Table XLI. Weight (In Grains) Of Water-Vapour In A Cubic Foot Of Saturated Air1 From

14° to 88° F.

Temperature, o F.

Grains of Water in cubic foot of Air to Saturation.

14

1.0

16

1.1

18

1.2

20

1.3

22

1.4

24

1.5

26

1.7

28

1.8

Temperature, o F.

Grains of Water in cubic foot of Air to Saturation.

30

2.0

32

2.1

34

2.3

36

2.5

38

2.7

40

2.9

42

3.1

41

3.3

Temperature, o F

Grains of Water in cubic foot of Air to Saturation.

46

3.6

48

3.8

50

4.1

52

4.4

54

4.7

56

5.0

58

5.4

60

5.8

Temperature. o F.

Grains of Water in cubic foot of Air to Saturation.

62

6.2

64

6.6

66

7.0

68

7.5

70

8.0

72

8.5

74

9.1

76

9.7

Temperature, o F.

Grains of Water in cubic foot of Air to Saturation.

78

10.3

80

11.0

82

11.7

84

12.4

86

13.2

88

14.0

1 When the air is thus saturated, the readings of the wet and dry bulb thermometers are the same; humidity less than 100 per cent is indicated by the lower reading of the wet bulb.

Evaporation in nature depend* not so much on the temperature as on the relative humidity of the air, and its consequent capacity for taking up more moisture; it is therefore greatly aided by wind and movement, which carry off the air as it tends to become saturated and replace it by drier. Evaporation tendste lower the temperature of the air and of the surface-soil, since all bodies in passing from the solid to the liquid, and still more from the liquid to the gaseous state, abstract heat from their surroundings, which they give oft' again during the reverse transition. Thus, a heavy fall of snow is followed by a perceptible rise of temperature, and the "raw" cold that accompanies a thaw is well known. Thus too. is produced the refreshing coolness that follows the use of a water-carl or hose on a hot sunny day, or of the wetted matting curtains in an Indian bungalow.

Rain is the result of the sudden condensation of vapour (or, more strictly speaking, finely-divided particles of water) contained in masses of warmer air in the form of fog or cloud, when these come in contact with colder air or impinge on the cold summits of mountains. It is thus heavier and more frequent on the western coasts, where the vapour-laden winds from the ocean first meet the mountains and hills of that part of Great Britain. On the eastern shores and counties the rainfall is less, since the west winds have already deposited much of their vapour as rain, and those coming from the north and east are for the most part drier.

The average annual rainfall in the British Isles is about 37 inches, the average in England and Wales being 3376, in Ireland 38.54, and in Scotland 46.56. But in each division of the kingdom there are great variations; thus, in the eastern counties of England the annual rainfall is only 25 inches, while in the hilly or mountainous and moorland districts of Devon, Cornwall, Wales, the Pennine Range, and Cumberland it varies from 40 to 80, and in particular localities it considerably more than 100 inch The extreme mean annual rainfalls in England are 15 inches at Thetford in Suffolk, and 145 at Seathwaite in Cumber-land. In Scotland and Ireland similar variations occur, the midland and eastern counties being, as a rule, the driest, and the western counties having a rainfall approximating to the English maximum.

The heaviest rainfalls in Great Britain, i.e. 80 inches and upwards, are met with in the Isle of Skye and the opposite mainland as far as Loch Linnhe; in the Lake District of Cumberland and Westmoreland; on the Snowdon range in North Wales; and over a small area among the South Wales Mountains in Carmarthen.

It must not be forgotten, however, that there are great differences in the rainfall of different years, especially in the south-west (as at Plymouth, where the extremes hitherto observed have been 45 and 100 inches, and at Exeter 30 and 90), end that, owing to local conditions, - chiefly the disposition of hills or headland*. - there are in the districts of heaviest rainfalls spots in which it is less than the average of those of the lowest; e.g. at Sidmonth in rainy Devonshire it is but inches, and at Cockermouth in Cambariaod it is only 22, though this town is but a few miles distant from Seathwaite. where the maximum annual rainfall in the British Isles is met with.

There is no relation between the rainfall and the humidity of the air, the latter depending chiefly on the nature of the soil; the heaviest rainfall is frequently found in mountainous places where a hard non-ahsorliciit rock gives but little opportunity for evaporation, and where, therefore, the amount of water-vapour in the air may be, in ordinary circumstances, relatively small. Instances in proof of this may be seen in Table XXII., which gives a summary of the rainfall, humidity, and temperature at certain well-known health-resorts, together with the nature

Table XLII. - Climate Of Health-Resorts

Place.

Subsoil.

Mean Winter Temperature in deg. F.

Mean summer Temperature in deg. F.

Mean Daily Range in deg. F.

Mean Annual

Rainfall in inches

Rainy

Days

Humidity percent

Regent's Park,

London Clay, .. ..

41.6

56.9

13.8

25.16

164

80

Margate, ..

