29 



(3) The motion of the float is communicated to a very delicately 

 poised lever, the slightest movement of which sends a current of elec- 

 tricity through properly disposed magnets, which either alone or act- 

 ing in conjunction with clockwork perform the real labor of moving 

 the pen mechanisms and preserving a proper condition in the equilib- 

 rium of the float. 



(4) A distinct class of barographs is obtained by constructing 

 mechanisms which measure and record the barometric oscillations by 

 weighing the changing quantities of mercury within a poised barom- 

 eter tube or cistern. The weighing is effected by suspending the 

 barometer tube or its cistern from the beam of a balance which is con- 

 tinuously i^reserved in a condition of equilibrium by the automatic 

 movement of a counterpoise traveling along the beam of the balance. 

 The movement of the counterpoise is effected either by clockwork, 

 or by electromagnets, or by both. 



(5) A modification of the weighing principle consists in substi- 

 tuting for the balance mechanisms coiled steel springs, by the deflec- 

 tion of which the changing weights are measured and recorded. 



The following descriptions, given in some detail, of representative 

 types of mercurial barographs that have been maintained in opera- 

 tion by the Weather Bureau will enable the student interested to 

 understand more fully how such mechanisms operate : 



70. Foreman^ s 'barograph. — This is shown in Figure 22. It belongs 

 to a class mentioned above under (3). Prof. G. W. Hough, director 

 of Dudley Observatory, Albany, N. Y., about 1862, perfected baro- 

 graphs recording on this principle, the form here figured being de- 

 signed by Mr. H. L. Foreman, who was at one time Professor Hough's 

 assistant. The glass siphon tube of the barograph is at the back at 

 B, and is only partly visible, the bend being hidden behind the record 

 cylinder A. The open end of the siphon tube is seen at C ; an iron 

 float rests lightly upon the surface of the mercury within, being sus- 

 tained by means of a fine wire t from the short end of the lever /, 

 which is delicately poised upon the steel knife edges at r. The long 

 end of this lever at K is tipped with platinum and placed between two 

 platinum-pointed screws, both of which nearly, but not quite, toucli 

 the tip of the lever when the latter is poised in proper equilibrium. 

 The upper screw is connected by the wire W with an electromagnet 

 at the back of the instrument ; a corresponding electromagnet, partly 

 seen at M, is connected by the wire W, with the screw just beneath 

 the tip of the lever. P is a strong clockwork driven by the cord T, 

 and permitted to run intermittently whenever released by the action 

 of the electromagnet at M. 



The clock movement D, regulated by the pendulum F, gives motion 

 to the recording cylinders A, A'. The cylinder A makes 1 revolu- 

 tion in 24 hours, whereas A' revolves at the much slower rate of 

 about 1 revolution in 16 days. 



The lever Z, together with the platinum-pointed screws and elec- 

 trical connections W, W, are all mounted upon a carrier R, which is 

 moved by the fine-threaded screw S and guided by the columns a, a. 

 The coarse-threaded screw S' is provided with the double-pen carrier 

 R', the screws S' and S, being geared with each other by means of 

 suitable wheels. We will supjDOse that the mechanisms have been 

 properly set so that the iron float is normally sustained upon the 



