6z THE POPULAR SCIENCE MONTHLY. 



move him or distract him; lie performs the most complicated op- 

 erations in the midst of the excitement of public representations. 

 He can even talk while mentally working ; he answers questions 

 properly, and even keeps up a regular conversation without dis- 

 turbance to his arithmetical operations. During his exercises he 

 is sometimes seen to lift his hand to his forehead or to close his 

 fist, or to draw imaginary lines with the forefinger of his right 

 hand in the palm of his left hand. These are little tricks of no 

 importance, that vary from one day to another. Finally, after an 

 interval which is always short, he says, " I am done," gives the 

 solution of the problem, and proves it for his own satisfaction. 



The two remarkable features in M. Inaudi's mental calcula- 

 tions are the complexity of the problems he undertakes, and, in a 

 less degree, the rapidity with which he finds the solution. Most 

 of the questions that are put to him involve the use of a consider- 

 able number of figures ; he can add in his head numbers com- 

 posed of twelve ciphers each ; he multiplies by one another num- 

 bers composed of eight or ten figures each ; he tells how many 

 seconds there are in an arbitrarily selected number of years, 

 months, days, or hours. These operations, to be well carried on, 

 require the subject to keep in mind the data of the problem and 

 the partial solutions till the moment when the definitive solution 

 is found. For so considerable a task M. Inaudi takes, they say, 

 an extremely short time — so short as to convey the illusion of in- 

 stantaneousness. It has been published on this subject that " he 

 adds, in a few seconds, seven numbers of eight or ten figures. 

 He completes the subtraction of two numbers of twenty-one 

 figures in a very few minutes, and finds as rapidly the square 

 root or the cube root of a number of from eight to twelve figures, 

 if the number is a perfect square or cube, but needs a little more 

 time if there is a remainder. He likewise finds, with incredible 

 celerity, the sixth or seventh root of a number of several figures. 

 He performs a division or a multiplication in less time than it 

 takes to announce it." M. Inaudi found in thirteen seconds the 

 answer to the question, How many seconds are there in eighteen 

 years, seven months, twenty-one days, and three hours ? 



But while M. Inaudi calculates rapidly, he is not much more 

 rapid than a professional calculator who is permitted to work 

 out his problems on paper ; M. Inaudi's merit is that he performs 

 his operations in his memory. 



His processes are not ours, and although he has been able to 

 read and write for four years and is acquainted with the ordinary 

 methods of calculation, he does not use them. M. Charcot caused 

 him to perform at the Salpetriere two divisions of equal diffi- 

 culty, one on paper according to our method, and the other in his 

 ( iwn way ; the second required four times less time than the first. 



