When the shutters fall a great rush of water occurs. To obviate this a valve is made in the upper half of the shutter. It consists of a miniature shutter on the same principle as the main shutter. The pivot of the main shutter is made at such a height that the shutter will not turn over when only a small depth of water flows over it. Instead of this the valve comes into operation. The valve also facilitates the raising of the shutter. Again, instead of the tripping bar, which would sometimes have to be of great length or be liable to damage owing to stones jamming in its teeth, the shutter can be released by pulling the strut upstream so that it falls into a second groove, down which it slides. When a tripping bar is used, its teeth can be so arranged that the shutters are released a few at a time, first singly, then in twos and threes. Sometimes there are gaps of a few inches between one shutter and the next, and the gaps can be closed by needles if necessary. Chanoine shutters can be very rapidly lowered, and they are used in France and in the U.S.A. in places where sudden floods occur. They are also used for navigation “passes” where most of the heavy traffic is downstream and where it is too heavy to be dealt with in a lock. A foot-bridge across the stream or across the navigation pass is always an assistance, but sometimes it cannot be used when there is much floating rubbish or ice. With a foot-bridge the cost is greater than that of a needle weir.[14] [Illustration: FIG. 45.] In the Bear Trap weir (fig. 45) the upstream shutter rests against the downstream one. Both are raised by admitting water from the upper reach, by means of a culvert, through an opening in the side wall, and they are made to fall by placing this opening in communication with the downstream instead of the upstream reach. This kind of shutter is only suitable for passes of moderate width, and it is rather expensive on account of the culverts.[15] Shutters with fixed supports are used on the Irwell and Mersey. A fixed frame is built across the stream (fig. 46) and the shutters are hinged to it. When the water rises to a certain height above its top, the shutter turns into a horizontal position, but as this causes a severe rush of water the shutter is usually raised by a chain attached to its lower end and worked from the bank. When in a horizontal position, it is held there by a ratchet. When the stream falls the ratchet is released and the shutter is closed by the stream. This kind of shutter cannot be used where there is navigation.