For approximate results, a chart may be used to take the place of a computation of efficiency. Fig. 39 shows such a chart based on the evaporation per pound of dry fuel and the heat value per pound of dry fuel, from which efficiencies may be read directly to within one-half of one per cent. It is used as follows: From the intersection of the horizontal line, representing the evaporation per pound of fuel, with the vertical line, representing the heat value per pound, the efficiency is read directly from the diagonal scale of efficiencies. This chart may also be used for efficiency based upon combustible when the evaporation from and at 212 degrees and the heat values are both given in terms of combustible. [Graph: Evaporation from and at 212° per Pound of Dry Fuel against B.T.U. per Pound of Dry Fuel Fig. 39. Efficiency Chart. Calculated from Marks and Davis Tables Diagonal Lines Represent Per Cent Efficiency] Boiler efficiencies will vary over a wide range, depending on a great variety of factors and conditions. The highest efficiencies that have been secured with coal are in the neighborhood of 82 per cent and from that point efficiencies are found all the way down to below 50 per cent. Table 59[57] of tests of Babcock & Wilcox boilers under varying conditions of fuel and operation will give an idea of what may be obtained with proper operating conditions. The difference between the efficiency secured in any boiler trial and the perfect efficiency, 100 per cent, includes the losses, some of which are unavoidable in the present state of the art, arising in the conversion of the heat energy of the coal to the heat energy in the steam. These losses may be classified as follows: 1st. Loss due to fuel dropped through the grate. 2nd. Loss due to unburned fuel which is carried by the draft, as small particles, beyond the bridge wall into the setting or up the stack. 3rd. Loss due to the utilization of a portion of the heat in heating the moisture contained in the fuel from the temperature of the atmosphere to 212 degrees; to evaporate it at that temperature and to superheat the steam thus formed to the temperature of the flue gases. This steam, of course, is first heated to the temperature of the furnace but as it gives up a portion of this heat in passing through the boiler, the superheating to the temperature of the exit gases is the correct degree to be considered. 4th. Loss due to the water formed and by the burning of the hydrogen in the fuel which must be evaporated and superheated as in item 3. 5th. Loss due to the superheating of the moisture in the air supplied from the atmospheric temperature to the temperature of the flue gases. 6th. Loss due to the heating of the dry products of combustion to the temperature of the flue gases.