METEOROLOGY. 



577 



tricts, respective! y known as 1, North Britain 

 (including from the Moray Firth to the middle 

 of Northumberland), along the coast; 2, Ire- 

 land, generally, around the coast ; 3, Central 

 (Wales to the Sohvay), coastwise ; 4, East 

 Coast (from Northumberland to the Thames) ; 

 5, South England (from the Thames round to 

 Wales), by the coast. At the most suitable 

 points along the coast in each of these divi- 

 sions, main stations are established, at which 

 a set of signals can be made to mariners within 

 sight of those points, by means of hoisting 

 upon separate poles or the same, a storm drum 

 and cone ; these at night are illuminated by 

 lights placed within them. By an arbitrary 

 and previously understood arrangement, the 

 order or combination of these as hoisted is 

 made to indicate a coming gale or tempest, its 

 probable severity, and the quarter from which 

 it will come. The warnings are telegraphed 

 from the central office in London, as directly 

 as possible to these stations, and to the prin- 

 cipal ports; from these, as centres, the in- 

 formation is then conveyed to other points 

 from which the signals cannot be seen. When 

 practicable, the signals are displayed first at 

 the point at which it is calculated the storm 

 will first be felt, and from this progressively 

 on to others, in accordance with its expected 

 course. The warnings are thus quite general- 

 ly conveyed to vessels in port and to craft near 

 the shores ; while from a large number of 

 points visible to a distance at sea, they are also 

 made known by the signals. It is still not ex- 

 pected that boats or ships shall delay sailing 

 for a gale that, after all, may not happen : the 

 real meaning of the signals is simply, "Be on 

 your guard " "A storm is probable " " Con- 

 sult your instruments and the signs of the 

 weather; " or, in certain cases, " Make imme- 

 diate preparation for a gale," &c. Persons 

 contemplating brief voyages are also thus 

 warned of occasions on which it is less safe to 

 embark than at others. In very many in- 

 stances, already, important testimony has been 

 borne to the value of the weather forecasts and 

 signals. The preparation of navigators within 

 sight of the coasts for coming storms, not other- 

 wise anticipated, is believed already to have 

 resulted in the saving of many lives and large 

 amounts of property. In the recent violent 

 gale on the English "coast, Jan. 17-19, 1863, it 

 is believed that the fatality and marine losses 

 would have been increased far beyond what 

 they actually were, had not the classes con- 

 cerned been warned beforehand by the Fitzroy 

 signal drums and cones displayed along the 

 coast. 



When admitting that the great system of 

 the trade winds is equivalent to ihe maintain- 

 ing of a certain general uniformity in the direc- 

 tions and character of the air currents for parts 

 of the earth in which it prevails, including 

 (though in less marked degree) even the tem- 

 perate zones, the directions of the prevalent 

 winds being generally the same for each region 



VOL. II. 3T 



or part in which the trades have an influence 

 it is nevertheless remembered that there are 

 continual causes of disturbance or local varia- 

 tion in this regularity, as by changing tempera- 

 tures of neighboring sections of the earth's 

 surface or of strata of air at different heights, 

 with changing amounts of evaporation, con- 

 densation, rain fall, &c., so that in fact at any 

 given time, and particulary in temperate lati- 

 tudes, air currents at the surface may be flowing 

 side by side, but in opposite directions ; or two, 

 three, or more such currents may be super- 

 posed, the one flowing above the other, and in 

 opposite directions or at any angle with each 

 other: these movements being attended with 

 collisions and intermixtures of bodies of air, 

 and followed by the results naturally due to 

 their unlike qualities, it will at once be seen. 

 that the interpretation of the continual varia- 

 tions of the barometer and thermometer along 

 with present weather signs, so as (if it be really 

 possible) to forecast with a good degree of 

 certainty the coming weather, becomes in itself 

 an extremely complex and difficult study. Of 

 course, it is impracticable to present here even 

 an outline, which should be of real value to 

 the reader, of the principles by which the fore- 

 castings of the weather are attempted. Those 

 principles necessarily include the rules for 

 interpreting the indications of the barometer, 

 but which Mr. Fitzroy has still further extend- 

 ed and particularized. Those who may be 

 interested to prosecute the subject in its details, 

 are referred to the " Weather Book," already 

 named, to Mr. Fitzroy's paper in the " Philos. 

 Magaz., " May, 1862 ; and to an abstract of his 

 rules for " reading the barometer,'' in the 

 ''Journal of the Franklin Institute," May, 

 1862 ; and of course, to the standard works on 

 Meteorology. 



It may be here remarked, however, as a 

 particular consequence of the general move- 

 ment of the trade winds, that in the northern 

 temperate regions a long-continued north wind 

 tends to become easterly, and a continued south 

 wind to become westerly. That is, the general 

 tendency of the winds in shifting is to "veer" 

 turn to the right, or by a direct rotation; 

 and it is much less common that they " back " 

 turn in the opposite course ; the probability 

 of the former change being to that of the latter 

 at least as 2 to 1. Not unfrequently, within a 

 lapse of time varying from 24 to 60 hours, the 

 surface wind, starting it may be in the south- 

 west or west, veers in this manner through the 

 complete circuit of the points of compass 

 (Dove's law of " rotation of the wind "). Great 

 changes or storms are usually preceded by a 

 fall of the barometer through an inch or more, 

 and within a comparatively brief time. The 

 tenth of an inch an hour may be taken as fore- 

 telling a tempest or very heavy rain. The 

 more rapidly such changes occur, the more 

 imminent and violent the approaching com- 

 motion. Mr. Fitzroy sums up as follows the 

 influence of the three causes known (electrical 



