1826.] of the Length of the Pendulum at the Equator, S9I 



the knife-edges with a piece of cloth, or flannel, saturated with 

 fine oil. This should be done before the observations are com- 

 menced, and after finishing them, the knife-edges are always to 

 rest in the Ys, when the pendulum is not used, but to be let 

 down on the agates, when the experiments are making. 



" The small telescope is to be placed at such a distance from 

 the pendulum, that the sides of the black slip at the bottom of 

 the pendulum shall just be embraced by the sides of the perpen- 

 dicular opening in the diaphragm ; large pickets with flat tops 

 may be driven into the ground at the proper distance, and the 

 bottom of the triangular stand supporting the telescope be 

 screwed to these, the telescope being kept at the same height 

 as it is in the Observatory, or a brick foundation may be laid 

 with pickets let in: the telescope must be slid horizontally, 

 until the left edge of the diaphragm becomes a tangent to the 

 edge of the disc on the same side ; and a mark must be made 

 to show the position of the telescope to the left; move the tele- 

 scope to the right, until the other side of the diaphragm is a 

 tangent to the part of the disc on the right ; here make a mark 

 also, bisect the distance of the two marks, and place the tele- 

 scope half way between. 



" The following is the mode of making the experiments : — The 

 barometer being up, and the thermometer hung near the middle 

 of the pendulum, lower the knife-edges gently upon the agates ; 

 adjust the perpendicular sides of the diaphragm most accurately 

 to the edges of the black slip, by the screws on the brass part of 

 the support ; then bring the on the arc, to coincide exactly 

 with the angular point of the pendulum. With the forefinger of 

 the right hand upon the edge of the black slip of the pendulum, 

 bring the point to about 1*3 upon the arc for measuring the 

 vibrations, and on the left of ; an instant before the pendulum 

 of the clock comes to its greatest height on the same side, with- 

 draw the finger horizontally ; take the height of the thermometer 

 and barometer, which register ; look through the telescope, an 

 assistant counting the clock, note the second and parts of a 

 second, when the disc is completely hid behind the black slip of 

 the pendulum ; also the instant when it re-appears ; let the 

 exact time, minutes, seconds, and tenths, of both be registered, 

 and the mean taken. Immediately after the second observation, 

 (the re-appearance of the edge of the disc) take the arc of vibra- 

 tion very carefully, by seeing how much the pendulum vibrates 

 on each side of 0, which also register. In about eleven or 

 twelve minutes, the disc will again coincide with the pendulum ; 

 the times of the same appearances as above must be noted and 

 registered ; and thus proceed until five mean coincidences are 

 obtained (as in the form). The thermometer being observed at 

 the beginning, at the third and fifth coincidences, and the baro- 

 meter at the beginning and end of each set. The pendulum 



u2 



