BOILERS. These are very important articles in all gardens where there are glass houses, and the best should always be selected. They are made in cast and wrought iron, both of which have their special advantages and disadvantages. The former are less liable to burn through when encrusted with any deposit from the water, but will crack with sudden changes of temperature, by reason of the granular form of the metal not allowing gradual contraction; the latter may burn through where there is any deposit of mud or other matter, but they will not crack, and will stand a greater pressure than those made of cast metal. It is, however, the better plan to use Boilers of wrought iron, as, with careful usage, they are less liable to break down in hard frosts than are the others. As the value of the plants, as well as that of the Boiler, has to be considered, should such a contingency occur, it is certainly advisable to reduce all risks to a minimum; and as the average life of a Boiler is from ten to fifteen years, a slightly larger first cost is not of very serious moment. The forms of Boilers are very diverse, and, in some of the patented forms, complicated. But, whatever the shape, the following points are essential: A clear and unrestricted waterway of not less than 2in. in thickness; the greatest exposure of surface to the direct action of the fire; a sufficient fire space; and a fire-bar area calculated to supply enough fresh air to the fire to support thorough combustion. The forms generally used for horticultural work are the following, or some modifications of them; and, however grand the name, their chief value consists in the attested heating capacity at a given cost: The Saddle Boiler, which is made of various sizes, with or without check ends, cross tubes, and other devices for increasing the heating surface, and also of a tubular form; the Upright Cylinder (a vertical section of which is shown in Fig. 260); the Upright Tubular Cylinder; the Horizontal Tubular; and the Cornish or Horizontal Cylinder. Combinations of the various forms, and complicated patterns of different kinds, are made for particular purposes; but in no case should large Boilers be used, unless they are recommended, by one competent to judge, as suitable for the purpose they are needed to fulfil. In every case, it is necessary that the Boiler should be fixed in a proper manner. It is also good policy to have Boilers about 30, or even 50 per cent., more powerful than is actually required, when they are new, as, from various causes, their heating power falls off in a year or two in many places; and, under ordinary work, it is not desirable to stimulate the action in order to command sufficient heat. There is no doubt that, for general purposes, some modification of the Saddle or Cylinder Boiler is by far the best, all things duly considered; but it is impossible to give any practical advice without a thorough knowledge of the requirements of any particular place. Gas Boilers are also useful for small places. These are made in many forms, and are, as a rule, in the shape of a cylinder, with a coned inside, against which the flames play. Some Gas Boilers have also a superheater attached; this exhausts the heat from the air which has passed through the Boiler. Another good form is made of horizontal tubes, which contain the water, the flames playing over and amongst them. An atmospheric burner of approved construction should be used where gas is the heating power. A sufficient supply of gas should be assured by using supply pipes of a good size. Care should be taken to keep these pipes free from water, and protected from frost by silicate cotton lagging, or some other good non-conductor. Except for very small places, however, gas apparatus is almost useless, and will never supersede fuel Boilers. For other particulars regarding Boilers, see Heating and Stoking.