10 



Description of an Automatic 



means, should their cogs arrive at the circumference of 

 W together, the machine would become blocked. 



This was easily remedied by introducing a circuit 

 " breaker" on one of the wheels a, a 1 . As soon as the 

 wheel a begins to move, the circuit is broken, and remains 

 so until it comes to rest again ; so that in case they should 

 both start at the same instant, the cog of a' being in ad- 

 vance, it would make its revolution and come to rest, be- 

 cause the circuit would be interrupted until a should come 

 to rest also. Since the circuit interrupter was attached, 

 there has been no difficulty from this cause. 



Fig. 3 is a perspective view of the apparatus as it is 

 when in operation. The frame work for supporting the 

 barometer tube and other mechanism is of black walnut 

 two inches thick, which is firmly fastened to the east 

 wall of the west transit room. This wall is built of brick, 

 and is two feet thick, so that the whole apparatus occupies 

 a very firm position. 



Having given a general idea of the mechanism for 

 causing the screw S to follow the motions of the baro- 

 metrical column, we will show how the curve of pressure 

 is recorded, as well as the printed results. 



The wheel W, fig. 2, which receives the impulses, has 

 40 teeth ; and the screw S, having 50 threads to the inch, 

 one tooth of the wheel W corresponds to the -j^Vs- of an 

 inch change in the barometrical column, or of an 

 inch change of pressure. To the wheel W is attached 

 another of nearly the same diameter, having 80 teeth ; 

 this wheel is geared into one of 40 teeth carrying an 80- 

 tooth wheel on the same axle. This second 80-tooth 

 wheel is geared into a 50-tooth wheel, which operates 

 the screw S', fig. 3, of 26 threads to the inch. To this 

 screw is attached an arm, carrying a pencil which traces 

 the curve of pressure on the revolving cylinder o. 



From this arrangement, the curve is magnified a little 



