CHAPTER V.--_The signal station._ Location of stations: General considerations 68 Backgrounds 70 Azimuth of stations 71 Altitude 71 Determination of background color 72 Choice of apparatus 73 Miscellaneous considerations 73 Intervisibility table 74 Finding a station 75 Operation of stations: Personnel 76 Calls and personal signals 78 Opening communication 79 Commencing the message 80 Sending and receiving 80 Breaking 80 Discontinuance of transmission 81 Acknowledgment of receipt 81 Station records 81 Formation of signals 82 Repeating the message 83 Signal practice 83 CHAPTER VI.--_Codes and ciphers._ Codes in use 84 Employment of codes 84 Cipher code 85 The War Department Code 86 Cipher code in field work 87 Field ciphers: Description and use 87 Forms of field cipher 88 Inversions 88 Concealment of terminations 88 Cipher apparatus: The cipher disk 89 The mathematical cipher 93 The route cipher 94 Cipher detection: Employment of cipher disk 96