4 



Description of an Automatic 



be represented by this process. It was found by experi- 

 ment that a motion of less than .0005 of an inch was readily 

 shown, a quantity within the limits of reading of a first 

 class standard barometer. 



The next step was to devise the proper mechanism for 

 repeating the motion thus transferred, and recording it in 

 some legible form. A finely cut screw was considered as 

 best adapted to measure such minute intervals of space. 

 To this screw a forward or reverse motion was given by a 

 double system of clock work, each operated by an electro- 

 magnet in connection with the float, and raising or lower- 

 ing the screw by intervals corresponding with the changes 

 indicated in the mercury column. 



In respect to the permanent record of results, it was 

 decided not only to attempt the production of a linear 

 diagram or curve of atmospheric pressure, as an interesting 

 method of presenting the recorded changes to the eye, but 

 to avoid the tedium and uncertainty of measuring up such 

 results, by producing at the same time a printed record of 

 such variation, to any extent deemed advisable. 



Having thus endeavored to give some conception of the 

 design and principal features of this method, I will proceed 

 to explain more fully the details of its execution as at pre- 

 sent arranged. 



In order to make any self-recording machine of this kind 

 practicable, we need to attend to two points. First, to 

 reduce the consumption of electricity to the smallest possi- 

 ble amount consistent with certainty in the results ; and 

 secondly, to secure the greatest amount of useful work with 

 the minimum of labor. We at once decided to adopt the 

 "make" circuit; for so long as there is no motion, there 

 will be no consumption of battery elements. The battery 

 which we have adopted for recording transits is essentially 

 that of Daniell; sulphate of copper being the exciting 

 agent. A battery of this kind will maintain sufficient 

 power for chronographic records for two or three months, 



