WIND -PRESSURE AND ATMOSPHERIC PR FISSURE.
[TO TER EDITOR OF TUN EPROTATOR.1
SIR,—In your last week's number you suggest that there may have been some error in the recorded pressure resulting from the lath storm in Sydney, as it is said to have reached only 117 lb. per square foot, or 5 per cent. on the ordinary pressure of the atmosphere. Will you allow me to point out that it may be misleading to compare a wind-pressure with the total pressure of the atmosphere, as the latter is perfectly balanced, and only acts as a compressing force on all matter on the earth's surface? All wind is, indeed, caused by a partial vacuum being formed OD some part of the earth's surface, and the phenomenon is due to the rushing of air from regions of high pressure to regions of low pressure. That a difference of pressure should ever be established sufficient to cause a pressure of 5 per cent, of the whole along the line of the inrushing stream is, I think, very extraordinary, when the fact is borne in mind that the air is perfectly free to flow in to the low-pressure region the moment a sensible difference is created.
The pressure you mention is indeed a most abnormally high one, the maximum wind-pressure allowed for as likely to influence structures in England being from 40 lb. to 64) lb. per square foot.
—I am, Sir, &c., C. F. THOMPSON. Preswylfa, near Cardiff, November 22.
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