202 
Proceedings of the Royal Irish Academy. 
and allied affections, in which the right heart and the veins are con- 
gested, and the left heart and the arteries comparatively empty. 
"With reference to the cnrative application of high atmospheric 
pressure alluded to in the last sentence I will make a few observations 
further on. 
In the building of the great bridge now in course of construction 
across the Mississippi at St. Louis, observations of the utmost value 
to science have been made upon this subject. 
Through the courtesy and kindness of my friend Dr. Thomas 
O'E-eilly, an eminent medical practitioner at St. Louis, I have been 
favoured with a copy of the chief engineer's Report, from which I shall 
borrow a few extracts bearing upon the subject of this Paper. The 
Eeport itself I shall have much pleasure in presenting to the Academy, 
as from it alone an adequate conception of the vastness of the work 
engaged in, and of the great engineering skill developed in its execu- 
tion, can be formed. 
In regard to the subject under discussion it may be confidently 
stated that never hitherto has the experiment of subjecting the human 
body to the operation of compressed air by immersion and respiration, 
been performed on so large a scale, and in so extreme a degree. 
The chief engineer, Colonel Eads, says : A column of water one 
hundred and ten feet six inches in height would be equal to a pressure 
of 47*96 pounds per square inch, assuming the weight of the water to 
be 62*5 pounds per cubic foot. The greatest pressure marked by the 
gauges was fifty-two pounds, and it is not probable i:hat the pressure 
in the air-chamber ever exceeded fifty, or fifty-one pounds." 
Effects of Compressed Air on the Men, 
The first symptom manifesting itself, caused by the pressure of 
the air, is painfulness in one or both ears. The Eustachian tubes ex- 
tending from the back of the mouth to the bony cavities over which 
the drums of the ears are distended, are so minute as not to allow the 
compressed air to pass rapidly through them to these cavities, and 
when the pressure is increased rapidly, the external pressure on the 
drums causes pain. These tubes constitute a provision of nature to 
relieve the ears of such barometric changes as occur in the atmosphere 
in T^hich we live. The act of swallowing facilitates the passage of 
the air through them, and thus equalizes the pressure on both sides of 
the drums, and prevents the pain. The pressure may be admitted into 
the air-lock so rapidly that this natural remedy will not in all cases 
relieve it. Ey closing the nostrils between the thumb and fingers, 
shutting the lips tightly, and inflating the cheeks, the Eustachian tubes 
are opened, and the pressure on the inner and outer surfaces of the 
tympanum is equalized, and the pain prevented. 
This method must be used and repeated, from time to time, as the 
pressure is let on, if it be increased rapidly. 'No inconvenience is felt 
by the reaction when the pressure is let off, as the compressed air 
