258 . MERCURIAL PENDULUM. 





as they do (I believe invariably) the farina of plants. I have 

 also frequently observed the combs of very late swarms to be 

 remarkably thin, and white, and brittle ; which are circum- 

 stances very favourable to the conclusion, that the wax is a 

 vegetable substance, for it would probably be less abundant 

 during autumn than in summer; and that portion which had 

 remained on the plants till late in the season would hence 

 become more colourless by exposure to light, as well as more 

 dry and brittle than when it first exuded ; but were it an 

 animal substance, there does not appear any reason, why it 

 should be more dry and brittle, or less abundant, in the au- 

 tumn, than in the spring and summer. Thti conclusions of 

 Mr. Hunter are, however, always drawn with so much cau- 

 tion, and he united so much skill and science with the great- 

 est degree of industry, that it is not without much hesitation 

 and diffidence, that I venture to put my opinion in opposition 

 to his authority. 



Elton, May 4, 1807. T. A. KNIGHT. 



III. 



Description of a Mercurial Pendulum. Communicated by Mr. 

 Barraud, of Cornhillj who has made several, and has been 

 highly satisfied with their performance in the Measure of 

 Time. 



Description of A HE whole length of the pendulum rod, from the rivet 



a mercurial that joins the spring to its top, to the end of the screw at L, 

 pendulum. 



fig. 1, PI. VII, is 33A inches, (say 34- inches). The side 



pieces of the frame M M are of steel, as thick as the 



rod, that is | of an inch, and not less. The top of the frame 



H consists of two pieces of steel, each J of an inch thick, 



shaped as in the drawing, and screwed over the ends of the 



side pieces M M. The inside height of the frame, from E to 



A, is 8^ inches, and the inside width between the pieces 



M M about 2| inches, so that the cylinder stands \ of an 



inch clear of them. The bottom piece N is \ an inch thick 



from 



