108 PROCEEDINGS OF SECTION A. 



the diagram I have only shewn the method of making contact 

 which depends upon an accurately made micrometer screw. I 

 have not deemed it worth while to do more, as the details are not 

 finally decided upon, all that is shewn is that the cylinder in 

 tuiniing round would gear into the one hundred wheel and so 

 push forward C until it makes contact, at the same time a weight 

 would be wound up, which, directly the wheel and the cylinder 

 ceased to be in gear, would turn the wheel back one turn and so 

 break contact. It is obvious that a series of screw contacts may be 

 placed round A, and that the cylinder would gear into the wheels 

 one after tlie other and make contact, as proposed. The intention 

 is to make the point of suppoi't for the pendulum level with the 

 surface of the ground, and put the recording parts in a well to 

 secure stability and freedom from rapid changes of temperature. 



The amount of paper given to each second of arc of change 

 may be made lai^ge or small at pleasure on the scale proposed. 

 Each second of arc will be represented by half an inch of paper, 

 and therefore the maximum i^ecord possible in one direction 

 would be 10", because five inches of paper is to be added to each 

 of the four screws or directions. In time, one hour will occupy 

 one inch, which will permit of each individual record being seen 

 if there are sixty in an hour, and the time it was made discovered, 

 because the time will be automatically marked on the cylinder 

 every hour by the standard clock in the Observatory. 



[Note added, December 3. Since the foregoing was written this 

 method of recording minute changes has been applied to record 

 the changes in length due to temperature of a piece of zinc tube 

 twenty inches long ; and the result is entirely satisfactory ; 

 changes of tempei'ature of ^ of a degree are distinctly shewn. 

 As this tube is held by a clamp fixed at one end of a piece of 

 plate-glass, and the screw contact at the other end, it is obvious 

 that the available expansion is only the difference between that 

 of the zinc and the glass. This is roughly, (the exact expan- 

 sion of the glass being unknown) -^q^\q-q of an inch, which is a 

 satisfactory confirmation of the experiments by hand.] 