Chalk, .. .. ..

41.8

56.4

10.9

22.98

165

81

Lowestoft, ..

Gravel, .. ..

40.8

54.8

11.4

24.09

173

83

Cromer, ..

Cragland Blue Clay, .. ..

40.6

55.4

12.0

27.73

154

85

Scarborough, ..

Loam on Clay, .. ..

40.8

541

10.0

26.68

195

83

Hastings,..

Sand on Sandstone, .. ..

42.2

57.0

11.8

29.26

137

82

Eastbourne,..

West-end Chalk, East Loam,

42.2

56.2

..

29.53

165

..

Brighton,..

West-end Clay, East Chalk,

42.5

57.1

11.2

30.43

163

78

Southsea,..

Bagshot Sand and Clay, .. ..

42.7

55.2

13.8

26.39

173

82

Ventnor,..

Lower Greenland, .. .. ..

44.2

57.7

10.5

28.33

167

80

Weymouth, ..

Sand, Shingle, Clay, .. ..

43.5

56.8

10.3

27.61

162

81

Teignmouth...

Marl, in parts Sand, .. ..

43.8

57.0

12.4

32.28

169

82

Torquay,..

Limestone, .. .. ..

43.5

55.9

10.8

31.72

177

80

Falmouth,..

slate on Quartz, .. ..

44.9

57.0

9.5

43.49

204

81

Ilfracombe,..

Shales on Sandstone, .. ..

44.9

57.0

8.4

31.53

204

81

Llandudno,..

Limestone, .. .. ..

42.7

55.2

10.1

27.52

175

79

Guernsey,..

Granite .. ..

45.5

57.6

9.1

33.28

192

85

Harrogate..

.. ... ..

39.5

53.4

12.7

..

..

82

Malvern,..

.. ... ..

41.2

56.3

12.8

..

..

80

Tunbridge Wells,

.. ... ..

40.9

56.0

14.4

..

..

81

The above averages are for the years 1882 to 1888 inclusive, except for Brighton, 1879.1 and for Harrogate, Malvern, and Tunbridge Wells, 1890.1897. All, except for Brighton, are from the records of the Meteorological Society. It must not be forgotten that the climatic conditions of different places, in the same county or district, vary largely; elevation, shelter, aspect, soil, and the proximity of water and trees, must all be taken into consideration.

of the subsoil in most cases; thus Falmouth, built on hard impervious rook, has nearly 60 per cent more rainfall than Cromer, but 5 per cent less humidity.

The dew-point is the temperature at which air in the course of cooling can no longer retain its water as an invisible vapour, but lets it fall in minute drops which, according to circumstances, take the form of dew, hoar-frost, fog, or mist. The warmer the air the more moisture does it take up from the soil, rivers, sea, etc While the sun is shining, the earth receives and absorbs more heat than it loses by radiation into space; but at night, unless radiation be checked by overhanging trees or clouds, and there be not enough wind to keep the lower air in constant movement, the earth rapidly cools, and in so doing brings down the temperature of the air in contact with it. When the temperature of the air falls below its dew-point, some of its watery vapour is deposited, either as dew or as hoar-frost - according as the dew-point is above or below 32° P., - and most abundantly on those objects, as grass and twigs, which are the best radiators. Hoar-frost is not frozen dew, but dew deposited in the form of ice from air whose dew-point is below freezing.

The formation of fog, unlike that of dew, is favoured by a moderate movement of the air, and indirectly by irregularity of the surface of the country. It results from the rapid cooling of large masses of air to below their dew-point, (l) by the mingling of volumes of warm humid air and cold dry air (provided the temperature of the mixture be below its dew-point), (2) by the sudden chilling of vapour-laden air when it impinges on cold hilltops or cliffs, (3) by the air passing over land cooled by evaporation, as wet clay soils, or (4) over the surface of lakes (provided the water in these be colder than the air). The fogs which form over running streams in frosty weather are, however, due to the condensation of the vapour rising from the warmer water; and in valleys and over damp meadows fog is often caused by the colder and therefore heavier air from the higher lands around flowing down and condensing the vapour in the warmer air in the valley, which, if there be little or no wind, may gradually assume the appearance of a lake of white mist

Though fog and mist, which differ only in the size of the coalescing drops of water, may, like dew, saturate the clothing of persons exposed to them, they are by no means necessarily indicative of dampness of site. No such suspicion attaches to those that cap the summits of high hills, or veil the cliffs and headlands of the southern and western coasts, or to the fogs to which the heights along the course of rivers are exposed; but the case is totally different when they mark the sites of ponds and water-courses that have disappeared through drainage, or haunt low-lying clay-lands and tracts where the ground water is near the surface.

In and near towns, where the air is charged with smoke, the particles of carbon attract and condense the moisture of the air, and form fogs which are marked by a yellow or brown colour and a pungent acid and tarry smell.