1852.] 



ADDBESS OF THE PRESIDENT OF THE BRITISH ASSOCIATION. 



86 



the existence of wliat may possibly prove tlie most instructive) 

 as it is certainly at first sight the least explicable of all the period- 

 ical magnetic variations with which we have become acquainted. 

 I refer to the concurrent testimony which observations at parts 

 of the globe the most distant from each other bear to the exist- 

 ence of a periodical variation or inequality, affecting alike the 

 magnitude and frequency of the disturbances or storms. The 

 cycle or period of this inequality appears to extend to about ten 

 of our years ; the maximum and minimum of the magnitudes 

 aflected by it being separated by an interval of about five yeai-s, 

 and the difiei'ences being much too gTeat, and resting on an 

 induction far too extensive, to admit of uncertainty as to the facts 

 themselves. The existence of a well-marked magnetic period 

 which has certainly no counterpart in thermic conditions, appears 

 to render still more doubtful the supposed connexion between the 

 magnetic and calorific influences of the sun. It is not a little 

 remarkable that this periodical magnetic variation is found to be 

 identical in period and in epochs of maxima and minima with the 

 periodical variation in the frequency and magnitude of the solar 

 spots which Mr. Schwabe has established by twenty-six years of 

 unremitting labour. From a cosmical connexion of this nature, 

 supposing it to be finally estabhshed, it would follow, that the 

 decennial period which we measm-e by our magnetic instruments 

 is, in fact, a solar period, manifested to us also by the alternately 

 increasing and decreasing frequency and magnitude of obscura- 

 tions on the surface of the solar disc. May we not have in these 

 phenomena the indication of a cycle or period oi secular change 

 in the magnetism of the sun, aft'ecting visibly his gaseous atmos- 

 phere or jshotosphere, and sensibly modifying the magnetic 

 influence which he exercises on the surface of our earth ? 



The determination of the figure and dimensions of the globe 

 which we inhabit may justly be regarded as possessing a very 

 high degree of scientific interest and value; and the measure- 

 ments necessary for a correct knowledge thereof have been long 

 looked on as proper subjects for public undertakings, and as 

 liighly honourable to the nations which have taken part in them. 

 Inquiries in which I was formerly engaged led me fully to concm- 

 with a remark of Laplace, to the eft'ect that it is extremely pro- 

 bable that the fii-st attempts were made at a period much anterior 

 .to those of which history has preserved the record ; the relation 

 which many measures of the most remote antiquity have to each 

 other and to the terrestrial circumference strengthens this conjec- 

 ture, and seems to indicate, not only that the earth's circumference 

 was known with a great degree of accuracy at an extremely 

 ancient period, but that it has served as the base of a complete 

 system of measures the vestiges of which have been found in 

 Egypt and Asia. In modern times the merit of resuming these 

 investigations belongs to the French nation, by whom the arc of 

 the meridian between Formentera and Dunkirk was measured 

 towards the close of the last century. The Trigonometrical 

 Survey of Great Britain commenced in 1783, for the specific 

 object of connecting the observatories of Greenwich and Paris, 

 was speedily exjjanded by the able men to whom its direction 

 was then confided into an undertaking of far greater scientific as 

 well as topograjshical importance, having for its objects on the 

 one hand the formation of correct maps of Great Britain, and on 

 the other the measurement of an ai-c of the meiidian having the 

 extreme northern and southern points of the Island for its ter- 

 minations. A portion of this arc, amounting to 2 ° 0' 50', viz., 

 from Dunnose in the Isle of Wight to Clifton in Yorkshire — was 

 published in the Fhil. Trans, in 1803. As the whole arc, 

 extending from Dunnose to Unst and Balta, the most northern of 

 the Shetland Islands, would comprise more than 10 ° , and as 

 nearly half a century had elapsed since the publication of the 

 earlier part of the Survey, it is not surprising that some degree 

 of impatience should have been felt, both by those who desired 

 the results for scientific use and by those who were interested for 



the scientific character of the nation, that the general results of 

 the survey applicable to scientific purposes shall at length be 

 given to the world. Accordingly, at the Birmingham Meeting 

 of the British Association in 1849 a Resolution was passed 

 appointing a deputation to confer with the Master-General of the 

 Ordnance, and a similar resolution was passed about the same 

 time by the President and Council of the Royal Society. By a 

 recent letter to my predecessor from Capt. Yolland, of the Royal 

 Engineers, who is intrusted with the direction of the publication, 

 I am enabled to have the pleasure of announcing that the 

 " printing of thg observations made with the Zenith Sector, for 

 the determination of the latitudes of stations between the yeaj-s 

 1842 and 1850 is finished, and will be presented in time for the 

 meeting of the British Association, and that the calculations 

 connected mth the triangulation are rapidly advancing towards 

 their completion." 



In the mean time, the great arc of Eastern Europe has been 

 advancing with unexampled rapidity, and to an extent hitherto 

 unparalleled. Originating in topographical surveys in Esthonia 

 and Livonia, and commenced in 1816, the operations, both 

 geodesical and astronomical, have been completed between Izmail 

 on the Danube and Fugleness in Finnmarken, — an extent of 25-J 

 meridional degrees. Next to this in extent is the Indian arc of 

 21° 21' between Cape Comorin and Kaliana; and the third is 

 the French arc already refen-ed to of 12"^ 22'. It appears by a 

 note presented to the Imperial Academy of Sciences at St. 

 Petersburgh by M. Struve, that a provisional calculation has been 

 made of a large part of the gTeat arc of Eastern Europe, and 

 that it has been found to indicate for the figure of the earth a 

 greater compression than that derived by Bessel in 1837 and 1841, 

 from all the arcs then at his command, — Bessel's compression 

 having also been greater than Laplace's previous deduction. It 

 is naturally with great pleasure that I perceive that the figure of 

 the earth deriyed by means of the measurement of arcs of the 

 meridian approximates more and more nearly, as the arcs are 

 extended in dimension, to the compression which I published in 

 1825 as the result of a series of Pendulum Experiments which, 

 by the means placed by Government at my disposal, I was 

 enabled to make from the equator to within ten degrees of the 

 pole, — thus giving to that method its greatest practicable extension. 



The observations hitherto made on the tides of the ocean have 

 been insuflicient to furnish such a connected knowledge of the 

 subject as would enable us to follow the course of the tide over 

 an}^ considerable portion of the ocean ; and in the opinion of 

 persons most competent to judge, it is only by systematic obser- 

 vation, specially directed for the purpose, that this connected 

 knowledge is likely to be obtained. The recent researches of 

 Capt. Beechy, which have given a new and imexpected view of 

 the tidal movements of the ocean, show how much yet remains 

 to be learnt respecting the tides even for the practical purjjoses 

 of navigation. 



The facts derived a few years since from the barometrical ob- 

 servations at St. Helena, showing the existence of a lunar atmos- 

 pheric tide, have been corroberated in the last year by a similar 

 conclusion drawn by Capt. Elliott, of the Madras Engineers, from 

 the barometrical observations at Singapore. The influence of 

 the moon's attraction on the atmosphere produces, as might be 

 expected, a somewhat greater effect on the barometer at Singa- 

 pore, in lat. 1. ° 19, than at St. Helena, in lat. 15. °57. The 

 barometer at the equator aj^peare to stand on the average about 

 0.006 in. (more precisely 0.0057, in lat. 1 ° .19) higher at the 

 moon's culminations than when she is six houre distant from the 

 meridian. 



We have received from our valued Corresponding Member 

 Prof. Dove, for presention to (his Meeting, an important continu- 



