NATURE 



301 



THURSDAY, FEBRUARY 17, 1876 



THE LISBON MAGNETIC OBSERVATIONS 



Annaes do Observatorio do Infanto D. Luiz. Magnetistno 

 Terrestre. Lisbon Imprensa Nacional. (1870, 1874) 



THAT in 1858 Portugal entered into "the Magnetic 

 Union of Nations, founded by Gauss and by Hum- 

 boldt in 1 832,'' was due to the enlightened solicitude of 

 the King Luiz I. for the progress of science. The 

 observatory founded by him at Lisbon, and placed under 

 the care of the late Ivlr. de Sihiera, has during the last 

 years been directed by Mr. de Brito Capello, who had 

 previously the oversight of the magnetic department, and 

 to whom we believe the results in the two parts of 

 Annals before us are chiefly due. 



The value of the Lisbon observations and of these 

 results is so much the greater that the neighbouring 

 country, Spain, has done nothing for terrestrial magnetism, 

 and even France, which stands so high in science, has 

 given us no such series of observations since Arago 

 originated them by the devotion of years to the move- 

 ment of the magnetic needle. 



The first part of the Annals before us (1870) treats of 



the results for the magnetic declination derived from direct 



observations made at 8 a.m. and 2 p.m. (1858- 1868), and for 



the diurnal variations (1864- 1868) from a photographic 



registering apparatus similar to that at Kew. The secular 



variation is first investigated. This mysterious movement, 



which we can attribute to no known cause, as it has no 



, known period, is shown at Lisbon by a mean approach of 



I the north end of the needle to the north at the rate of 5 "9 



1 a year during ten years, but varying from a minimum of 



I 4''8 in 1858-9, to 8'*2 in 1866-7. An annual law has also 



i been found consisting of an oscillation of i''5, the needle 



' being nearest the north in June, and farthest from it in 



December and January. 



The diurnal variation at Lisbon resembles to a con- 

 siderable extent that obtained at more northern stations 

 in Europe. The periodic variation of the mean range of 

 this oscillation, first discovered by Dr. Lament, which 

 occupies from ten to twelve years, has also been found at 

 Lisbon. This investigation has its importance increased 

 by the fact that the greatest and least mean oscillations 

 happen at the same times as the maximum and minimum 

 frequency of solar spots. The coincidence first indicated 

 by Sir E. Sabine, with reference to magnetic disturbances, 

 was remarked independently by Dr. Rudolph Wolf, of 

 Zurich, who has made it the subject of an extensive and 

 valuable series of investigations. Dr. Lament, com- 

 mencing with Cassini's observations, has found the mean 

 duration of the magnetic period to be io'43 years, while 

 Dr. Wolf obtains 1 1 "I I years from a longer, though per- 

 haps less certain, series of solar spot observations. Since 

 4 here can be no doubt that the yearly variation of sun- 

 spots and of the amplitude of the diurnal oscillation of 

 he magnetic needle follow the same law and depend on 

 he same cause, every new determination of the epochs of 

 ' naxima and minima is of value, fixing points which wiU 

 ietermine the mean duration and variable length of the 

 leriod, and thus probably lead us to a knowledge of the 

 Vol. xhi. — No. 329 



common cause. The Lisbon observations give 1859*9 

 and 1867-0 as epochs of maximum and minimum, agree- 

 ing very nearly with those derived from the Munich and 

 Trevandrum observations. In the determination of the 

 mean duration, everything depends on where we com- 

 mence. If we begin with Arago's magnetic observations, 

 the mean duration is about 107 years ; if we take the 

 most accurate results for the sunspot area from Messrs. 

 De la Rue, Stewart, and Loewy,we find 11 "2 years, nearly 

 that deduced by Dr. Wolf. Evidently a much longer 

 series of accurate observations is required to determine 

 the length of a period which has varied between 8 and 

 12*3 years within the last half century ; though we believe 

 Dr. Lament's result to be nearly true. 



As the Observatory establishment was not sufficient 

 for the long calculations required for the investigations 

 connected with the lunar diurnal variation, the latter was 

 sought approximately from the effects of the lunar action 

 in diminishing or increasing the solar diurnal oscillation 

 at different days of the moon's age. It was thus found 

 that the diurnal oscillation from 8 a.m. to 2 p.m. was 

 greatest when the moon was three and eighteen days old, 

 while it was least when she was ten and twenty-four days 

 old. This indicates a semi-diurnal oscillation due to the 

 moon's action, with an amplitude of 1' nearly, having the 

 maximum westerly positions when the moon is near the 

 upper and lower meridians. 



It is remarked by Mr. CapeUo, with reference to dis- 

 turbances, that in many cases the observations taken as 

 disturbed (those differing from the normal position by 

 2'*26 or more) were, properly speaking, not disturbed 

 observations, but belonged to very regular curves, in 

 which the morning minimum was more marked or the 

 afternoon maximum was less so than usual. Whereas 

 disturbances are shown by serrated curves. This remark 

 is quite exact, and the fact becomes even more marked in 

 lower latitudes. The investigation for the lunar variation 

 just noticed will show one cause for this variation of the 

 amplitude of the diurnal oscillation which is greater the 

 greater the lunar action. 



The second part of the Annals before us (1874) contains 

 the discussions for the other magnetic elements. IMr. 

 Capello finds, from nine years' observations, that the 

 horizontal force of the earth's magnetism was greatest in 

 July, least in September, with a secondary maximum in 

 November, and minimum in February. This result de- 

 pends on few observations, but approaches considerably 

 to that obtained first at Makcrstoun. The very marked 

 minimum found for the month of September induced Mr. 

 Capello to examine whether this might not be due to the 

 action of disturbances, which in general diminish the 

 earth's magnetic force ; it results from this discussion 

 that though the disturbances have a considerable effect 

 in increasing the amplitude of the annual oscillation, yet 

 the maximum in December is best marked in years of 

 least disturbance. This result he confirms by an exami- 

 nation of the Kew observations, and it agrees with that 

 deduced from each of a series of years' observations of 

 the bifilar at Makerstoun. Mr. Capello also finds that 

 the Munich observations show the most marked minimum 

 in September and October, in years of least disturbance. 

 It is an important fact, confirmed by the results from 

 many observatories, that the horizontal force of the 



