in said web and even web breaks as hereinbefore mentioned no influence could be exerted on the aforedescribed phenomena by the known or traditional procedures because even when potentially affecting the pressure level these methods merely raise the pressure level on the whole this produces no differential pressure across the web which is the object of the invention achieved by directing to critical points the pressure effect produced by a pulse of air flow in trial machine experiments carried out by the inventor pressures up to 550 pa have been measured under operating conditions of air blown in the directions f 1 and f 2 at the sides of the web w in the absence of extra equipment there is a sub atmospheric pressure of about 10 to 20 pa at the side of the web w in the region a 1 correspondingly there is a slight over pressure in the region a 2 depending on the machine speed measurements performed by the inventor in actual practice on production machines indicate that substantially lower differential pressures have a significant effect on the behavior of the web w in the critical regions discussed the air blowing operations of the air directing device of the invention thus make it easy to produce an over pressure which cancels the pressures tending to detach the web w from the felt 3 on the side of said web fig2 shows a system for connecting a doctor blade in accordance with the invention which enables volumetric flows to be used for blowing air which exceed the replacement air quantity consistent with the air flow balance in fig2 a pipe 20.