His ear was also equally exact. He could readily distinguish the 5th part of a note. By the quickness of this sense he could judge of the size of a room and of his distance from the wall. And if ever lie walked over a pavement, in courts or piazzas which reflected a sound, and was afterwards conducted thither again, he could tell in what part of the walk he had stood, merely by the note it sounded.
Dr. Saunderson had a peculiar method of performing arithmetical calculations, by an ingenious machine and method, which has been called his Palpable Arithmetic, and is particularly described in a piece prefixed to the first volume of his Algebra. That he was able to make long and intri cate calculations, both arithmetical and algebraical, is a thing as certain as it is wonderful. He had contrived for his own use a commodious notation for any large numbers, which he could express on his abacus, or calculating table, and with which he could readily perform- any arid'. .roe tical operations, by the sense of feeling only, for which reason it was called his Palpable Arithmetic.
His calculating table was a smooth thin board, a little more than a foot square, raised upon a small frame, so as to lie hol low, which board was divided into a great number of little squares, by lines inter secting one another perpendicularly, and parallel to the sides of the table, and the parallel ones only one-tenth of an inch from each other ; so that every square inch of the table was thus divided into 100 little squares. At every point of inter
section the board was perforated by small holes, capable of receiving a pin ; for it was by the help of pins stuck up to the head through these holes, that he express ed his numbers. He used two sorts of pins, a large and a smaller sort ; at least, their heads were different, and might ea sily be distinguished by feeling. Of these pins he had a large quantity intwo boxes, with their points cut off, which always stood ready before him when he calcula ted. The writer of that account de scribes particularly the whole process of using the machine, and concludes: " He could place and displace his pins with in credible nimbleness and facility, much to the pleasure and surprise of all the be holders. He could even break off in the middle of a calculation, and resume it when he pleased, and could presently know the condition of it, by only drawing his fingers gently over the table."