Adding, subtracting, multiplying, dividing, and the working out of complex mathematical problems by machine seems wonderful until we stop to reflect that mathematics is the most precise and mechanical of all sciences. In the simpler forms these machines are mere counting mechanisms in which the counting is done very rapidly by the aid of intermeshing gears. The adding machine is in no sense possessed of any reasoning power, but blindly obeys the simplest of mechanical laws. There are rows of wheels with numbers running from 0 to 9 printed on their peripheries. One wheel represents “digits,” the next “tens,” the next “hundreds,” etc. The wheels are interconnected by means of gearing so that when the digits wheel makes a complete turn, the tens wheel makes 1-10th of a turn, and when the tens wheel completes a rotation the hundreds wheel makes 1-10th of a turn. Keys numbered from “0” to “9” are provided, which respectively turn each wheel through angles from 1-10th to 10-10ths of a rotation. Thus suppose the digit wheel has already been turned through 9-10ths of a rotation and registers the figure “9,” and the “8” key is depressed, the wheel will be given an additional turn of 8-10ths of a rotation and will register the figure “7,” but the tens wheel to the left will also move through 1-10th of a rotation, so that the two wheels will register “17.” Such is the underlying principle of the adding machine, but various refinements are added. For instance, the numbers that are being added are recorded in print, and the total sum of the numbers is not printed until the operator desires to foot up the column. More complicated, of course, are the mathematical machines which work out involved equations, but they are all based on simple mechanical operations. In the Weather Bureau at Washington there is a tide-predicting machine, which has been called a “great brass brain.” Its brass gears may be set to allow for all the varying factors of apparent solar and lunar motions, and they will work out the tide for any past or future data in a few moments, solving mechanically a mathematical problem that, by hand, would take hours and hours of weary figuring. HEARING AND TALKING WITH A MECHANICAL “EAR”