TRANSACTIONS OF SECTION A. 331 



in strong black lines. At one end of this narrow black field a broad black field 

 is formed large enough to accommodate the vernier arc. The vernier arc is a 

 thin white line traced on this dark field and split up by black divisions into ten 

 segments. The diffraction grating produces two flanking images of this arc, 

 which intersect each other and the original arc in a point determined by the 

 angular position of the grating. As the same angular position of the grating 

 causes the image of the scale to be projected upon the narrow dark field in a 

 position which may be displaced upwards or downward from the true position 

 of the scale, it follows that the displacement of the scale may be measured by 

 observing the point of intersection on the vernier arc. In this way very accu- 

 rate micrometer readings may be obtained. 



This form of micrometer possesses many advantages over those in present 

 use. In the first place, it is comparatively inexpensive to produce, and should, 

 therefore, be much cheaper than any other instrument of equal accuracy. In 

 the next place, as the scale lines do not cross the whole breadth of the field 

 they do not obscure the object under measurement. It is, therefore, immaterial 

 how bold the scale line is drawn, since measurements can be made to its edge. 

 Thus the reading of the scale is facilitated. Lastly, the scale can be read in 

 the field of the instrument where the vernier arc is displayed and there is no 

 divided screw head or other external part to be examined for this purpose. 



Department op Cosmical Physics. 



The following Papers and Report were read : — 



1. The Thunderstorms of July 28 and 29, 1911. 

 By Dr. W. N. Shaw, F.R.S. 



On the afternoon of July 28 the western districts of London were visited by 

 a severe thunderstorm, and in the afternoon and evening of the following days 

 thunderstorms occurred over nearly the whole of England and Ireland. The 

 London storm was accompanied by a squall of wind which reached fifty-four 

 miles per hour at South Kensington, and the storms on July 29 were preceded 

 by a violent squall which raised clouds of dust particularly noticeable in South 

 Wales. There were also marked oscillations of the sea on the south-west coasts. 



Meteorograms from records taken at the Meteorological Office at South Ken- 

 sington were shown for both storms. That for July 28 was of the ordinary 

 thunderstorm type — a sudden rise of pressure, which gradually fell off, accom- 

 panied by a drop of temperature of more than 20° F., a wind squall of fifty-four 

 miles per hour with a change of direction probably from E. to S.W., and a 

 rain shower giving an inch of rain in less than twenty minutes. For the follow- 

 ing day the sudden rise of the barometer was followed by violent fluctuations : 

 there was little fall of temperature, not much wind, and little rain. 



Reproductions of barograms and anemograms from all parts of the country 

 were exhibited in order to show the shape of the ' crochets d'orage ' and the 

 time of its occurrence at different stations. 



They showed that the disturbance of July 28 was confined to the neighbour- 

 hood of London. That of July 29 was very widespread. 



The most characteristic curve was that from Watlington, Oxon (Mr. W. H. 

 Dines), which showed that the 'crochet d'orage' occurred while the passage of 

 a depression was in progress, apparently just before the minimum was reached. 

 The 'crochet' of disturbance gave an M -shaped figure, which it took about 

 two hours to complete. A somewhat similar figure is noticeable at many other 

 stations in the region to the south and west. 



The weather maps showed that the storms of July 29 were incidental to the 

 setting in of a south-westerly current of air, replacing an easterly current which 

 had formed the northern part of a shallow low-pressure area to the south-west 

 of Britain. By the following Sunday morning the new south-westerly current 

 formed the eastern section of a well-developed cyclonic depression with its 

 centre off the west of Ireland. 



The barograms showed that the ' crochet ' disturbances could be arranged 



