| | | | | Smoke rising perpendicularly | 6 | 5 | 1 | +------------------------------------------+--------------+------+------+ Among the animals whose movements give weather warnings few are more trustworthy than the leech. The reader may verify this by placing one in a broad glass bottle, tied over with perforated leather, or bladder. If placed in a northern aspect, the leech will be found to behave in the following manner:— 1. On the approach of fine or frosty weather, according to the season, it will be found curled up at the bottom. 2. On the approach of rain, snow, or wind, it will rise excitedly to the surface. 3. Thunder will cause it to be much agitated, and to leave the water entirely. PERIODS.—M. Köppen states, as the result of his examination into the chances of a change of weather, that _the weather has a decided tendency to preserve its character_. Thus, at Brussels, if it has rained for nine or ten days successively, the _next_ day will be wet also in four cases out of five; and the chance of a change decreases with the length of time for which the weather _from_ which the change is to take place has lasted. In the case of temperature for five-day periods, the same principle holds good;[17] for if a cold five-day period sets in after warm weather, we can bet two to one that the next such period will be cold too; but if the cold has lasted for two months, we can bet nearly eight to one that the first five days of the next month will be cold too. The chance of change is, however, greater for the five-day periods than for single days. Similar results follow for the months, but here again the chance of change shows an increase. [Footnote 17: “Recent Progress in Weather Knowledge,” by R. H. Scott, F.R.S.] “If we revert to the instance first cited, that of rain, the result is, _not_ that if it once begins to rain the chances are in favour of its never ceasing; all that is implied is, that the chances are against its ceasing on a definite day, and that they increase with the length of time the rain has lasted. The problem is similar to that of human life: the chance of a baby one year old living another year is less than that of a man of thirty. “The practical meaning of all this is, that although we know that a compensating anomaly for all extraordinary weather exists somewhere on the earth’s surface, _e.g._, the very common case of intense cold in America, while we have a mild winter in Britain, there is no reason as yet ascertained to anticipate that this compensation will occur at any given place during the year. In other words, when definite conditions of weather have thoroughly established themselves, it is only with great difficulty that the courses of the atmospheric currents are changed.”