32 



mercury. When, therefore, it is desired to set the recording pens or 

 check their positions in relation to the true air pressure, it is neces- 

 sary to make a reading of the standard barometer. In the barograph 

 next described the effect of temperature is inappreciable, and the 

 actual height of the mercurial column may be directly measured at 

 any time, thus dispensing with the extra barometer required with 

 Foreman's barograph. 



71. Marvin's no^'Tnal hrn'ograph. — This instrument is shown in 

 Figures 23 and 24. It belongs to the class mentioned above under 

 (4), wherein the mercurial column is directly weighed upon a 

 balance. 



The glass tube which, with the top portion of the mercurial col- 

 umn, may be seen at B, is freely suspended by the hook h from the 

 balance A. The point of the tube dips into the mercury contained in 

 the cistern C, which is suspended by a gimbal joint from the columns 

 d^ d^ by means of the metal tube B, which forms a sheath and protec- 

 tion for the glass barometer tube proper. The weight of the barom- 

 eter tube on the short arm of the beam A is balanced by the rolling 

 carriage W and fixed weight (not shown) on the end of the long 

 arm of A. Whenever a change occurs in the height of the mercurial 

 column, the weight changes, and the carriage W must be moved to a 

 new position if equilibrium is to be preserved. In order to make the 

 motions of the carriage W automatic a platinum-tipped contact 

 spring is attached to the balance beam at the extreme end r of the 

 long arm. The slightest displacement of the beam from its position 

 of equilibrium causes the spring to move into contact with one or the 

 other of two platinum-pointed screws, shown enlarged in Figure 24 

 at -m., mf . These are electrically connected, respectively, with the 

 magnets M and M', so that when the spring r makes contact with m 

 or m', an electric battery being in proper connection, a current is 

 caused to flow through the corresponding electromagnet, the action 

 of which causes the pin N or W to engage the teeth of the notched 

 wheel D in such a manner as to revolve it tooth by tooth. The long 

 screw S, Figure 23, carries the wheel D, fixed at its end so as to be re- 

 volved thereby. The threaded carrier W', fitted to the screw S, is 

 connected by a double universal linkage to the rolling carriage W. 

 The electromagnets thus act very directly through the wheel D and 

 the screw S to automatically move the carriage W into such positions 

 as may be required to maintain the equilibrium of the balance ; that 

 is, to prevent the contact spring on the beam from remaining con- 

 tinuously in contact with either screw m or m' . 



The motor mechanisms act in such a manner that, whenever the 

 equilibrium is disturbed and the electric circuit closed, the armature 

 of whichever electromagnet is affected makes stroke after stroke, 

 revolving the wheel D until the equilibrium is restored. Generally 

 one or two strokes only are necessary, representing a change in the 

 carriage corresponding to only the ten-thousandth part of an inch of 

 pressure, as explained below. 



The continuous record of the pressure, as indicated by the succes- 

 sive positions of the rolling carriage, is obtained in a very direct and 

 simple manner. A suitable spring adjustably attached to the threaded 

 carrier W, is fitted with a pen p, Figure 23 , and traces the pressure 

 curve upon a large cylinder, not shown in the figure, but mounted 



