eS 
258 
and their frequency he estimates at twen- 
ty or thirty iu a second ;:and he adds that 
the resemobla: yce of the muscular 
tions to the sound of carriages at a dis- 
tance, arises nut so much from the qua- 
lity of the sound as froman z agreeme: it In 
frequency with an aver age ot the tremors’ 
usualy prodaced by the number of stones 
in tie reguiar pavement of London pass- 
ed over by carriages moving quickly. 
if the number of vibrations be twenty- 
four in a second, and the breadth of each 
stone be six inches, the rate of the car- 
riage would be about eight miles in an 
hour, which agrees with the truth of the 
faers on which the estimate is founded. 
The doctor was led to theinvestigation 
of the cause of sea-sickness from w hat he 
himseif experien¢ed im a voyave.. He 
first observed a peculiarity in his inode of 
respiration, evidently connected: wiih the 
motion of the vessel: 
ions were not taken with the accus- 
tomed unifurmity, but were interrupted 
by irregular pauses, with an appearance 
of Ww atching ior some favourable oppor- 
tunity for making: =) succeeding effort: 
atid it seemed as if the act of mspiration 
‘were. in some manner to be guided by’ 
the tendency of the vessel to pitch with 
an uneasy) motion. ‘This action, he 
thought, affected the system by its influ- 
ence on thé motion 6f the blood, for, at 
the same instant that the chest is di- 
lated forthe recepuion of air, its vessels 
become aiso mare open to the reception 
of the Klood, so that the return of blood 
from the he: Ay is more free than at any 
other period of complete respiration. 
But by the act ef expelling air from the 
Jungs, the ingress oi the biood is so far 
obsiructed, 
Lrain is exposed by the trepai, a succes- 
sive turgescence and subsidence of the 
mn is seen.In alternate motion with the 
iferent states of the chest. Henee, 
a aps, in. sevcre head-aches a degree 
of temporary relief is obtained by occa- 
sional - compicte mspirati OuS: in sea- 
sickness also the act of.inspirauion will 
have some tendency to reheve, if regu- 
jated so as to counteract any temporary. 
pressure cf Lleod upon the ‘brain, The 
srincipal uneasiness is felt during the 
subsidence of tbe vessel by Bs. sein 2 off 
the wave ov whichit rests. tis during 
this subsidence ihat the biocd ee a ten- 
dency to press with unusual farce upon 
the brain, 
reasoning, and by what is known to occur 
in the barometer, which, when carnedd 
Royal Socrety 
wibra-: 
that bis respiva- 
that when the surface of the 
This fact is elucidated by. 
of London. binds 
out to sea in acalm, rests at the same 
height at which it would stand on the 
[Oct f 
es 
shore ; but when the ship falis by the — 
ubsidence of the wave, the mercury is 
seen apparently to rise in the tube that 
contains it, because a portion of its gra- 
vity is then employed in occasioning its 
descent along with the vessel; and ac 
cordingly, if it were gonfined ina tube. 
ciosed at bottem, it would no longer press. 
with its whole weight upor the lower end. 
In the same manner, and: for the same 
reason, the-blood no longer presses down= 
wards. wait its whole weight, and will be 
driven- upwards by the elasticity which 
“before was merely sufficient to support it. 
The sickness occasioned by swingmg 
may be explained in the same way. It 
is in descending -forwards that this sen- 
sation is perceived ; 
has the greatest tendency to move from 
the feet towards the head, since the line 
Joming them is in the direction of the- 
motion, but when the descent is back~ 
wards, the motion is transverse to the 
line of ‘the body, it occasions little incon= 
venience, because the tendency to propel 
the blood towards the head is meonsi-, 
derable. Dr. Wollaston thinks that the 
contents of the intestines are also aifect- 
ed by the same cause as the blood; and 
if these have any direct disposition eG res 
gurgitate, this consequence will be in na 
degree counteracted by the process of 
respiration. “ In thus referring,” says 
our author, “ the sensations of sea-sick- 
hess in so great a degree to the agency _. 
of mere-mechanical pressure, I feel con- 
firmed by considering the consequence of 
an Opposite motion, which, by tou quick- 
ly withdrawing blood from the head, oe- 
casions a tendency to faint, or that ape 
proach to fai.ang which amounts to a 
mementary giddiness with diminution of 
muscular power. Ata time when I was 
much fatigued by exercise, I had occa. 
sion to run to some distance, and seat 
myself under a low wall for shelter from 
avery heavy shower. In rising suddenly 
feos this posftion, I was attacked with 
such a See of giddiness, that I invo- 
luntarily dropped into my former pos- 
ture, andy was instantaneously relieved by. 
return of blood to the bead, from every 
sensation of uneasiness. Since that time, 
the same affection has frequently occur~ 
red to mein slighter degrees; and Lhave 
observed that ft has ie onder similar 
circumstances of rising suddenly from an 
inclined position, 
previous fatigue, sinking down 
; niediately. 
for ihen the blood . 
afier some degree of - 
n.again ime - 
= 
. 
