74 



earlier at Toronto and St. Helena than at the Cape of Good Hope 

 and Hobarton. The comportment of the two equinoctial months, 

 March and September, at the Cape of Good Hope and St. Helena is 

 pointed out as presenting a remarkable contrast to that of the two 

 solstitial groups which have been described, and at the same time a 

 still more remarkable contrast to each other, March being at almost 

 all the hours on the West, and September on the East, of the mean 

 line. 



In conclusion the author points out one or two practical conside- 

 rations suggested by the facts under notice : — 



1. That as, in the Annual Variation represented in the plate, the. 

 same months occupy positions on opposite sides of the mean line at 

 ditferent parts of the twenty-four hours, the mean annual variation, 

 or that which is shown by the mean values in each month taken from 

 all the observation hours, must be merely a residual and not an abso- 

 lute quantity; and that consequently natural features must be more 

 or less masked in deductions in which only mean values are brought 

 into view. In fact, as has been shown in the published volumes of 

 the observations at St. Helena and Hobarton, the mean annual va- 

 riation at those stations is so small as to be scarcely sensible. But 

 when we resolve these mean results into their respective constituents,, 

 viz. the annual variation at each of the observation hours, there is 

 then at once disclosed to us an order of natural phenomena, very 

 far from inconsiderable in amount, systematic in general aspect, and 

 apparently well deserving the attention of those who are occupied 

 in the delightful and highly intellectual pursuit of tracing the agen- 

 cies of nature. 



2. We perceive in the variations of the position of the several 

 months in the annual range, the necessity of paying regard to the 

 period of the year, as well as to the period of the day at which ob- 

 servations have been made which do not include long intervals, and 

 from which, nevertheless, inferences are drawn in respect to secular 

 change. Such observations, when not those of a fixed observatory, 

 are usually made at some hour in the day-time, when it needs only 

 a glance at the plate to perceive that annual as well as diurnal 

 variation-corrections are required, unless the month as well as the 

 hour are the same in the earlier and later observations. A table of 

 corrections for every hour of the day to the mean value in each 

 month — corrections derived, as in the instances now before the So- 

 ciety, from a series of strictly comparable observations continued for 

 several years — should be considered, not merely as a desirable, but 

 as an almost indispensable provision, in countries where magnetic 

 surveys are conducted with the degree of perfection of which they 

 are now susceptible. 



3. The subjoined Notice by Monsieur A. Quetelet, Foreign 

 Member of the Society, is entitled "Sur les ondes atmospheriques." 



J'ai eu I'honneur de deposer, dans la seance precedente, un travail 

 imprime sur les pressions et ondes atmospheriques, fesant partie d'un 

 ouvrage sur le climat de la Belgique. Si je me permets d'appeler, 



