spring, the frame being mounted on horizontal trunnions. The function of this device is not clear; in the words of the text, its object is “to illustrate an unfamiliar application of a known principle,” but the employment of the apparatus does not seem to lead to any results of importance.
§ 232. Langley's Experiments. “The Resultant Pressure Recorder.”—This instrument is designed, in the words of the Memoir, “for the purpose of obtaining graphically the direction of the total pressure on an inclined plane (in practice a square plane), and roughly measuring its amount.” The instrument consists of a beam (Fig. 145) hung symmetrically at its centre in gimbal joints, and carrying at its outer extremity the “wind plane,” that is, the plane under investigation, and at its opposite end a tracing point or pencil adapted to record on a sheet of diagram paper, arranged at right angles to the beam itself. A co-ordinate combination of tension springs is employed to hold the beam radial to the whirling table, and the whole is accurately counterpoised so that the plane is virtually weightless; thus, so long as the apparatus is at rest the pencil point is central or at the co-ordinate zero, but when the whirling table is in motion the total reaction on the plane is measured, both as to direction and magnitude, by the resulting displacement of the pencil point. In order to obviate friction the pencil is held away from the recording paper until the desired velocity is reached, when it is released by means of an electro-magnet. Due precautions are taken to ensure proper calibration; to this end the entire spring system is carried in a revoluble frame shown in the figure.
The method of employment is described as follows: “The wind plane is set at an angle of elevation a disc of paper is placed upon the recording board and oriented so that a line drawn through its centre to serve as a reference line is exactly vertical. The whirling table is then set in motion, and when a uniform velocity has been attained a current is passed through
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