It is hard to believe that this unit was successful, so many factors militating against success, which depends upon the exactly similar jet from each of the five holes. But it is possible that in large geometrical pieces it was at least of use to give an additional effect in what, owing to the lack of variety of the fireworks of the time, must have been rather a monotonous repetition of a few effects. It also would enable small blank spaces to be filled in on set pieces. In a sun or star of the ordinary type, that is of radiating cases, the commencement of the jets must be as far apart as the length of two of the cases, which length is governed by the required time of burning. This leaves a blank centre; the five-pointed star, however, if working correctly, has the jets radiating from a point. Many of the earlier writers classified fireworks under the heads:—Fireworks for the ground, for the air, and for the water. Those falling in the latter division are only variations of those for the ground, that is to say, a gerb, fountain or other firework is fitted with a float, such as a block of wood, and functions floating on the surface of the water, the effect being greatly enhanced by the reflection. [Illustration: From “The Universal Magazine,” 1764.] It is not proposed to deal separately in this work with aquatic fireworks unless they have some essential difference from their parallel type for land display. One unit, of the rocket class, which is so distinct is the “skimmer.” This is in effect a stickless rocket with the cap (which is empty) fastened at an angle to the line of the main case. When fired the skimmer, as its name implies, skims over the surface of the water, with occasional dives under the surface in an erratic course. It requires for its safe display a considerable area of water. These are known by French pyrotechnists as “genouillères,” from their shape. Ruggieri and Frézier describe what they call “plongeons.” These are gerbs charged in the ordinary way, except that before each scoop of composition a small quantity of mealed powder is added. This produces a jerky burning, the recoil of each puff of powder driving the gerb beneath the surface of the water; the jet of fire, of course, is sufficient to prevent water entering the case while so submerged. These, and other earlier writers, in their section devoted to aquatic fireworks, give directions for firing ordinary land fireworks on the water, which would almost appear to have been included with the idea of filling space. One item which is generally included consists of directions for firing rockets under water. Jones, under this heading, gives the following directions: “TO FIRE SKY ROCKETS UNDER WATER.