1876.] S. Kurz — O71 the Vegetation oftlie Nicolar Islands. 193 



medium sjDeed it is about one-half as sensitive, whilst at a very low speed of 

 rotation the two become almost equally sensitive. So far as can be judged, 

 the only difference between the two instruments must be in the amoimt of 

 exhaustion ; but whether a more perfect vacuum will produce a more sensi- 

 tive instrument or the contrary, I cannot pretend to say. The results here 

 described, are of course, only applicable to the " Light Mill ;" but I should 

 imagine that the other kind of radiometer, where the effect is measured by 

 torsion, would probably give more constant results. I do not even now 

 despair of the Radiometer being of some use in Photometry, although I 

 very much doubt if it will ever supersede the old Bunsen method -wdth the 

 standard candles. 



7. A SJcetch of the Vegetation oftlie Nicolar Islands. 

 By S. KuRz, Esq. 



(Abstract.) 



The Nicobar Islands are geologically divisible into two groups, the 

 the Southern, which belongs to the brown-coal formation, and the Northern, 

 where alluvial deposits are pierced by plutonic rocks. This geological 

 division coincides with the botanical one, at least in its broad features ; 

 the islands of the northern group being characterized by extensive grass- 

 heaths, while those of the southern group are forest-clad to the very suumiit. 

 The vegetation divides into the following five groups : — 



1. The mangrove-forests, which grow on the swampy alluvium at the 

 debouchure of the rivers. 



2. The beach forests, which occupy the calcareous sand of the beaches 

 and are the chief zone in which not only the cocoa-nut palm grows but on 

 which the Nicobarese build their huts. 



3. The troj)ical forests, growing on different strata. Of these, two 

 varieties are for the j)resent sej^arated : 



a. The coral-reef -forests, which grow on the upraised coral-lands ; 



And 



1). The true tropical forests, growing on plutonic rocks and polycistina- 

 clay. Those growing on polycistina clay are alone fully treated, while those 

 on plutonic rocks and on calcareous strata had to be omitted for several 

 reasons. 



4. The grass-heaths. These occupy the hillocky plateaux of the 

 islands of the Northern group and offer many peculiarities. 



5. The marine vegetation, which is restricted to a few phanerogamic 

 plants, while seaweeds and other algae arc abundantly represented. 



Cultivation is little represented on these islands and, therefore, not 

 separately treated, but the botanical constituents of the forests are given 



