Hollow-tile wall and concrete and stone have factors about the same as for the furred brick wall. [Illustration: SIDE ELEVATION] 4. Add to this the number of B. T. U.’s lost per hour through the windows. This is determined by multiplying the area of the windows by the heat loss in B. T. U.’s per hour for each square foot of window, which is 78.8 for single windows, and where storm-windows are added it is 31.5 B. T. U.’s. 5. This total sum is the number of B. T. U.’s lost through walls and windows for each hour. 6. To this must be added the heat lost by leakage through the window-cracks. This is secured by measuring the length of window-cracks on the side which has the greatest length of crack and multiplying this by 168, or the number of B. T. U.’s lost each hour for each linear foot of window-crack. For very tight windows reduce above to 84. 7. The total of all the above gives the number of B. T. U.’s lost each hour from the room when the outside temperature is zero and the inside is 70 degrees Fahrenheit. Knowing the quantity of heat lost per hour, a radiator must be installed which will supply this amount per hour. As the average steam-radiator supplies about 250 B. T. U.’s per hour from each square foot of its surface, the number of square feet required for a radiator to be installed in the room can be found by dividing 250 into the number of B. T. U.’s which were found to be lost from the room each hour. A hot-water radiator gives off about 150 B. T. U.’s per hour for each square foot of surface, so that the radiator is generally about one-third larger than the steam-radiator. Knowing the required number of feet of radiation for the radiator, the proper size can be selected from the manufacturer’s catalogue. By lumping the total number of square feet of radiation for all the radiators throughout the house together and adding 35 per cent to this to make up for loss through pipes and under-rating of boilers, the size of the boiler can be selected from the catalogue to fit this need. To estimate the size of a warm-air furnace, the total quantity of heat lost from all the rooms of the house should be calculated in the same way, and then 25 per cent added to allow for cold attics and exposure. This quantity should then be multiplied by 2.4 and divided by 8,000 to find the number of pounds of coal which will be required to be burned per hour. By dividing this amount by 5, the grate area of the required furnace can be found, and the correct size selected from the manufacturer’s catalogue. X LIGHTING AND ELECTRIC WORK _Modern Developments_