1863.] Mr. B. Stewavt— Squalls ofSOth Oct. and2lst. Nov. 51 



II. On the Sudden Squalls of 30tli October and 21st November 

 1863/' By Balfour Stewart, M.A., F.R.S., Superintendent of 

 the Kew Observatory. Received December 10, 1863. 



The 30tli of October was windy throughout, and in the afternoon there 

 was a very violent squall. 



The barograph at the Kew Observatory, as will be seen from Plate 1. 

 which accompanies this communication, records a very rapid fall in the 

 pressure of the atmosphere, which appears to have reached its lowest point 

 about 3*" 9"^ P.M., G. M. T. At this moment, from some cause, possibly a 

 very violent gust of wind, the gas-lights in the room which contained the 

 barograph went out, and were again lit in a quarter of an hour. During 

 this interval the barometer had risen considerably ; and indeed the baro- 

 graph curve, although unfortunately incomplete, presents the appearance 

 of an extremely rapid rise. It may therefore perhaps be supposed that 

 there was a very sudden increase of pressure accompanied with a violent 

 gust of wind at the moment when the gas went out, which would be about 

 3^ 9°" P.M., as above stated. 



In a paper communicated to the Royal Society on November 23, 

 Mr. Glaisher has remarked that at Greenwich the time of maximum de- 

 pression of the barometer was 3^ 30"^ p.m., while at the RadclifFe Observa- 

 tory, Oxford, it was 2^' 30°^ p.m. This would indicate a progress of the 

 storm from west to east, in accordance with which Kew should be some- 

 what before Greenwich as regards the time of maximum depression. This 

 anticipation is therefore confirmed by the record of the Kew barograph 

 which has been given above. 



The indications of the Kew self-recording electrometer during this squall 

 show that about 2'^ 39°" p.m. the electricity of the air, which before that 

 time had been very slightly negative, became rapidly positive, then quickly 

 crossed to negative, became positive again, and once more crossed to nega- 

 tive about 3^ 3™ P.M., recrossing again from strong negative about 3^^ 51°^ 

 P.M., after which it settled down into somewhat strong positive. 



It is well, however, to state (what may also be seen from Plate I.) 

 that the variations of this instrument between 3^ 3™ p.m. and 3^ 51"^ p.m. 

 were so rapid as not to be well impressed upon the paper. 



At Kew there is often occasion to move the dome, so that we cannot 

 well have an instrument which records continuously the direction of the 

 wind ; but we have a Robinson's anemometer, which records the space tra- 

 versed by the wind, and thus enables us to find its velocity from hour to 

 hour, though not perhaps from moment to moment. A reference to 

 Plate I. will show an increase in the average velocity of the wind during 

 this squall. 



A somewhat similar squall took place in the afternoon of Saturday, 

 November 21st, about 4 o'clock. 



In this case the Kew barograph presents a rapid (and, in the curve, 



