THE SANITARY INFLUENCE OF FORESTS 15 
Shelter belts are of great economic importance, but this 
view of the subject scarcely comes within the scope of this 
book. I may advert to one or two points of interest. 
The utilisation of mountain pasture by sheep is a large 
industry. In the Highlands of Scotland extensive areas 
of the sheep farms are covered with bracken and are 
consequently of no value at present for grazing. Bracken 
land can always be covered with trees. It is believed that 
of the total acreage of mountain pasture in the Highlands, 
at least 10 to 20 per cent could be afforested without 
producing any reduction in the number of sheep, merely 
by planting up the bracken areas. Furthermore, the 
shelter provided would enable the grass to grow better and 
the sheep to thrive more, and would actually in the end 
increase the output of mutton considerably. 
Throughout Ireland in the central plain and other low- 
lying districts, and in parts of Scotland, there are extensive 
tracts of peat, which were formed after the retreat of the 
Ice Sheet, by the gradual filling up of the shallow lakes 
that were due to the disturbance of the natural drainage 
by glacial deposits. The chilly effects of these peat-bogs is 
well known (6). They act as centres of cold, often giving 
rise to severe spring frosts in the adjoining farm land, so 
that early kinds of potatoes cannot be grown, while grass is 
later in spring in the pasture fields. In order to mitigate 
these effects, peat-bogs should be surrounded with plantations 
which can be easily and cheaply established in the cut- 
over margin of the bog. This part of the peat moss is 
either already drained or can be drained at a small expense ; 
and on it spruce, maritime pine, Scots pine, and larch can 
be profitably grown (7). 
It was formerly believed that the leaves of the trees, by 
their absorption of carbon dioxide from the atmosphere and 
their exhalation of oxygen, made the air in a forest healthier ; 
but Ebermayer showed that the oxygen exhalation of a 
forest in proportion to the needs of human respiration is 
insignificant, and is perhaps offset by the increase of carbon 
dioxide which results from the decomposition of organic 
