great accuracy be required, owing to the nearly total impossibility of securing a perfectly unexceptionable exposure for the thermometers. The indications of these instruments will vary more or less, with the pattern and material of the stand in which they are placed, with their height above the ground, their proximity to trees or buildings, and even with the nature of the soil. and the character of the vegetation which covers it.

We must commence with some notice of the thermometer itself, of which instrument a most interesting history has been published by M. Renou in Paris,' and we shall describe some of the principal forms of thermometers at present in use. An ordinary thermometer consists of a fine glass tube with a bulb blown on one end, and is partly filled with some liquid, usually mercury or spirit of wine. This liquid expands on being heated, and contracts again on being Annuaire de la Société Météorologique de France, 1876, p. 19.

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cooled. When it expands it passes up along the tube. By the amount of this expansion the temperature is measured by means of a scale marked off on the tube. The mode of making and filling thermometers so as to exclude all air from the bulb and tube is given in text-books of physics, as in Balfour Stewart's 'Elementary Treatise on Heat' (1866). Other conditions being equal, the finer the tube in relation to the bulb the more sensitive will the instrument be, foi the scale will be more open.

The actual inventor of the thermometer is unknown. At the end of the seventeenth century two forms of instruments for measuring temperature were recognised -Spirit thermometers with closed tubes (Florentine thermometers), and Air thermometers, in which the open end of the tube was plunged in water (Dutch thermometers). It is generally stated that Galileo was the real inventor of the first-named form of thermometer, but Robert Fludd, in a work published in 1638, expressly stated that he had found the instrument described in a manuscript dating more than fifty years back. This date is at least twenty years before Galileo went to Florence, and began to publish, for his first paper is dated circa 1609.

The great improvements in the instrument, which, in its simplest form, corresponds in principle exactly with the Florentine pattern, have been the substitution of mercury for spirit as the thermometric liquid, and the reference of the scale to the fixed points of boiling water and melting ice. All three of these improvements are placed by M. Renou to the credit of Englishmen, the first two having been suggested by Halley in the Philosophical Transactions' for 1693; while the

third, the proposal to use the freezing-point of water as the starting-point of the scale, was made by Robert Hooke in 1667.

The advantages presented by the use of mercury as a thermometric liquid are the great distance between its melting-point and its boiling-point, amounting to about 700° on the Fahrenheit scale; the regularity of its expansion for the greater part of that range; its high conductivity and low specific heat,' which enable it to indicate more rapidly than other substances would the temperature to which it is for the time exposed.

The only conditions under which mercurial thermometers cannot be employed in meteorology are those of extreme cold, inasmuch as at the temperatures experienced in winter in very cold climates, such as those of Canada or Siberia, mercury freezes, or at least is so near its point of congelation that its expansion is not so regular as is the case between the limits of temperature met with in warmer climates.

Various plans have been proposed for the graduation of thermometers; of these three have come into extensive use: those of Fahrenheit, Linnæus, and Réaumur. In Fahrenheit's thermometer, used at all British and American stations, the interval between the freezing and boiling points is divided into 180 degrees, the freezing-point being 32° and the boiling-point 212°.

In the Centigrade thermometer, in general use in Europe, which was invented by Linnæus,2 not, as usually stated, by Celsius, that interval is divided into 100 degrees, the freezing-point being 0°, and the boilingpoint 100°.

Conductivity is the rapidity with which heat is communicated from particle to particle of a substance. Specific heat is the amount of heat required to raise 1 lb. of a substance one degree, in terms of that necessary to raise 1 lb. of water one degree. 2 See note, p. 49.