THE SIX GATEWA YS OF KNOWLEDGE. 267 



performs depends chiefly on the oxygen taken 

 in. If the air has only three quarters of the 

 density it has in our ordinary atmosphere here, 

 then one and one-third times as much must be 

 inhaled, to produce the same oxidising effect on 

 the blood, and the same general effect in the 

 animal economy ; and in that way undoubtedly 

 mountain air has a very different effect on living 

 creatures from the air of the plains. This effect 

 is distinctly perceptible in its relation to health. 



But I am wandering from my subject, which 

 is the consideration of the changes of pressure 

 comparable with those that produce sound. A 

 diving bell allows us to perceive a sudden in- 

 crease of pressure, but not by the ordinary sense 

 of touch. The hand does not perceive the dif- 

 ference between 1 5 Ibs. per square inch pressing 

 it all around, and 17 Ibs., or 1 8 Ibs., or 20 Ibs., or 

 even 30 Ibs. per square inch, as is experienced 

 when you go down in a diving bell. If you go 

 down five and a half fathoms in a diving bell, 

 your hand is pressed all round with a force of 

 30 Ibs. to the square inch ; but yet you do not 



