of the block plate 13 has at least one torsion spring 14 to automatically restore the opened block plate 13 back to the closed state the block plate 13 is tilted toward the bottom of the vertical water tank 10 an opening of the vertical water tank 10 has a guiding portion 15 the guiding portion 15 has a confining section 151 extending toward the interior of the vertical water tank 10 and a guiding section 152 extending outward from the vertical water tank 10 the confining section 151 can confine the float balls 60 in a region on the surface of water an automatic water supply device 16 is installed at the opening of the vertical water tank 10 and used to control the water level a rotary control device 17 is installed at one end of the guiding section 152 and used to guide the movement of the float balls 60 the propulsive device 20 is a hollow structure having a holding portion 21 to receive the float balls 60 and an inlet 22 at a lateral thereof the propulsive device 20 has one end coupled to the aperture 12 to interconnect with the vertical water tank 20 the propulsive device 20 has a hydraulic propeller 23 at the other end the propulsive device 20 is tilted upward to the hydraulic propeller 23 whereby the float balls 60 are gathered in the region.