The heating plate is mounted flush with the of the cover plate of the box. The plate i the bottom of a tin tank in which boiling of water is achieved by electrical each of 600 W, thus immersion of two heaters, providing the constant temperature of the plate. The walls of the boiling tank are insulated. All the openings have width equal to that of the box. The exit opening is located close to the heating plate, as shown in Fig. 1b. As for the entrance opening, made in the partition, it was either at the bottom or at the half-height of the box. The distance between the partition and the sidewall of the box is 50 mm, the height of the window is 50 mm, and the exit opening is also 50 mm wide.