There is no standard method of rating the heating capacity of boilers of this kind and as a consequence, boilers of different makes--for the same rating--are not the same in actual heating capacity. The boilers are sold by their makers in sizes that are intended to furnish heat sufficient to supply a definite number of square feet of radiating surface. The ratings are quite generally too high for the weather conditions of the Northwest. A common practice with contractors is to select boilers for a given plant 50 per cent. and even 100 per cent. larger than those rated by the manufacturers for the same amount of radiation. Some manufacturers sell their boilers at honest ratings but they are exceptions. In specifying the capacity of a house-heating plant it is common practice to require the boiler to be of such size as will easily heat a definite number of square feet of radiating surface. The radiators are required to possess sufficient radiating surface to keep the house at 70°F. in any weather. In the absence of any rules or specifications for determining the heating capacity of the boiler, the only means of securing a satisfactory plant is to require a guarantee of the contractor to install a boiler such as will fulfil the conditions stated above. =Boiler Trimmings.=--Attached to the boiler and required for its safe operation are a number of appliances that demand special attention. The office of each part should be thoroughly appreciated and the mechanical construction should be fully understood. An intimate acquaintance with the details of the plant, helps to make its operation satisfactory and adds to the efficiency with which it can be made to perform its duty. =The Water Column.=--In Fig. 18 the water column is shown at _C_. It is attached to the boiler by pipes at points above and below the water line, so as to allow a free passage of the water of the boiler to the interior. The water line should be 3 or 4 inches above the top heating surface. Attached to the water column is the _gage-glass_, the _try-cocks_ _T_ and _T_ and the _steam gage_ _G_. The object of the gage-glass is to show the height of the water in the boiler. It is shown in place on the boiler in Figs. 16 and 18 and in detail in Fig. 19. The lower part of the gage-glass occupies a position on the boiler about 2 inches above the top heating surface. When the boiler is working, the level of the water should always be visible in the glass and should stand normally one-third to one-half full. [Illustration: FIG. 19.--The water gage.] The water gage is attached to the water column by two brass valves _V_. The valves are provided so that in case the water glass should be broken the openings may be closed. The ends of the glass are made tight by “stuffing-boxes” marked _C_, in the figure. The packing _S_ is generally in the form of rubber rings but greased wicking may be used if necessary as in the case of valve-stems.