Ancient Alarm System in the Forbidden City

Surrounding the halls and palaces in the Forbidden City, there are many stone fences with beautiful baluster stigmas. The baluster stigmas are in the shape of lotus petals with 24 string lines which symbolize the 24 solar terms. Therefore, they are also called 24 solar term baluster stigma. If look carefully, you can find that some of the baluster stigmas are hollow from top to bottom, like hollow gourds. In fact, it is a special alarm device named “shi bie la”, which can quickly transmit dangerous information to all parts of the Forbidden City. How clever the ancient Chinese were by combining the decorative and alarming functions perfectly!
 

Shibiela is set in the following areas: Gate of Supreme Harmony (Taihemen), Gate of Heavenly Purity (Qianqingmen), Gate of Earthly Purity (Kunningmen), West Prosperity Gate (Xihuamen), East Prosperity Gate (Donghuamen) and some other important areas of the Forbidden City.
 

In the past, the soldiers on duty all carried a small Tongjiao, which is a trumpet made of copper and looks like a small horn. In case of danger, such as enemy invasion or fire, they would insert the Tongjiao into the hole and blow hard. The stigma is capable of producing a very sharp, far-reaching sound. If all these stigmas blow, the whole Forbidden City can hear them.

Below is the sound signal transmission route supposing the invaders come from the Meridian Gate:

Step 1: Seeing the invaders, the guards of Meridian Gate will ring the alarm bell set in the gate tower and the sound will be transmitted to the Gate of Supreme Harmony Square.
 

Step 2: When the guards of Gate of Supreme Harmony Square hear the alarm bell, they will sound Shibela. Shibela, which is set in surrounding areas, will transmit the alarm sound to the West Prosperity Gate, East Prosperity Gate, Hall of Supreme Harmony, Hall of Central Harmony, Hall of Preserving Harmony, and Gate of Heavenly Purity.

Step 3: As soon as the guards of Gate of Heavenly Purity hear the alarm, they will continue to blow Shibiela. And then the alarm is sent to the Gate of Good Fortune, Gate of Thriving Imperial Clan and Gate of Earthly Purity.

Step 4: After knowing the alarm, the guards of Gate of Earthly Purity will pass the message to the Gate of Divine Prowess through Shibiela. At this time, all parts of the Forbidden City get to know the invasion.

According to the estimation, it will take no more than 1 minute to complete the above four steps. That is to say, if any invaders are found at the Meridian Gate, the guards at all positions of the Forbidden City can hear the alarm within 1 minute.
 

On September 15, 1813, Tianli Sect’s two groups of believers invaded the Forbidden City from the West Prosperity Gate and East Prosperity Gate respectively. Seeing them, the soldiers guarding the east side of Gate of Supreme Harmony Square immediately sounded Shibela. The dangerous information was transmitted to all parts of the Forbidden City in a short while. Nearly a thousand soldiers from the firearm camp rushed to fight with the believers and protected the Forbidden City.
 

The alarm cannon built on the Baita Mountain was very close to the Forbidden City. As long as a token of dangerous information was set out from the Forbidden City, the gunner would immediately shoot towards the sky. When the guards stationed in or around the capital heard the sound of the cannon, they would gather as soon as possible to resist the invaders in time.

 Further Reading:
Amazing Drainage System of the Forbidden City
Arches on the Wall of Forbidden City