394 Chronicles of Science. [July, 



cation is that which has reference to the relation between the varia- 

 tion of the magnetic declination and the frequency of solar spots. 

 In 1859, Professor Wolf had proposed the following formula for 

 this variation (v) at Christiania : — 



v = 0'-0413 r + 4'-921, 



where r is the " relative number representing sun-spot frequency." 

 The following Table, in which v represents the calculated v the 

 observed variation, indicates what cannot but be regarded as a very 

 satisfactory confirmation of this formula : — 





1864. 



1865. 



1866. 



1867. 



1868. 



1869. 



V .. 



.. 6'-87 



6' -26 



5'-64 



5' -25 



6' -58 



8' -39 



v' .. 



.. 6-00 



5-72 



5-70 



5-69 



6-65 



7-82 



The observations of the magnetic variation were communicated to 

 Professor Wolf by Messrs. Mohn and Fearnley. It will be seen at 

 once that the minima of the two series closely accord. Doubtless, 

 with the progress of observation, the empirical formula stated above 

 will have to be slightly modified. 



Mr. Birt supplies further notes on the visibility of the spots 

 upon the floor of the lunar crater Plato. It seems too difficult to 

 separate the different qualities of vision, the various conditions of 

 the atmosphere, and still more the peculiarities of different telescopes, 

 from the variations of visibility considered by Mr. Birt, to form any 

 satisfactory conclusion on the subject. 



Professor Cayley supplies two valuable papers on the geome- 

 trical theory of solar eclipses. The mode in which he treats the 

 subject is too rigidly mathematical to admit of being presented in 

 these pages. It may be remarked, however, that in his first paper 

 he discusses, in a most lucid and interesting manner, the problem of 

 determining the stereographic projection of the curve of two dimen- 

 sions which is the intersection of a cone and a sphere, showing that 

 this projection is a bicircular quartic. 



4. BOTANY. 



Cross-fertilization. — Dr. Ogle has continued his observations on 

 the various contrivances in the structure of the flower to promote 

 cross-fertilization. The purpose of the nectary is to attract insects ; 

 any noticeable irregularity of the corolla is also usually connected 

 with the visits of insects, compelling them, when in search of the 

 nectary, either to impinge on the anthers or on the stigma, and thus 

 carry the pollen from one flower to another. Adrien de Jussieu had 

 remarked the connection which exists between the presence of nee- 



