Mathematical and Physical Papers. 231 



various meridians. The globe will show the apparent revolution 

 of the plane of inertia, and the time will be twenty-four hours di- 

 vided by sine of latitude. 



The first attempt by the author of this paper to make an accurate 

 Foucault pendulum was at Washburn College, some six or seven years 

 ago, where one of the ventilating shafts in what is now called Rice 

 hall was used, and where one could get a length of more than fifty 

 feet, but the arc of vibration could be not more than one and one- 

 half feet. The greatest difficulty was the narrow space for making 

 adjustments, and yet some results were shown to this Academy 

 which appeared to be of interest. The plan was adopted of marking 

 the oscillations with a needle on a plate of smoked glass, and then 

 blue-prints were made of these pendulum markings for the first 

 time in experiments with the Foucault j/endulum. One of these 

 prints, made six or seven years ago, is still preserved. 



In the state-house, near the Academy rooms, is a space which 

 seems peculiarly well suited to the Foucault experiment. It is in 

 one of the light shafts, and the Executive Council were ready to 

 grant the use of this waste space for these' experiments, provided 

 no expense to the state was incurred. The light shaft in question 

 is a space of about ten by fifteen feet, and extends from the first 

 floor to the roof, where it is surmounted by a skylight resting on a 

 system of iron trusses and rafters, and the rafters are covered with 

 sheets of plate roofing glass. 



The pendulum wire passes through a hole drilled through this 

 glass, and then through a steel plate by a hole just large enough to 

 admit a No. 15 piano wire. The wire is wound on an iron cylinder 

 an inch in diameter, with a crank and a rachet wheel, furnished 

 with a dog to hold it in any desired position. The length of this 

 pendulum as determined by its rate of oscillation is about seventy- 

 three feet, and the bob is a cylinder of lead terminating with a 

 cone at the bottom, and weighs sixty-eight pounds. The lead is cast 

 around a stout brass tube and is turned on a lathe to a symmetrical 

 form. The brass tube coming through the lower extremity of the 

 bob has inserted in it three concentric glass tubes, the inner one 

 being just large enough to admit easily a No. 1 sewing needle. 

 This smaller tube is narrowed by heating until it is able to let the 

 point of the needle pass through one-fourth of an inch. It is now 

 free to play up and down through this distance, and to trace on 

 the smoked glass below the course of the pendulum through its 

 circular arc. The time of a double oscillation is very close to three 

 and one-half seconds. To start the pendulum, it is pulled to one 



