I believe there is one or more errors in the circuit, unless the photo cell is really a 12 volt solar panel, but that probably won't work either.

You need to add a ten ohms or more fixed resistor in series with the 5k variable, otherwise the 5 k and/or the transistor will be damaged if the variable resistor is turned to minimum resistance. There does not seem to be a path for any significant current though the 12 volt siren so it will not work as an alarm.

The ground symbol does nothing, except suggest the best ground connection, if some one thinks a ground is needed. It would however be important if there is a second path to ground of something connected to this circuit.

I believe the first answer is in error because the person has treated the +9V as a battery with + on the left side, when it is in fact +9 volts above the ground symbol at the bottom connected in two directions to bias the adjustable pot and supply the siren.

If my interpretation is correct there is path for current through the transistor and the siren directly down the middle. The photo cell is drawn as a variable resister so the 5k pot and it control the voltage at their junction and thus with the base of the transistor. The suggestion of an added 5 ohm may be valid, it a bit beyond my weak knowledge.

The circuit is flawed in that the connection of 9v should not be shown that way -

This is not a REAL EARTH GROUND used on AC power circuit. It is just a COMMON in any circuit. Besides, the circuit was wrong on the 9V power source drawing by a fool. +9V is connected to both Collector and top terminal from potentiometer. Negative connects to the Ground symbol.

indeed, schematic error. the '9v' just means: 'this point is 9v'

You achieve it by connected the minus of a 9v battry to the earth sign, and the +sign at the '9v' point

The siren should go off when the resistance of the photocell is really high, but it will be fed with only about 8v instead of the 12v....

I see a "earthing/ground" sign in the circuit i don't know how to do this please help me with this if u could please verify this circuit is correct.......