The figures on preceding page simply form a reasonable average, and allowance must be made for exposure, etc. Each square foot of direct radiating surface gives off to the air around it about 11⁄2 Thermal Units per hour per degree of difference between the temperature of the steam and that of the surrounding air. This is eqvivalent to about 1/4 of a pound of steam per hour, or, in other words, about 4 to 44 square feet of surface to each pound of steam to be condensed. HOT WATER HEATING. Circulation of water in a hot water-heating system is a movement of hot water from the boiler towards the radiators where it parts with some heat, and a consequent movement of colder water from the radiators to the boiler to become reheated. Without circulation, we cannot convey heat from the boiler to the radiators. The velocity of circulation depends chiefly upon : (a) the difference between the mean density of the water in the ascending current and the mean density of the water in the returning current; (b) the vertical height of circuit above the boiler; (c) the resistance to the flow due to friction, change in direction, vena-contracta. etc. The theoretical velocity of circulation can easily be computed by formulas, but we refrain from giving any, because, first, the theoretical velocity is only an imaginary thing and of little benefit to the practical man; and, second. because the actual velocity bears no definite ratio to the theoretical. The sizes of pipe should be governed by the amount of radiating surface they have to supply, the height of the radiators above the boiler, and the number of changes in the direction of the several currents. It is considered safe practice to allow from 50 to 100 square feet of direct radiation for every square inch of cross-section in the pipe. If the pipes are short, straight and high, 1 to 100 would be allowed; if long, crooked or low, 1 to 50 or more, according to the condition, would be employed. The sectional area of the mains should approximate the sum of the area of its branches, but in many cases, particularly where there is no indirect work, the sizes may be a little less. SIZES OF MAINS AND BRANCHES. 212* 66 3" 32 4' 42" 5" 6" 7" 8" 66 66 66 66 two 11⁄2" and one 14", or one 2" and one 11⁄4". All flow and return pipes should be of the same size. All pipes must rise from the boiler to the radiators with a pitch of at least 1" in 10 ft. To determine the amount of radiating surface necessary to easily warm a building in all kinds of weather, and to proportion it so that each room will have the required temperature at the same time, is a very important item and must be carefully considered. No formula, rule or table, can be given which will be applicable to all cases, but for ordinary buildings having the average wall and glass exposures, the table on following page is found in practice to give good results, when used with good judgment. 1 The above ratios are for an average temperature of 160° Fah. in the radiators. For direct-indirect radiation allow not less than 25 per cent. extra. For indirect radiation allow at least 50 per cent. additional surface. Due allowance must be made for leakages through loose doors, windows, etc. When extended surface indirect radiators are used, allow 30 per cent. more than the manufacturer's ratings. The expansion tank capacity should be at least of that of the entire apparatus, if it is an open tank. We do not advise the use of closed expansion tanks. COMPASS SURVEYING. The bearing of a line is the angle which it makes with the direction of the magnetic needle. The length of a line, together with its bearing, is termed the course. To take the bearings of a line, set the compass directly over a point in it, at one extremity, if possible. This may be done by means of a plumb bob suspended from the compass. Bring the compass to a perfectly level position. Let a flagman hold a rod carefully plumbed at another point of the line, preferably the other extremity, if he can be distinctly seen. Direct the sights upon this rod and as near the bottom of it as possible. Always keep the same end of the compass ahead; the north end is preferable, as it is readily distinguished by some conspicuous mark, usually a fleur de lis, and always read the same end of the needle, that is, the north end of the needle if the north point of the compass is ahead, and vice versa. Before reading the angle, see that the eye is in the direct line of the needle so as to avoid the error which would otherwise result from parallax, or apparent change of the position of the needle, due to looking at it obliquely. The angle is read and recorded by noting, first, whether the N. or S. point of the compass is nearest the end of the needle being read; second, the number of degrees to which it points, and third, the letter E. or W. nearest the end of the needle being read. Let A B in Fig. 1, be the direction of the magnetic needle, B being at the north end. Let the sights of the compass be directed along the line CD The north point of the compass will be seen to be nearest the north end of the needle which is to be read. The needle, which has remained stationary while the sights were being turned to CD, now points to 45° between the N and E points, and the angle is read north forty-five degrees east (N. 45° E.). FIG. 1. A sure test of the accuracy of a bearing is to set up the compass at the other end of the line, i. e., the end first sighted to, and sight to a rod set up at the starting point. This process is called backsighting. If the second bearing is the same as the first, the K35+78 27+50 GA 20+38 N° 25°W BAN reading is correct. If it is not the same, it shows that there is some disturbing influence at either one or the other end of the line. To determine which of these two bearings is the true one, the compass must be set up at one or more intermediate points, when two or more similar bearings will prove the true one. The magnetic meridian is the direction of the magnetic needle. The true meridian is a true north and south line, which, if produced, would pass through the poles of the earth. The declination of the needle is the angle which the magnetic meridian and the true meridian make with each other. Example of the Use of the Compass in Railroad Work.-Suppose CAD in Fig. 2 to be a railroad in operation, and that it has been decided to run a compass line from the point A along the valley of the stream X Y to the point B. Bellford The bearing of the tangent A D cannot be determined by setting up the compass at A on account of the attraction |