228 Royal Society, 



botanical relations, as it appears highly probable that the matiere 

 incrustante is the same in all phanerogamous plants, as they yield 

 furfurole. On the other hand, the matiere incrustante in the Algae, 

 mosses and lichens, as it yields fucusole and not furfurole, though the 

 same in each of these classes, is evidently different from that of pha- 

 nerogamous plants. The matiere incrustante of ferns appears how- 

 ever to be dissimilar from either of the others, as it yields an analo- 

 gous but peculiar oil. 



April 25. — " On the means adopted in the British Colonial Mag- 

 netic Observatories for determining the absolute values, secular 

 change, and annual variation of the Magnetic Force." By Lieut.-Col. 

 Edward Sabine, R.A., For. Sec. R.S. 



The determination of the mean numerical values of the elements 

 of terrestrial magnetism in direction and force at different points of 

 the earth's surface (the force being expressed in absolute measure, 

 intelligible consequently to future generations, however distant, and 

 conveying to them a knowledge of the present magnetic state of the 

 globe), and the determination of the nature and amount of the se- 

 cular changes which the elements are at present undergoing, are, as 

 the author states, the first steps in that great inductive inquiry by 

 which it may be hoped that the inhabitants of the globe may at some 

 date, perhaps not very distant, obtain a complete knowledge of the 

 laws of the phenomena of terrestrial magnetism, and possibly gain 

 an insight into the physical causes of one of the most remarkable 

 forces by which our planet is affected. 



After stating the inadequacy of the instruments originally pro- 

 posed by the Royal Society, to the attainment of all the objects for 

 which they had been designed, the author refers to the modifications 

 which had been introduced, in the instruments and methods of ob- 

 servation for the determination of the absolute values, and the secu- 

 lar changes of the horizontal component of the magnetic force. He 

 then gives the series of the results of the monthly observations at 

 Toronto from January 1845 to April 1849 as relatively correct ; and 

 from this series, regarding each monthly determination as entitled 

 to equal weight, and taking the arithmetical mean of all the values 

 as the most probable mean value, obtains 3*53043 as the mean value 

 of the horizontal force at Toronto, with a probable error of + "00055; 

 and the probable error of +'0040 for each monthly determination. 



This is on the most simple hypothesis, in which neither secular 

 change nor annual variation is supposed to exist. The monthly re- 

 sults however distinctly indicate a secular change, and by means of 

 them, on the hypothesis of a uniform secular change, the author 

 deduces '0042 as the annual decrease of the horizontal force during 

 the period comprehended by the observations, the value of the force 

 on the 1st of March 1847, the mean epoch being 3*53043, with a 

 probable error of +'00025. 



For the purpose of deducing the values of the total magnetic force 

 and its secular change from those of the horizontal force, it is neces- 

 sary to know the magnetic inclination corresponding to the epoch 

 and its secular change. From the observations of the inclination, 

 75° 16'*09 is deduced as the value of this element on the 1st of March 



