Thus the speeds at which the force and the heat of the explosion travel are ascertained. Another important fact which needs to be found is the amount of the force, or the pressure, at different points. For this purpose pressure-gauges can be connected to the gallery at the desired spots by means of flexible tubes. This flexible tube is necessary in order that the vibration of the steel shell, due to the explosion, shall not be communicated to the instrument. The pressure, finding its way along the flexible pipe, raises a piston against the force of a spring, and the distance to which it is raised forms, of course, a measure of the pressure inside the gallery at the point to which the tube is connected. The pressure is recorded by the action of the piston in moving a style which just touches against the surface of a moving paper. There are three styles in all marking this paper. The first is the one just mentioned. The second is held down on to the paper by an electro-magnet energised by current flowing through a fine wire stretched across the gallery just where the explosion originates. This fine wire is broken at the moment of the explosion, whereby the current is cut off and the style raised. It therefore makes its mark until the moment the explosion occurs, and then leaves off. The end of that line, therefore, shows the time of the explosion. Meanwhile the first style is drawing a straight line, but as soon as the pressure begins to be felt by the pressure recorder this style moves and the line slopes upward. Upward it goes as the pressure increases, until it has reached its height, after which it descends, until the style is drawing a straight line once more. Thus the rise and fall of the line represents the rise and fall of the force of the explosion.