170 
SUCCESSFUL USE OF OIL TO CALM ROUGH SEAS. 
Tropical plants ... 18,000 species. 
Half-hardy ' ... 18,000 ,, 
Hardy ... ... 8,000 ,, 
Alpines ... ... 2,000 ,, 
46,000 species. 
Drawbacks of a Botanic Garden. 
It should be borne in mind that a Botanic Garden, from 
the nature of the case, differs in many respects from an 
ordinary garden. Many plants, which it is not worth while 
for a gardening firm to keep in stock are interesting from their 
structure or associations. The way to make a fine-looking 
collection of orchids, or ferns, or palms, is to select a few of 
the finest kinds, and grow several plants of each, eliminating 
from the collection altogether the less showy and ornamental 
kinds. It adds very much to the labour and thought required 
from the cultivator if a botanical arrangement is followed. 
For instance, take a genus like Ranunculus : some of the 
species grow naturally in swamps, some in pastures, some in 
woods, and some in waste ground. If these are grown side 
by side, the cultivator needs to individualise each species in 
order to treat it properly. A. class of plants which are 
specially interesting to botanists are those which, either from 
their size or texture, cannot be properly represented by 
herbarium species, such as palms, cycads, tree ferns, and 
succulent plants. 
(To be continued.) 
ON THE SUCCESSFUL USE OF OIL TO CALM 
ROUGH SEAS.* 
BY W. P. MARSHALL, M.I.C.E. 
[Note. —The particulars given in this paper have been mainly 
collected from various published notices.] 
The idea of making use of oil to calm and smooth the 
surface of water is an old one, as illustrated by the well- 
known old saying about throwing oil on the troubled waters; 
but it has been only in recent years that the subject has 
received any serious attention, and it was previously treated 
as merely fanciful and imaginary. The fact has, however, 
* Transactions of the Birmingham Natural History and Micro¬ 
scopical Society, February 14th, 1888. 
