S88 M. A. Seccbi on the Periodical Variations 



One was established at Toronto in Canada, and another at Van 

 Diemen's Island ; these two points were chosen as being near to 

 the two points of maximum magnetic force, and because, being 

 almost antipodal to each other, they were eminently suited to 

 show the differences which the opposition of the seasons in the 

 two hemispheres might introduce in the variations. 



A third obsei*vatory was placed at the Cape of Good Hope as 

 the southern extremity of the African continent, and a point 

 important for the large amount of secular change taking place in 

 the magnetic elements. Lastly, the fourth was established at 

 St. Helena as a point situated within the tropics, and at a small 

 distance from the magnetic and geographic equators, as well as 

 from the line of minimum force. 



The system of observations established at these four places was 

 in the highest degree such as science might demand, and as should 

 form a true monument to posterity. The instruments were all of 

 the best dimensions and greatest precision, not only for the decli- 

 nation, but also for the inclination and intensity of the force. 

 They included a large declinometer, a bifilar magnetometer for 

 the horizontal force, a dipping-needle, a differential balance mag- 

 netometer for the vertical fBrce, and a series of smaller supple- 

 mentary instruments, with which the magnetic constants might 

 be determined from time to time without touching the large 

 magnets, which experience had shown ought never to be dis- 

 placed. All these instruments were furnished with mirrors or 

 collimators and microscopes, and were observed from a distance 

 with telescopes so as not to disturb their action. 



The observatories were erected in open situations and without 

 iron fastenings. All were furnished with proper astronomical 

 instruments for the exact determination of time, and with a very 

 complete series of meteorological instruments to be observed at 

 the same times as the magnetical ones. Precise determinations 

 were made from time to time of the different constants required 

 for reducing the magnetic observations, such as the torsion of 

 the thread, the coefficient of the variations of the force of the 

 bars corresponding to variations in their temperature, the azi- 

 muths, meridian marks, &c. 



Even this rough outline of the system must surprise the reader, 

 and it may be safely said that there was never undertaken for 

 the study of any physical problem a larger scheme or one less 

 restricted by considerations of cost. It was arranged that the 

 action of the different observatories w^s to be simultaneous ; 

 their observations were made at first every two hours, and after- 

 wards at every hour of exact Gottingen mean solar time, besides 

 observing at least every five minutes whenever any extraordinary 

 perturbation was perceived either in the needle or in the atmo- 



