272 PROFESSOR C. PIAZZI SMYTH ON 



Lightning max. in July = 72, and Lightning min., between February and March = 7. 



Tables XIX, and XX treat similarly of Aurora. The former giving the number of 

 days on which it is seen per average month = 1 "6 ; and the latter recording the number 

 of stations reporting it, viz., 7. 



The former table comes curiously and very decidedly to a double maximum ; viz. , 

 in October = 2*1, and in February = 2*0. While the latter table confirms the former by 

 coming also to a double maximum in the same months; viz., in October =11, and in 

 February = 11. 



Again the former table comes to its minimum in June and July = 0*1; but partly 

 owing to lengthened twilight days ; while the latter table comes also to its minimum in 

 June and July, for probably the same reasons, and averages then per month less than 1. 



Wherefore combining the two kinds of return by multiplying into each other, we have 

 the first maximum of Aurora in any whole winter season, occurring in October and 

 registering 23 ; the second maximum in March = 22. And the one, or at least the utter 

 minimum of the whole year between June and July, with a record of something less than 

 1. While the comparative or necessary minimum between the two maxima, occurs in 

 December, and reaches in its two components 2 x 7 = 14 on the whole return. 



The double annual maxima of Aurora are thus well marked out by the minima on 

 either side of them, and are all the more interesting from being so absolutely opposed in 

 date and character to the single annual maximum which Lightning obtains in the cycle of 

 a year. That is to say, Lightning has its one annual maximum in July, just when Aurora, 

 but under forced conditions, reaches its one most conspicuous minimum of the year. 



While at the same time the dates of Aurora reaching its two maxima for any winter 

 season, are not corresponded to inversely by both of those being dates for Lightning falling 

 to a minimum ; though the Spring maximum of Aurora does occur at the beginning of a 

 nearly three-month's minimum of Lightning, and corresponds rather to the period of 

 dry North- East winds. 



So that on the whole, Aurora and Lightning, noth withstanding that they may both 

 claim electricity in large part for their luminous manifestations, are left by meteorology 

 at singular points of variance with each other, just as spectroscopic analysis declares their 

 molecular constitution entirely different. 



Thus far all these preceding 20 Tables of 32 years' collection are the continuation of the 

 older 20 tables of the first 16 years, under the same titles and the same numbers. But 

 before we introduce their summings up for one mean epoch of time, as was done in the 

 old Table XXL, to be re-introduced presently as Table XXXI. , — let us describe the new 

 Tables already alluded to, and to be numbered from XXI. to XXX. inclusive. 



3. Newly prepared Tables for New Derivations from the Old Observational 



Meteorology. 



Thus of the new Tables XXL and XXII., the former of the Mean monthly Maxima, 

 and the latter of the Mean monthly Minima, of Temperature in the shade, — it should be 



