VII. RECORDING INSTRUMENTS. 589 



The pressure-plate is circular, Las an area equal to 1 square foot, and is 

 kept constantly face to wind by a vane (or by windmill fans) ; as it is driven 

 forward by the wind, its motion is conveyed by means of a wire to a pencil, 

 which continuously records every movement on paper revolving by clockwork 

 below. 



The peculiarity of this instrument is, that, instead of employing springs as 

 the resisting medium, which from their proximity to the pressure plate and 

 consequent continual exposure to the weather, cannot always preserve the 

 same strength and elasticity, the resistance is furnished by a system of leverage. 

 This consists of two eccentric curves of different sizes rigidly connected 

 together and revolving on the same axle, of which the curvatures decrease 

 respectively in opposite directions, so that the effect is doubled. Round the 

 larger one a cord is carried, from which hangs a fixed weight. Kound the 

 smaller one a chain is passed, which is connected directly with the pressure 

 plate, and also with the recording pencil. As the motion is direct, the spaces 

 moved through by the pencil and by the pressure plate are exactly equal to 

 another one and also to the primeter of the smaller curve. 



One great advantage of this apparatus consists in its close proximity to 

 the recording pencil, and in fact that the resistance is always the same, as 

 every part of the instrument except the pressure plate is under cover and 

 free from exposure to the weather. Another peculiarity of this instrument is 

 that the plate is furnished with a conical back, so as to diminish the error 

 arising from the formation of a partial vacuum behind it in strong winds. 



The direction of the wind is also continuously recorded by Beckley's 

 method, on the same paper as the pressure. 



288O. Whewell's Anemometer. Elliott Brothers. 



Consists of a delicate wheel with vanes and endless screw, working a series 

 of cog wheels, which communicate with an ordinary lead pencil, that registers 

 the force and direction of the wind on a vertical japanned cylinder. The 

 cylinder is enclosed in a wooden case to prevent it from being injured by 

 exposure to the weather. 



2882. Registering Aneroid Barometer, for showing at a 

 glance the various fluctuations that have taken place in the baro- 

 meter. Elliott Brothers. 



2883. Hewlett's Portable Anemograph, an instrument 

 which records the varying direction and force of the wind in 

 the form of a map. Elliott Brothers. 



2885. Self-recording Aneroid for Hall or Library ; a 

 graphic delineation of the change in pressure of the air for each 

 week can be seen at a glance. Francis PastoreUi. 



Fixed nearly in line, in a case, an aneroid, an eight day clock, and a revolving 

 cylinder that occupies the central position ; it is covered with metallic paper 

 ruled for the days of one week, with the barometric scale in inches and 

 tenths ; upon this is marked every hour the pressure of the atmosphere ; the 

 markings have the appearance of a curved line, and the rise and fall of the 

 barometer is seen at a glance for every hour of each day for one week, when 

 a new paper has to be placed on the cylinder. The index hand on the dial 



