In all measurements of density, and therefore of T. O. P., the _rate of burning_ must be considered. If a slow burning material be compared with a rapid one, the former will not reach its true maximum density, as a great deal of the smoke may settle out during the time of burning. Comparisons of T. O. P. are significant only when made on smoke mixtures of the same type and in about the same quantities. MEASUREMENT Two methods of measuring the effectiveness of a smoke cloud have been devised, one, the smoke box, which measures the obscuring power directly by observing at what distance a lamp filament is obscured by intervening smoke, the other, the Tyndall meter, which measures the intensity of the scattering of the light. The earliest measurements of smoke intensity are perhaps those of Ringelmann (_Revue Technique_, =19=, 286), who devised the well known chart of that name, intended mainly for measuring intensities of black smoke issuing from a chimney at a distance. The first measurements for military purposes are probably due to Bertrand, who made numerous comparative studies with his “salle opacimetrique.” This was a room 23 × 14 × 3.6 meters, with 7 windows. Two doors, one provided with 3 oculars 2 cm. in diameter, gave access to the room. On the other door, opposite the first, were hung several black signs. Six pairs of columns were spaced along the room at measured distances. When a smoke is produced in the room, the black paper signs first become invisible, then the door itself, and finally the columns, pair by pair. They reappear in the reverse order, and as a measure of relative opacity Bertrand took the time elapsing between the detonation and the reappearance of the farther door. [Illustration: FIG. 89.—Tyndall Meter.] [Illustration: FIG. 90.—Cottrell Precipitation Tube.] =Smoke Box.= The smoke box, used by the C. W. S., was constructed of wood with tight joints, and had a moveable brass rod running through it to which was attached a small size 25-Watt Mazda lamp. The density of each smoke introduced in the box is determined by moving this lamp back and forth until a point is reached when the pattern of the filament can just be distinguished by the observer looking in at the glass window, external light being excluded by a black cloth. The thickness of the smoke layer between the glass window and the light is recorded as the measure of the smoke density. For field tests, a larger box, 6 × 8 × 8 feet (288 cubic feet) was constructed. The observation light was moveable in a line connecting the mid-points of opposite sides of the box. To insure uniform distribution of smoke, a fan with 18-inch blade revolved at any desired speed between 60 and 250 r.p.m. With this, results are obtained indicating both the original density and its stability.