THE SPIRIT-RAPPERS 



latter is estimated by Professor Fessenden to be 

 40,000 times as sensitive as the best type of coherers. 



Again, the range of impressions which we get 

 from lifting an object in the hand is rather small. 

 An ordinary chemist's balance is about twenty 

 million times as sensitive. It will weigh down to 

 the two hundredth part of a milligramme. If we 

 had as delicate a sense for gravity effects, we should 

 notice the difference in the weight of our hands in 

 lifting them an inch off the table, or, to employ 

 an ingenious comparison of Professor Wiener's, 

 we should be able to feel with our bodies the pres- 

 ence of a lump of gold buried in the earth not 

 too deep as we walked over it. Kohlrausch and 

 Topler have devised barometers so sensitive to vari- 

 ations of air-pressure that a person walking through 

 an open doorway, at the opposite side of a room, 

 will set the indicator swinging. It will betray a 

 variation of the hundredth part of the millionth 

 of an atmosphere. So, too, in comparison with 

 our ability to count about ten or eleven per second, 

 Feddersen has been able to devise an instrument 

 which will count down to the hundredth part of a 

 millionth of a second. 



Wherever we turn we shall find instruments 

 which surpass each and all of our senses in a most 

 humiliating way. Without them we should know 

 very little of the world about us. Lacking them, 



305 



