PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 437 



The motion of the piston, for a motion of two inches or even six 

 inches of the crank, would be very slight, and I believe that if 

 the steam should not be admitted while the crank is passing that 

 distance, it would be an advantage, provided sufficient cushion- 

 ing is provided for. 



Mr. Rowell. — The back pressure, which is the greatest at the 

 dead centre, has not yet been discussed at all. 



Mr. Tillman. — If the back pressure is increased, it will be so 

 much diminution of the direct pressure. 



Mr. Fisher. — When the lap of the valve is long, the exhaust 

 opens before the piston gets to the end of the stroke, and practi- 

 cally back pressure ceases before the end of the stroke. 



Mr. Rowell. — It is at the commencement of the stroke, when 

 the steam is least effective, that the back pressure is greatest. 

 In Mr. Babcock's illustration, at the last meeting, of the angle 

 of friction, the surfaces were assumed to be flat. But in the 

 friction of the crank-pin and shaft, the surfaces are circular, so 

 that the friction is the same at any angle. 



Mr. Tillman. — If we allow the piston to move at all without 

 the pressure of the steam, we lose just so much of the force of 

 the steam when it is admitted, for it fills up that space without 

 performing- any work. The true way is to let on the steam as 

 soon as possible ; because, although the friction is increased, the 

 vast preponderance of the force applied tends to move the 

 engine. 



Mr. Parkhurst, as an illustration of the theory of the crank 

 motion, exhibited a drawing showing a plan for varj-ing the 

 length of the crank so that equal motions of the piston should 

 produce equal angular motions of the crank, and hence a uniform 

 power. 



Mr. Tillman. — Such a motion of the piston would jerk the 

 engine all to pieces. The varying velocities of the crank are 

 admirably adapted in this respect to a reciprocating piston. 



AMERICAN GEOLOGY. 



Dr. Stevens resumed his series of remarks upon this subject : 

 Many parts of New England, the whole of Labrador, a large 

 part of tlie northern portion of North America, and portions of 

 the Atlantic States, are underlaid by the rocks commonly called 

 primitive, but now often called azoic, from their containing no 

 relics of animal life. Granite is generally supposed to be the 



