the rocket with a clay diaphragm evidently suggested that the choking of the case might be dispensed with where the composition was less fierce, the necessary reduction of the orifice being produced by a clay diaphragm with a central hole of sufficient size. This method is followed with the gerb, fountains, and flower pot, and in the firework known to pyrotechnists as "fixt"; this unit is largely used in display work to form the fringe frame or lattice effect of a set piece. "Fixt" are made in 1 oz. and 2 oz. sizes, and contain a composition of approximately one part of steel filings to four of mealed gunpowder and finish with a bounce. The origin of the name is uncertain: it may refer to their use on fixed pieces in contradistinction to one revolving, or—as is most probable—was first used to distinguish between a fixed and a moving rocket.
The time of the introduction of the clay choke is uncertain. Jones, writing in 1765, although using clay in the heading of rockets, still choked all cases, but Mortimer (1824) uses it, although Ruggieri (1821), whilst doing the same, appears to think choking preferable.
The former gives instructions for charging the clay solid and boring the central hole; Ruggieri, however, uses a nipple like a much shortened rocket spindle, in which he agrees with the modern practice. This method is also utilised at the present time for small-sized rockets.
Of the fireworks of the fountain class, probably the first to develop from the crude rocket form were the gerb and flower pot. The gerb, or Chinese tree, contains a composition of saltpetre, sulphur, charcoal and iron borings, with the addition—if more force is required, as for instance to turn a device—of mealed gunpowder. Early makers used mealed powder alone and "iron sand," or cast-iron reduced to powder by hammering. This composition is known as Chinese fire, and, as its name implies, was introduced into Europe from the
