Registering and Printing Barometer. 



3 



the movements of one machine may be reproduced in an- 

 other with no greater expenditure of force than is requisite 

 for electrical contact. In the cases cited, however, the 

 motion to be reproduced is sensibly uniform and in the 

 same direction. For the solution of our problem, a me- 

 chanism is demanded that shall repeat the changes of the 

 original in every form, whether the motion be uniform or 

 variable, forward or reverse. 



The feasibility of this plan was discussed with my friend 

 Mr. Thomas Simons as early as the year 1862, and some 

 steps were then taken to apply it to the thermometer. I 

 may here express my acknowledgments to Mr. Simons for 

 valuable suggestions in the construction of the present 

 machine. Various plans were considered for effecting the 

 electrical contact with the fluctuating medium which is 

 the basis of this method. It w r as at first proposed to do 

 this at the surface of the mercury in a syphon barometer, 

 by means of a platinum wire which should be carried con- 

 tinually toward the mercury surface by suitable mechan- 

 ism, and on touching the surface, a galvanic current would 

 be formed which should operate by an electro-magnet on 

 the mechanism so as to reverse the motion of the wire and 

 break the circuit. This would be immediately restored 

 by the normal movement of the mechanism, and thus the 

 point of contact would be kept oscillating at the surface 

 continually. The consumption of battery power by this 

 plan would have been considerable, and it was thought the 

 oxidization of the mercury by the electric circuit would in 

 time be appreciable. It was therefore concluded to make 

 the connection outside of the barometer tube, by means of 

 a float resting upon the mercury column. By this plan 

 there is no demand of action from the battery until some 

 change takes place in the barometer, and a considerable 

 saving of battery elements is effected. 



Attention was then given to determining the degree of 

 delicacy with which changes of the mercury surface could 



