*THE POPULAR SCIENCE MONTHLY.*

move him or distract him; he performs the most complicated operations 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 disturbance 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 calculations 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 considerable number of figures; he can add in his head numbers composed of twelve ciphers each; he multiplies by one another numbers 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 instantaneousness. 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 Salpêtrière two divisions of equal difficulty, one on paper according to our method, and the other in his own way; the second required four times less time than the first.