The three principle defenses, in order of effectiveness, are sliver, iron, and salt.
Defences against ghosts have been discovered, created, and manufactured since the Problem began. The main ones used by agencies and the general population are silver, iron, salt, lavender, and running water. There are numerous ways these defenses have been used in products.
Silver is generally used by agencies as a chain net to subdue a Source with (any object used to subdue a Source is also called a Seal). Some rapiers are made from silver. Most people use silver in the form of adornments such as silver scarf pins.
Iron is a strong and commonly used defense. Agents have iron rapiers and chains, while the public uses iron wards, in their homes as iron plates around their house and doorways, iron gnomes in their gardens, iron baby bracelets and crib decorations, as well as a variety of other uses.
Salt is primarily used by agents in salt bombs or to scatter on a Source. Agents may also use salt to in suspicious areas or around their iron chains.
Lavender's smell is effective against ghosts, mostly the weaker ones. It is used by agents in small vials of lavender water, and most people wear dried sprigs of lavender, burn lavender at night time, or hang lavender outside their house.
Other defenses include curfew and Ghost-lamps. Curfew is enforced by the British government, and begins shortly before nightfall. The population is strongly encouraged to be indoors during this time, protected by their home defenses. Ghost-lamps are extremely bright street lamps installed in heavily-populated areas. They turn on for short intervals of time throughout the night to discourage ghosts. Running water is also effective against ghosts; much activity in high density areas is done near the large bodies of moving water, such as the River Thames. A large portion of the England's trading is done on the Thames, by boat. Houses and shops often have runnels of water that they fill at nightfall. George Cubbins is said to have survived an encounter with a Spectre by turning on a garden hose and standing behind it.