378 Royal Society : — 



and masters of merchant vessels should be correctly and thoroughly 

 instructed in the methods of distinguishing in all cases between the 

 rotatory storms or gales, which are properly called Cyclones, and 

 gales of a more ordinary character, but which are frequently accom- 

 panied by a veering of the wind, which under certain circumstances 

 might easily be confounded with the phenomena of Cyclones, though 

 due to a very diflferent cause. It is recommended therefore that the 

 instructions, proposed to be given to ships supplied with meteorolo- 

 gical instruments, should contain clear and simple directions for 

 distinguishing in all cases and under all circumstances between these 

 two kinds of storms ; and that the forms to be issued for recording 

 the meteorological phenomena during great atmospheric disturbances 

 should comprehend a notice of all the particulars which are required 

 for forming a correct judgment in this respect. 



Thunder-storms. 



It is known that in the high latitudes of the northern and southern 

 hemispheres thunder-storms are almost wholly unknown ; and it is 

 believed that they are of very rare occurrence over the ocean in the 

 middle latitudes when distant from continents. By a suitable classi- 

 fication and arrangement of the documents which will be hencefor- 

 ward received by the Board of Trade, statistical tables may in pro- 

 cess of time be formed, showing the comparative frequency of these 

 phenomena in different parts of the ocean, and in different months 

 of the year. 



It is known that there are localities on the globe where, during 

 certain months of the year, thunder-storms may be considered as a 

 periodical phenomenon of daily occurrence. In the Port Royal 

 Mountains in Jamaica, for example, thunder-storms are said to take 

 place daily about the hour of noon from the middle of November 

 to the middle of April. It is much to be desired that a full and 

 precise account of such thunder-storms, and of the circumstances in 

 which they appear to originate, should be obtained. 



In recording the phenomena of thunder and lightning it is de- 

 sirable to state the duration of the interval between the flashes of 

 lightning and the thunder which follows. This may be done by means 

 of a seconds-hand watch, by which the time of the apparition of the 

 flash, and of the commencement (and of the conclusion also) of the 

 thunder may be noted. The interval between the flash, and the com- 

 mencement of the thunder, has been known to vary in different cases, 

 from less than a single second to between 40 and 50 seconds, and 

 even on very rare occasions to exceed 50 seconds. The two forms 

 of ordinary lightning, viz. zigzag (or forked) lightning, and sheet 

 lightning, should always be distinguished apart; and particular atten- 

 tion should be given both to the observation and to the record, in the 

 rare cases when zigzag lightning either bifurcates, or returns upwards. 

 A special notice should not fail to be made when thunder and light- 

 ning, or either separately, occur in a perfectly cloudless sky. When 

 globular lightning (balls of fire) is seen, a particular record should 

 be made of all the attendant circumstances. These phenomena are 

 known to be of the nature of lightning, from the injury they have 



