1178 



METEOROLOGY. 



[WE ATH ER-FORF.C1ST8. 



which is M yet the mort uncertain branch of science, 

 neb a principle must be acted on without tlio leant 

 beaUtion, if we would not commit ourselves to crude 

 theories, and, possibly, to positively dangerous in- 



To the general inquiry, "On what meteorologic con- 

 ditions or changes are the forecast* baaed?" Admiral 

 FiUroy replies in the following terms :* " They depend 

 on considering the atmosphere as a lighter ocean, having 

 currents ; elastic expansibility ; equilibrium ; momentum 

 or inertia ; chemical alterations, and extreme sensibility 

 to heat or cold, its chirf motors ; and on knowing the 

 statical condition of the air, in the oceanic envelope at 

 many places simultaneously ; likewise, again similarly 

 after certain intervals of time, by which inter-com- 

 parisons are made, showing the relative conditions and 

 causations, whence dynamic effects originate. These 

 dynamic motions are proportional to disturbances of 

 level, like those caused by a head of water ; to inequalities 

 of temperature, and consequent chemical changes, with 

 more or less electric action. They are our winds, and 

 may be soft or gentle, or as heavily boisterous as in a 

 tempett: of which differences, through all degrees, in- 

 strumental means and telegraphy now give available 

 information. To utilise their indications adequately, a 

 central office should know the natural and general 

 atmospheric movements, with their disturbing causes, 

 even as a pilot knows the varieties of streams and eddies 

 in a wide estuary. 



"The whole map of a region (say the British Islands) 

 should be outlined in the mind, as the estuary, with its 

 shoals, is mentally visible to the pilot. The normal 

 tendency of the whole atmosphere (in our latitudes) to 

 more eastward, while crossed or variously interfered with 

 by polar or tropical currents, that in course of seasons 

 cause every variety of wind and weather, should be 

 always considered ; and then, with due allowances made 

 for gradual advances from westward, for effects of land 

 and differences of temperature, good forecasts may be 

 generally drawn." 



Hitherto we have referred solely to the principles on 

 which the meteorologist should be guided ; but we shall 

 now afford a number of practical instructions, for the use 

 of those who desire to undertake weather-forecasting. 

 The following rules are chiefly selected from the Baro- 

 meter Manual, published by the Board of Trade ; 

 a work which, whilst of trifling cost, will be found an 

 admirable compendium, and a guide for making ob- 

 servations and registering results. It is highly possible 

 that some who peruse these pages, reside in localities 

 where the observations of an intelligent person may be of 

 great use to those whose livelihood is obtained at sea, 

 Ac. ; and a careful attendance to the following suggestions, 

 with the possession of a good barometer and thermo- 

 meter, may be of essential service. Care must be taken 

 to correct the results obtained, for elevation above the 

 sea-level, temperature, <fec., if they are to be compared 

 with those of other observers. Rules for these cor- 

 rections have been already given in the preceding 

 chapter. 



RULES FOR PREDICTING CHANGES IN THE 

 WEATHER. 



1 . If the mercury, standing at 30 inches, rise gradually 

 whilst the thermometer falls, and dampness becomes 

 lew, N.W., N., or N.E. wind; less wind; or less rain 

 and snow may be expected. 



a fall take place, with a rising thermometer, and 

 increasing dampness, wind and rain may be expected 

 from aB., 8., or 8.W. : a fall in winter, with a low 

 thermometer, foretels snow. 



. imi-iidiiig N. wind, before which the barometer 

 ofUn rua, may be accompanied with rain, hail, or snow, 

 and so forms an apparent eice/iHim to the above rules, 

 for the barometer always rises with a north wind. 



4. The barometer being at 20J inches, a rise foretell 



less wind, or a change of it northward, or leu wot. But 

 if at 29 inches, a fast Jir.it rise precedes strong winds or 

 squalls from N.W., N., or N.E. ; after which, a gradual 

 rise, with falling thermometer, indicates improving 

 weather ; with a steady or rising thermometer, a S. or 

 S.W. wind will follow, especially if the rise of the baro- 

 meter has boon sudden. 



6. A rapid barometric rise indicates unsettled, and ;\ 

 rapid fall, stormy weather, with rain or snow ; whiM 

 a steady barometer, with dryncss, indicates continued 

 fine weather. 



6. The greatest barometric depressions indicate gales 

 from the 8.E., S., or S.W. ; the greatest elevations 

 foretel wind from N.W., N., or N.E., or calm 

 weather. 



7. A sudden fall of the barometer, with a westerly 

 wind, is sometimes followed with a violent storm from 

 the N.W., N., or N.E. 



8. If the wind veer to the S., during a gale from E. to 

 S.E., the barometer will continue to fall until the wind 

 is near a marked change, when a lull may occur. The 

 gale may afterwards be renewed, perhaps suddenly and 

 violently ; and if the wind then veer to the N. W. , 

 N. , or N. E. , the barometer will rise, and the thermometer 

 fall 



9. The maximum height of the barometer occurs 

 during a N.E. wind, and the minimum during one from 

 the S. VV. ; hence these points may be considered as tho 

 poles of the wind. The range between these two heights 

 depends on the direction of the wind, which causes, on 

 an average, a change of half an inch ; on the moisture of 

 the air, which produces, in extreme cases, a change of 

 half an inch ; and on the strength of tho wind, which 

 may influence the barometer to the extent of two inches. 

 These causes, separately or conjointly with the tem- 

 perature, produce either steady or rapid barometric 

 variations, according to their force. 



Allowances should invariably be made for the state of 

 the barometer and thermometer, for some days as well as 

 some hours previously to forecasting, because sudden 

 changes are always indicated antecedently to their 

 occurrence. The longer and steadier a change is in 

 coming, the longer the expected weather, either wet or 

 fine, will last. 



Sometimes severe weather from the S., not lasting 

 long, may cause no great fall, because followed by a 

 duration of N. winds ; and, occasionally, the barometer 

 may fall during N. winds and fine weather, because a 

 continuance of S. wind is about to follow. 



The table of average temperatures, corrected for the 

 place of observation, but given for Oreenwich at page 

 1172, is to form the standard of comparison with the 

 actual temperature at the time of observation, in con- 

 nection with tho barometrical indications. If, at 8 or 9 

 A.M., the thermometer is much higher than the corrected 

 average of the place, a southerly or westerly wind (tro- 

 pical current) may be expected ; if considerably lower, 

 then a northerly wind (polar current) is indicated. 



The preceding rules, of course, solely refer to observa- 

 tions with instruments ; but the following external signs 

 (chiefly selected from the Manual) of impending changes 

 should be taken collaterally, because they will frequ 

 serve as interpreters of obscure indications given by tho 

 barometer, itc. In connection with the following, our 

 remarks on well-known phenomena, depending on tho 

 moisture of the atmosphere, f may also be read ; and the 

 general observer can employ both, when the ordinary 

 meteorological instruments are not accessible. It will be 

 seen that the objects are purely natural, and that tho 

 effects noticed really depend on certain influences, which, 

 affording the phenomena as signs, are also their pro- 

 ducing causes. 



CLOUDS. The following appearances depend chiefly on 

 the refractive power of vapour, either of tho cloud, or 

 suspended invisibly in the air,X and, occasionally, on its 

 comparative or nearly entire absence. 



A rosy sky, at sunset, presages fine weather ; an 



t The ttudrnt may read, with thin, the articles on Refraction and Colour, 

 In the Section on Light. See pp. 4i, 49, tt itf. 



