increase condensation, and if they exist round the north-western, northern, and eastern edge of this continent they will act as obstacles to the winds at the earth's surface, and will thus still further decrease the pressure over the centre of this continent. The clouds over this area will be of the Inversion type in the warmer hours, and the higher clouds of Inclination and Interfret in the nocturnal cold hours. In the figure the lowest currents are represented by thin black arrows, the middle currents by firm black, and the upper currents by dotted black arrows.

Looking now at the arctic continent, we see that the currents in the highest and lowest strata of the air cannot pass directly into the area of denser air, but are deflected to the right by the rotation of the sphere, and tend to move clockwise round it. This increases the pressure at the earth's surface over the interior of the continent. The vapour-charged westerly winds which reach the southern coasts of the continent will precipitate some of their moisture as snow or rain. Should mountains exist of sufficient altitude, these will precipitate vapour from the upper currents. Moreover, if these mountains exist round the southern and south-eastern edge of the continent, by checking the eastward escape of the westerly winds at the earth's surface and the southerly outflow of the middle currents from the interior, the pressure of these lower and middle layers will be increased at this portion of the continent. Here we shall have "Interfret" action at no great elevation producing beds of Stratus Quietus.

125. We have not as yet considered the seasonal effects produced upon the atmosphere of our imaginary globe by annual changes in the sun's apparent path. These effects will be of a simple character.

Suppose the axis of this sphere to be inclined at an angle of 66° 32' to the plane of its orbit. The sun will, of course, be vertical at different times of the year to all portions of the surface lying between latitudes 23° 28′ N. and 23° 28′ S. Over each of these latitudes, therefore, there will be a single summer solstice, while between them the sun will be vertical twice in the year. We shall therefore have two belts lying near each tropic, along which, so far as they are not interfered with by our supposed intra-tropical continent, there will be one season of the year marked by the greatest heat and the greatest amount of cloud and rain. When this belt is nearest to the northern tropic, the northern belt of high pressure which, as hitherto supposed, lay over latitude 30° N. will now lie much farther to the northward, and the corresponding belt in the southern hemisphere will lie nearer to the equator. A similar change will take place in these belts when the sun is nearly vertical over the southern tropic.

In the former season, which will be the summer of the northern hemisphere, the under currents, which blow from the south-east out of the southern belt of high pressure, will cross the equator, and, being deflected in the opposite direction in the northern hemisphere, will become south westerly currents. Also, in the summer of the southern hemisphere, the

north - easterly under currents blowing from the northern belt of high pressure will, after crossing the equator, be deflected to the left of their course, and become north-westerly currents.

To these seasonal winds, as they actually occur on the earth's surface, the name "Monsoons" is given. The upper currents of the atmosphere, flowing out of the shifting belt of low pressure, will, in crossing from one hemisphere to the other, move in directions the reverse, generally speaking, of those of the under currents. At and near the equator there will be a transit of the belt of low pressure twice in the year, and here there will be two rainy seasons separated from one another by the longest interval at the equator itself, whereas they immerge into one another so as to form a more prolonged season near the tropics.

126. Now, owing to the time occupied by the various heating processes taking place in the earth and its atmosphere, the maximum effects of solar radiation are not felt at the time when the sun's rays are most vertical. To this delay we give the title

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Heat-lag." This heat-lag is noticeable at most localities on the earth's surface in the diurnal variation of temperature. The highest temperature is, on the average, experienced about two hours after noon, and the diurnal maximum of Inversion currents is reached generally between one hour and two hours and a half later still. Over fairly extensive oceanic surfaces between the latitudes 23° 28′ N. and 23° 28′ S. the maxima of the rains occur soon after the maxi

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mum temperature is attained. In the temperate zones the warmest week in the year occurs about one month after midsummer over low-lying land surfaces, whilst the coldest week in the year is about one month after midwinter. But at high altitudes and in high latitudes the annual heat-lag is much greater.

127. Before proceeding to touch upon the actual distribution of pressures and winds over the globe, it will be as well to summarise what has been stated in this chapter.

1. Convection currents over an imaginary fixed sphere, having a thermal equator and thermal poles, will, in the lower strata of the atmosphere, be directed from the poles towards the equator with a slight upward component, and in the upper strata will travel from the equator towards the poles with a slight downward component of motion.

2. The existence of vapour will modify and somewhat intensify such a circulation.

3. Land surfaces will displace the regions of thermal inequality, and therefore the atmospheric circulation between them.

4. Rotation of the sphere on its axis from west to east will cause bodies moving over its surface to be deflected, or to cause pressure, towards the right in the northern and towards the left in the southern hemisphere. There will be a belt of maximum pressure at the earth's surface at about latitude 30°, which equally divides the area of each hemisphere. From this belt the lower currents will flow from the north-east in the northern, and from the south-east

in the southern hemisphere into the equatorial calmbelt. Also from this belt, in the lowest strata of air, currents will flow from the south-west in the northern and from the north-west in the southern hemisphere, towards the polar calms. The upper currents will flow uniformly in directions parallel to these latter currents from the equator to the poles. These eastward deflections will diminish the atmospheric pressure near the poles and increase it in the direction of the equator.

5. Axial rotation will increase atmospheric pressure over continents in the higher latitudes and diminish it over continents in the lower.

6. Mountains will displace the centres of high and low atmospheric pressures.

7. Inclination of the sphere on its axis will displace the zone of highest temperature and alter all the atmospheric currents of the globe, causing the intra-tropical currents to cross the equator periodically and to be deflected in an opposite direction.