THE SIX GATEWAYS OF KNOWLEDGE. 273 



sound, and that is air pressure. We have not a 

 complication of motions in various directions. We 

 have not the complication that we shall have to 

 think of presently, in connection with the sense 

 of force ; complication as to the place of ap- 

 plication, and the direction, of the force. We 

 have not the infinite complications we have in 

 some of the other senses, notably smell and taste. 

 We have distinctly only one thing 'to consider, 

 and that is air pressure, or the variation of air 

 pressure. Now when we have one thing that 

 varies, that, in the language of mathematics, is 

 "one independent variable." Do not imagine 

 that mathematics is harsh and crabbed, and re- 

 >ulsive to common sense. It is merely the 

 etherealisation of common sense. The function 

 of one independent variable that you have here 

 to deal with is the pressure of air on the tym- 

 panum. Well now, in a thousand counting-houses 

 and business offices in Birmingham and London, 

 and Glasgow and Manchester, a curve, as Pro- 

 fessor Cayley pointed out, is regularly used to 

 show to the eye a function of one independent 



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