How should we imagine a mechanical brain? One way to think of a mechanical brain is shown in Fig. 2. We see here a railroad line with four stations, marked _input_, _storage_, _computer_, and _output_. These stations are joined by little gates or switches to the main railroad line. We can imagine that numbers and other information move along this railroad line, loaded in freight cars. _Input_ and _output_ are stations where numbers or other information go in and come out, respectively. _Storage_ is a station where there are many platforms and where information can be stored. The _computer_ is a special station somewhat like a factory; when two numbers are loaded on platforms 1 and 2 of this station and an order is loaded on platform 3, then another number is produced on platform 4. [Illustration: FIG. 2. Scheme of a mechanical brain.] We see also a tower, marked _control_. This tower runs a telegraph line to each of its little watchmen standing by the gates. The tower tells them when to open and when to shut which gates. Now we can see that, just as soon as the right gates are shut, freight cars of information can move between stations. Actually the freight cars move at the speed of electric current, thousands of miles a second. So, by closing the right gates each fraction of a second, we can flash numbers and information through the system and perform operations of reasoning. Thus we obtain a mechanical brain. In general, a mechanical brain is made up of: 1. A quantity of registers where information (numbers and instructions) can be stored. 2. Channels along which information can be sent. 3. Mechanisms that can carry out arithmetical and logical operations. 4. A control, which guides the machine to perform a sequence of operations. 5. Input and output devices, whereby information can go into the machine and come out of it. 6. Motors or electricity, which provide energy. THE KINDS OF THINKING A MECHANICAL BRAIN CAN DO There are many kinds of thinking that mechanical brains can do. Among other things, they can: 1. Learn what you tell them. 2. Apply the instructions when needed. 3. Read and remember numbers. 4. Add, subtract, multiply, divide, and round off. 5. Look up numbers in tables. 6. Look at a result, and make a choice. 7. Do long chains of these operations one after another. 8. Write out an answer. 9. Make sure the answer is right. 10. Know that one problem is finished, and turn to another. 11. Determine _most_ of their own instructions. 12. Work unattended.