decrease in the velocities of the currents through which the particles fall affects the shape of the cloud. Of course the above method assumes uniform velocities, which is only true if we make our intervals of time indefinitely small. Now great difference of velocity always indicates disturbance of some kind in the atmospheric equilibrium. Hence simple Cirrus,

which has very little slant, and hangs nearly vertically, is of itself by no means an indication of bad weather, but rather the opposite.

The curved form of the brushed-out portion of a Cumulo-nimbus may be drawn in a similar manner to the above, but the particles being here emitted in an upward direction, the curve will be reversed.

96. The true detached Cirrus, which has no tendency to distribution in parallel bands, does not often either accompany or immediately precede

extensive atmospheric disturbance. It may, however, usually be taken as an indication of previous and often very distant local showers. In fine summer weather, with a rather high and steady atmospheric pressure, a local thunderstorm frequently leaves a mass of Cirrus in the sky, which may remain for two or more days suspended in the atmosphere, and might float for a distance of several hundred miles from the spot at which it originated.

97. The largest and most elevated Cirrus clouds being usually relics of distant thunderstorms, are most common in our latitudes in summer. Cirrus, however, is not uncommon in our winter season, and is often then an indication of more disturbed weather, because the Cumulo-nimbi, from which it is developed, are not present in winter except within or in the neighbourhood of cyclonic circulations of the lower atmosphere. Thus it is that if a sheet of low Stratus Quietus, a cloud eminently of the winter anticyclone, be observed to have far above it a little Cirrus or Cirro -filum, we must regard it with suspicion.

The velocity of simple Cirrus is generally much less than that of Cirro-filum, for whenever the velocity of the currents which carry the cloud is very great the ice particles arrange themselves "end-on" in bands or belts, and the cloud is then no longer simple Cirrus.

98. A few rough rules with regard to the influence of the direction of Cirrus on the weather experienced in the British Isles, although the laws on which they

depend have not yet been treated of, may perhaps here be useful and instructive.

Cirrus moving from N. or N.E. with a high barometer is a sign of settled weather in summer and of temporarily fine weather in winter. With a low barometer it is a sign of marked improvement in weather.

Cirrus moving from E., a rare occurrence, is a sign of fine weather in winter, but of unsettled weather in summer.

Cirrus moving from S.E. (but it rarely does so with a low and unsteady barometer) is a sign of improving weather in winter, and in summer frequently indicates coming thunderstorms.

Cirrus moving from S. is generally an indication of unsettled weather, especially in summer. The temperature is then usually above the mean for the time of year.

Cirrus from S.W. indicates unsettled and sometimes stormy weather in winter. In summer it often precedes thunderstorms, but with a high barometric pressure and a high temperature it frequently has no disturbing influence, and is then usually replaced by Cirro-macula.

Cirrus from W. is commonly in summer a symptom of fair weather, but it is less so in winter.

Cirrus from N.W., when not tending to the form Cirro-filum, is an indication of temporary fine. weather, especially in summer.