398 Royal Imtitution : — 



westward, usually above and at an angle with the polar stream or 

 current of air, often mixing with it, but at times separately pene- 

 trating downward, then sweeping and warming the earth's surface, 

 uncombined with the polar current, even while feeling its approach- 

 ing influence — and thus, as it were, forcing passages between 

 streams of chilling polar air that at the same time are moving in 

 opposite and nearly parallel directions. 



Sometimes their opposition is so equal, and equilibrium is so com- 

 plete, that a calm is the result, no sensible movement horizontally 

 along the earth's surface being perceptible. 



Self-registering barometers show the alterations in tension, or, so 

 to speak, the pulsations on a large scale, of the atmosphere by hourly 

 marks ; and the diagram expresses to a practised observer what the 

 " indicator card " of a steam cylinder shows to a skilful engineer, or 

 a stethoscope to a physician. 



Our own islands have very peculiar facilities for meteorological 

 communication by telegraph between outlying stations on the sea- 

 coast and a central place, all being at nearly the same level, and 

 nearly all comparatively uninfluenced by mountain-ranges. 



And now the results are, that having daily knowledge of weather 

 (including ordinary facts of a meteorological nature) at the extreme 

 limits and centre of our British Islands, we are warned of any great 

 change taking place, the greater atmospherical changes being mea- 

 sured by days, rather than by hours. Only local changes, however 

 violent they may be occasionally (and dangerous in proportion to 

 their suddenness and violence), only such changes are unfelt at a 

 distance, and do not influence great breadths, say hundreds of miles 

 in area, of atmosphere. 



Extensive changes, showing differences of pressure above or below 

 the normal or mean level, amounting to an inch or thereabouts, are 

 certain to be followed by a marked commotion of the elements in the 

 course of a few days. If the fall has been sudden, or the rise very 

 rapid, swift but brief will be the resulting elementary movement ; if 

 slow or gradual, time will elapse before the change, and the altered 

 state of weather will take place more gradually, but last longer. 



Notice may thus be obtained and given a few hours or a day, 

 or even some days, before any important change in the weather 

 actually occurs. 



Having such knowledge, it obviously follows that telegraphic 

 warning may be sent in any direction reached by the wires, and 

 that occasionally, on the occurrence of very ominous signs, baro- 

 metric and other, including always those of the heavens, such cau- 

 tions may be given before storms as will tend to diminish the risks 

 and loss of life so frequent on our exposed and tempestuous shores. 



It has been proved also lately that storms, indeed all the greater 

 circulations of atmosphere between the tropics and polar regions, 

 have an eastward motion bodily while circulating around a centrical 

 area. Within the tropics it is otherwise, or westward, till they 

 recurve. 



In answer to a question from the Royal Commissioners on Lights, 



