The general surface winds of the globe, like those of cyclones, generally cross the isobars at angles more or less acute. It is probable, however, that all the winds of the world above 1000 metres, or even lower, follow the isobars very closely, and that the energy of cyclones both large and small is dissipated mainly by friction at or near the earth's surface. Although the energy represented by such movements is very large, it would be easy to overrate the strength of the forces required to keep them going or to start them. We must regard the circulation of the atmosphere much as we do the movement of the planets, moons, comets, &c. of the solar system; but the retarding frictional forces are greater in the case of the atmosphere than they are in the case of the solar system.

I have ventured to show what I regard as the general directions of the slow movement of the winds across the isobars in figs. 6 & 7.

In the case of fig. 6, the arrows show the assumed very slow movement to or from the centre of the cyclone during its growth. The velocity of the wind along the isobars may be high. In fig. 7 the arrows also show the assumed steady movement to and from the centre of the cyclone. The velocity along the isobars may also be great.

At the

The temperature distribution in the atmosphere appears to be the result of heating mainly in two ways. upper surface there may be arrested many kinds of radiation. (undulatory and material) and also cosmic matter moving at high velocities. It is thus heated at its upper surface, and this heat passes downwards and gives us the conditions of temperature found in the stratosphere. Light and heat rays pass through the atmosphere until they reach clouds in the lower atmosphere or the earth's surface. From here the heat rises and the temperature conditions of convective equilibrium are established in the lower atmosphere.

I have suggested that the great heating of the upper surface of the atmosphere over the poles is primarily due to the electrons shot out by the sun, which, being caught by the earth's magnetic field, are directed towards the poles, the air in the neighbourhood of which they heat and probably ionize. But we have to account for the local heating of the upper surface of the atmosphere required to produce cyclones. It may be due to pencils of high-velocity cosmic matter; for the optical properties of the upper atmosphere, as indicated by

*Phil. Trans. vol. xxxi., April 1916.

the varying sunrise and sunset effects, are otherwise difficult to explain.

The energy of impact of the cosmic dust need not be as great as the energy of the cyclone produced; for the winds and isobars of a cyclone are to a large extent a modified arrangement of existing isobars and winds.

Cyclones according to this theory must travel with the winds of the upper atmosphere. That cyclones originate at high levels and extend downwards would seem to be implied by the following remark of Shaw and Lempfert*: "This disturbance moved slowly in a north-westerly direction and finally passed away to the North Sea. In the early stages small secondary minima of pressure developed near the primary minimum, and the process of travel appears to consist in the formation of a secondary' in front of the storm, and the filling up of the original minimum."

The local heating of the upper portion of the stratosphere which is considered to result in the formation of travelling cyclones, is regarded as being produced rapidly and as dying away slowly. In the case of the polar fixed cyclones the heating of the upper atmosphere must be a continuous process or nearly so; the heat is always passing downwards as rapidly as the air moving into the lower portion of the cyclone raises the upper surface of the stratosphere, and the vertical distribution of temperature in the atmosphere remains nearly constant.

One would expect the weather conditions to be much less. variable if they depended wholly upon the physical features of the earth's surface and the radiations received from the sun. Indeed the Trade Winds, and even the general circulation of the atmosphere, are fairly regular. It is to wandering cyclones that our short-period weather variability is due, and the want of regularity in the manner of their occurrence would be only what might be expected if they were caused by irregular streams of cosmic matter. Indeed, our atmosphere probably protects us from a bombardment from space, not only of matter but of many undesirable radiations. However, the energy received from the sun is probably somewhat irregular in its amount owing to the formation and disappearance of spots on the sun, and some relationship undoubtedly exists between weather condition variations and sun-spot periods.

*Met. Office Pub. 174, p. 45.

