Description of a Mercurial Pendulum. 103 



wrote out directions for its construction and for its adjustment, 

 which he communicated to me in the year 1804, together with 

 the investigation by which he had computed the invariable di- 

 stance of the centre of oscillation: and as I believe these direc- 

 tions have been communicated to only a few of his acquaintance, 

 and being informed bv some time-keeper makers that they will 

 be of great service — from these reasons, and in testimony of the 

 gratitude I feel in the memory of my late learned friend, I have 

 communicated the whole, both directions and investigation, to the 

 public. Thomas Firminger. 



Description of' a Mercurial Pendulum constructed hy 

 Mr. Gavin Lowe. 

 " Fig. 2 (PI. II) shows the top of the pendulum rod, made of 

 steel \ of an inch thick, ■§ of an inch wide, and rounded at G to 

 receive the fork of the crutch, so that there may be as little slack or 

 play as can be perceived. Fig. 3 is the bottom, seen vertically, 

 of the frame that holds the glass cylinder (P. fig. 2.),which is filled 

 with quicksilver Gy^ inches from the bottom of the glass inside up 

 to P. C F is the spring at top, one inch long and pretty stiff. G is 

 the point where the crutch sliould embrace the pendulum rod. 

 The crutch should be a fork of steel the same as the ]jendulum rod 

 is. From C to G should be about 7 inches. L is the bottom of the 

 steel rod, and -^^ of an inch of it to be turned into a screw that 

 has 40 threads or turnings to an inch. The whole length of the 

 rod, from the rivet that joins the spring to its top at F, to the 

 end of the screw at L, to be 33-»^- inches. The side pieces of the 

 frame M M to be of steel as thick as the pendulum rod, that is 

 I of an inch, but not less than -j-'^. The top of the frame H con- 

 sists of two pieces of steel each ^ or -'^^ of an inch thick, shaped 

 as in the drawing, and screwed over the ends of the side bars 

 MM. The inside height of the frame from E to A to be S-j-V 

 inches ; and the inside width between the bars M M, about 2{ 

 inches, so that the glass may stand f of an inch clear of them. 

 The bottom piece N is of cast brass about \ an inch thick from 

 E to R, and formed as in the figure (3), with abase which may 

 be hollowed down | of an inch, so as to fit the bottom of the glass. 

 The nut K is \ of an inch deep, and the diameter of its circle 

 from m to n is l^V i'lt-h, having the upper edge divided into 

 28 equal parts, and figured 0, I, 2, 3, or at each 7th division ; 

 each of those parts is very nearly equal to \" of time in 24 

 hours. A small bit of brass, shown in profile in fig. 4, to be 

 screwed on below H, to jioint to the divisions on the nut K. 

 A i)rass lid is made to fit the mouth of the glass fredy or witli- 

 out pinching ; the edge of the lid has two notches -„- of an inch 



G A deep. 



