340 History of Mechanical Inventions and 



with respect to each other, are unequal in size and thickness ; they gra- 

 dually become thinner towards the circumference, aud more particularly 

 towards the posterior or larger extremity. The lobes are marked trans- 

 versely by 18-30 distinct costae, which become more elevated as they ap- 

 proach the raised zig-zag line. These costae do not terminate at the 

 edges, but are continued at the anterior extremity, on the under surface. 

 Faint longitudinal impressions are observed, at unequal distances, crossing 

 the transverse costae nearly at right angles. The inequality of the lobes 

 is not always constant; in one specimen, (See Plate VI. Fig. 11.) the smaller 

 lobe is compressed in such a manner as to produce a crest, and approxi- 

 mating nearly to the shape of a trilobite. The under surface is ex- 

 tremely irregular. The edges, as before mentioned, are very thin, and are 

 elevated about two lines above the inferior surface. This, however, is not 

 uniform throughout the whole circumference ; the anterior portion, com- 

 prising one-third of the fossil, is without such a raised border. In this 

 part, the dorsal costae are continued beneath, and meet each other at 

 angles of about 45°. The line of junction has not yet been rendered visible 

 by the most careful dissection. The posterior portion of the under sur- 

 face presents a series of concentric lines, interrupted by a carina directly 

 beneath, and in the direction of the dorsal furrow. These fossils were at 

 first supposed to be remains analogous to the Trilobites, but are at present 

 referred to the Productus of Sowerby — Dr Dekay. 

 See Plate VI. Fig. 11, 12, 13, and 14. 



Art. XXVII— HISTORY OF MECHANICAL INVENTIONS 

 AND PROCESSES IN THE USEFUL ARTS. 



1. Method of giving the Epicycloidal Form to the Teeth of Wheels. By 

 Peter Lecount, Esq. Midshipman, R. N. in a Letter to the Editor. 



H. M. S. Queen Charlotte, Portsmouth Harbour, 

 Sir, Dec. 5, 1822- 



I have to apologize to you for not having earlier sent you the account of 

 the method I propose to use in forming the teeth of the wheels and pinions 

 of chronometers in the shape of an epicycloidal curve. The fact is, I have 

 been waiting to get a sight of the engine now used for cutting watch 

 wheels, in order so to adapt my plan to that instrument as to leave it ca- 

 pable of performing all its former offices, and that the machinery for giving 

 the epicycloidal form may be put to it or taken away at pleasure. 



In the instruments now used for cutting the teeth of watch and 

 chronometer wheels, there is a circular brass plate (called the dividing 

 plate) about seven inches in diameter, having a steel axis in its centre 

 (called the mandrill) five or six inches long ; this is fixed in a frame with 

 the axis perpendicular, the wheel to be cut is fixed to the top of the man- 

 drill, and the plate is turned round the required distance for each tooth by 



