House-to-house communication is made easy through use of two bells of the ordinary gong type.
Place a small gong on the upper-story windowsill of the house next door. Do the same with one of your own windows. Attach a stout cord running to the other window-sill, as well as your own, so each boy will have a cord to do his own signaling, and a gong to receive signals from his friend. The cord is attached to a knocker, which raps the gong when the cord is pulled.
A simple code of numbers can be developed-one strike of the gong could mean "Meet me in the backyard right away"; two, "Meet me at the secret hideaway," etc., etc.
Doorbells of the buzzer type can also be used, with dry cell batteries, with wire properly insulated for outdoors.
A simple method of voice communication can be used with the aid of a length of rubber hose or ordinary water pipe.
In the illustration shown, a lookout in a tree-hut wanted to communicate with his "ground forces," so devised a "voice pipeline" for the purpose.
Such a hose or pipe can be run up the side of a tree, or through a pile of fallen autumn leaves, mound of earth or a snow fort.
When you wish to send a message, tap on the pipe or shake the hose, to attract attention. The other boy then puts his ear to the tube as you speak into it, and hears you clearly-and secretly. When he wants to get in touch with you, the process is reversed.
Perhaps the simplest of all "invisible inks' is ordinary lemon juice. Start with a clean pen, and dip it into a small cup of lemon juice instead of regular ink.
When the message is to be read, the one to whom it is sent merely holds the paper carefully over a flame or heat, whereupon the writing comes to life in a dark brown color, clearly legible.
Another method is the use of cobalt chloride crystals, which are easily procurable.
Drop a few crystals into a small amount of water, and when the crystals have all dissolved, add enough water to the mixture to make it pale red. Use this solution in the same manner as the lemon juice. The writing should not be visible, but if it is, simply add more water to your solution.
The person receiving your message will hold the paper over a flame or heat, and the message you have inscribed will immediately appear in blue.