If the driving force of cyclones originates in the upper portion of the stratosphere where it is locally heated, then this heated air must be carried along with the stratosphere wind, and the course of the cyclone on the earth's surface should indicate the direction of flow of the stratosphere wind above. The generally easterly movement of cyclones favours this assumption; but the track of cyclones from south to north in North America seems to require explanation. In this connexion it is interesting to note that the dust of the Krakatoa eruption adhered closely to the area of the equatorial trade winds, except over the Atlantic, where the dust was carried to the north apparently along the American Cyclone track.

XXVII. Resonance Spectra of Iodine. By R. W. WOOD, Professor of Experimental Physics, Johns Hopkins University".

SING

[Plates VI.-VIII.]

INCE the appearance of the last paper on this subject (Phil. Mag. ser. 6. vol. xxvi.), the study of these interesting spectra has been continued without interruption, and some new and very important relations have been brought to light. As has been shown in previous communications, the vapour of iodine in vacuo, when excited to luminosity by the light of the Cooper-Hewitt mercury lamp (glass), emits a spectrum consisting of a series of doublets, with a separation of about 1.5 AU., very regularly spaced along the spectrum and separated by intervals of about 70 AU. These intervals increase gradually, however, as we pass away from the green mercury line, at which point the doublet series has its origin, until, in the extreme red, the distance between the last two doublets observed is about 102 ĂU., and the separation of the components of the doublet has increased to 2.8 AU. By the use of dicyanine plates the series has been followed to its termination at wave-length 7685 and the wave-lengths of the seven new doublets accurately measured. The doublets are not all of uniform intensity, and some are missing entirely, and it is the connexion between this circumstance and the way in which the doublet series is related to the band absorption spectrum, that is the most interesting point brought out by the recent investigations. By varying the conditions of the experiment it has been found possible to excite by the green mercury line not only the doublet series, but a simplified * Communicated by the Author.

system of fluted bands, few in number and regularly spaced if the iodine is in vacuo, increasing in number and complexity if a gas of the helium group is mixed with the iodine, or if more than a single iodine absorption line is excited by the mercury lamp. It is probable that the lines forming the doublets are themselves constituents of the fluted bands, and the transfer of energy from one part of the vibrating system to another, as a result of collisions between iodine and helium molecules, enables us to build up, so to speak, the complicated system of fluted bands shown in the absorption spectrum, out of a number of simpler systems which can be excited separately. This constitutes a very great advance in the analysis of band spectra, and brings us a step nearer to the point at which we can picture some idea of the vibrating mechanism.

In the more recent work, a method of illumination has been employed which is distinctly superior to any previously used, and as it is well adapted to purposes of demonstration I shall describe it in some detail. The iodine tubes which I now employ are of soft glass, about 40 cm. long and 3 cm. in diameter. One end is blown out into a thin bulb, taking care to avoid having the thick drop near the centre of the bulb. This is best accomplished by drawing off the tube in an oblique direction, which brings the drop-formed by the melting down of the pointed end-well to one side. If this is not done the drop is apt to form a small lens on the surface of the bulb exactly on the axis of the tube.

The other end is drawn down, and a few flakes of iodine introduced into the tube. It is a good plan to provide the tube with a lateral branch, by which the density of the vapour can be controlled, though this is not necessary for demonstration purposes. The iodine flakes are now brought into the bulb, or to the bottom of the lateral tube, and the tube joined to a Gaede pump, interposing a U-tube immersed in liquid air or solid CO2, or a tube filled with caustic potash, to keep the iodine out of the pump. During the exhaustion it is a good plan to heat the walls with a bunsen flame, except where the iodine is located. Then allow the tube to cool down to the temperature of the room, and heat the portion where the iodine is located. The flakes will sublime rapidly and crystallize on the cooler portions of the wall. The tube is now sealed off from the pump and the drawn-down end painted black for a distance of a few centimetres. For the illumination I used a very simple modification of the "light furnace" described in the earlier paper.

The iodine tube is fastened alongside of and in contact Phil. Mag. S. 6. Vol. 35. No. 207. March 1918.

S