CH. XIIl] CALCULATING PRODIGIES 275 



later.relied largely on the auditory sense to enable him to recollect 

 numbers. " For my own part," he wrote, in later life, " though 

 much accustomed to see sums and quantities expressed by the 

 usual symbols, yet if I endeavour to get any number of figures 

 that are represented on paper fixed in my memory, it takes me 

 a much longer time and a very great deal more exertion than 

 when they are expressed or enumerated verbally." For instance 

 suppose a question put to find the product of two numbers each 

 of nine digits, if they were " read to me, I should not require 

 this to be done more than once ; but if they were represented 

 in the usual way, and put into my hands, it would probably take 

 me four times to peruse them before it would be in my power 

 to repeat them, and after all they would not be impressed so 

 vividly on my imagination." 



Bidder retained his power of rapid mental calculation to the 

 end of his life, and as a constant parliamentary witness in 

 matters connected with engineering it proved a valuable 

 accomplishment. Just before his death an illustration of his 

 powers was given to a friend who talking of then recent dis- 

 coveries remarked that if 36,918 waves of red light which only 

 occupy one inch are required to give the impression of red, and 

 if light travels at 190,000 miles a second, how immense must be 

 the number of waves which must strike the eye in one second 

 to give the impression of red. " You need not work it out," 

 said Bidder, " the number will be 444,433,651,200,000." 



Other members of the Bidder family have also shown 

 exceptional powers of a similar kind as well as extraordinary 

 memories. Of Bidder's elder brothers, one became an actuary, 

 and on his books being burnt in a fire he rewrote them in six 

 months from memory but, it is said, died of consequent brain 

 fever; another was a Plymouth Brother and knew the whole 

 Bible by heart, being *able to give chapter and verse for any 

 text quoted. Bidder's eldest son, a lawyer of eminence, was 

 able to multiply together two numbers each of fifteen digits. 

 Neither in accuracy nor rapidity was he equal to his father, 

 but then he never steadily and continuously devoted himself 

 to developing his abilities in this direction. He remarked 



18—2 



