234 
Report on the Trade in Animals. 
between the direction of the wind 
and the course of the steam- 
boat. If the wind is, for instance, 
ahead of the steamboat, its ven- 
tilating force is increased by the 
accelerated velocity due to the 
speed of the vessel, say 10 miles 
an hour. If, on the contrary, the 
wind is abaft the steamboat, its 
ventilating force is diminished 
to even a greater extent, because 
the steamboat will be then 
making more way. However, 
it may be reckoned that the ven- 
tilating force of wind (of a given 
velocity) on the hold of a cattle- 
steamer varies to the extent of 
that due to a current of air tra- 
velling at the rate of 20 miles 
per hour. 
The influence of the direction 
of the wind on the ventilation 
of the hold is not confined to 
the circumstance just stated. It 
is well understood that proper 
ventilation cannot be obtained 
without an outlet for the noxi- 
ous gases, as well as an inlet for 
the fresh air. The ventilation of 
the hold of a ship is therefore 
as dependent upon a proper out- 
let or " upcast " as a proper inlet 
or " downcast." The upcast is 
almost always obtained by leav- 
ing the hatchways open (see Fig. 
3), and not unfrequently this 
opening is made to do double 
duty by having a canvas wind- 
sail put down the centre of it. 
Of course, if the wind is abaft 
the steamer, it assists in blowing 
away the vitiated and heated air 
that seeks to ascend, partly in con- 
sequence of the action of the down- 
cast, but chiefly in obedience to 
the force of gravity. If, how- 
