THUNDER-STORMS. 539 



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 are not at all agreed, and the observations made on its phenomena 

 (made at different stations) do not accord in a satisfactory man- 

 ner. In fact, it appears as if the indications of the instruments 

 are due to local canses, so that they* do not lend themselves to any 

 useful generalizations. When a thunder-storm is actually raging 

 in the neighborhood of a station, the indications of electrometers 

 thereat are most erratic and violent, but it can not be said that any 

 electrometer enables us to perceive the approach of a storm one 

 whit earlier than we are able to do by careful watching of the 

 clouds. As regards forecasting thunder-storms, this can be done 

 in a general sort of way ; but it is not practicable to predict which 

 villages or parishes, or even counties, will be visited. When the 

 daily weather charts are drawn, if we find that there is an uneven- 

 ness in the isobaric lines — that is, if these are wavy, or bulge out 

 irregularly — we know that thunder-storms are likely to burst 

 somewhere or other over the country, but that is all we can say. 

 At each station the barometer is unsteady— the mercury moving 

 up and down in the tube — during the actual continuance of the 

 storm ; but this oscillation of the mercurial column has nothing 

 to do with the irregularity in the isobaric lines above mentioned. 

 Forecasting these storms is, therefore, always an uncertain and a 

 thankless task, for local success is rarely attained. 



Among the earliest symptoms of the approach of a thunder- 

 storm is the appearance on the western horizon of a line of cumu- 

 lus (" wool-pack ") clouds, exhibiting a peculiar turreted structure. 

 I say on the western horizon, for most of our changes of weather 

 come from that quarter, and it has been proved that thunder- 

 storms, like wind-storms, advance over the country, generally, 

 from some westerly point. This bank of clouds moves on, and 

 over it appear first streamers and then sheets of lighter upper 

 cloud — cirrus, or " mare's-tail " — which spread over the sky with 

 extreme rapidity. The heavy cloud mass comes up under this 

 film, and it is a general observation that no electrical explosion or 

 downfall of rain ever takes place from a cloud unless streamers 

 of cirrus, emanating from its upper surface, are visible when the 

 cloud is looked at sideways from a distance. 



Thunder-storms are generally accompanied by falls of hail as 

 well as rain, and these hailstones assume the form of lumps of ice 

 — some even as large as hens' eggs, and weighing several ounces, 

 having been known to fall. The stories of masses of hailstones, 

 weighing many pounds, having been found after storms, are ex- 

 plained by the fact that the hailstones, after they have fallen, 

 may have frozen to each other and formed a solid lump on the 

 ground. Large hailstones are composed of alternate layers of 

 clear crystalline and white porous ice, and many of them consist 

 of an aggregate of smaller hailstones which have attached them- 



