8 
Dr. Wollaston on Sea-Sickness. 
removed, the blood would be no longer supported by its 
vessels ; but both would fall together with the same velocity 
by the free action of gravity; and the same contraction of 
the vessels which before supported the weight of the blood 
would now occasion it to press upon the brain, with a force 
proportional to its former altitude. 
In the same manner, and for the same reason, during a 
more gradual subsidence of the deck, and partial removal of 
support, there must be a partial diminution of the pressure of 
the blood upon its vessels, and consequently, a partial re- 
action upon the brain, which would be directly counteracted 
by a full inspiration. 
The consequence of external motion upon the blood will 
be best elucidated by what may be seen to occur in a column 
of mercury similarly circumstanced. 
A barometer, when carried out to sea in a calm, rests at the 
same height at which it would stand on shore ; but, when the 
ship falls by subsidence of the wave, the mercury is seen 
apparently to rise in the tube that contains it, because a por- 
tion of its gravity is then employed in occasioning its descent 
along with the vessel ; and, accordingly, if it were confined 
in a tube closed at bottom, it would no longer press with its 
whole weight upon the lower end. In the same manner, and 
for the same reason, the blood no longer presses downwards 
with its whole weight, and will be driven upwards, by the 
elasticity which before was merely sufficient to support it. 
The sickness occasioned by swinging is evidently from the 
same causes as sea-sickness, and that direction of the motion 
which occasions the most piercing sensation of uneasiness, is 
conformable to the explanation above given. 
