316 On Printing Presses and their Theory, 



It thus appears that the power gained is about thirty-eight 

 times greater at the end than at the beginning of the pull. 

 While the re-action of the elastic substances which fornn 

 the tympan is small, the mechanical advantage is small, and 

 the platen is brought down rapidly ; but as the resistance 

 increases, the power gained undergoes somewhat more than 

 even a proportional increase, so that during the last mo- 

 ments of the revolution, the pull actually grows somewhat 

 easier. In consequence of this, although the bar is stopped 

 suddenly by the action of the check, nothing of that violent 

 jar is produced on the arm, which is so serious an incon- 

 venience in the common press ; and to relieve which most 

 pressmen find it necessary to sacrifice a part of the force 

 exerted by inserting an elastic heading over the tenons of 

 the summer. 



Let us now compare for a moment the mechanical ad- 

 vantage furnished by this combination with that furnished 

 hy the screw of the ordinary press. In all presses ahke, 

 the perpendicular motion of the platen may be regarded as 

 a constant quantity. It must necessarily rise a sufficient 

 distance to allow the thickness of the tympan frame to pass 

 freely under it. The distance allowed for this purpose ap- 

 pears to be in general about | of an inch. But in addition 

 to this, in the screw press, we must allow at least } of an 

 inch for the spring of the summer ; making the vertical dis- 

 tance described by the interior relatively to the exterior 

 screw half an inch. Then supposing the length of the pull 

 to be no greater than in the Lever press, the mechanical ad- 

 vantage gained will be uniformly 44 : 1. But if we sup- 

 pose, as is generally the the case in fact, that the distance 

 described by the hand is greater by about a foot, (although 

 the increase of the distance is in reality only an exchange of 

 bne disadvantage for another,) the power gained will h& 

 uniformly 66 to 1. Hence, on the most favourable hypothe- 

 sis, the strength of the pull at the last point, independently 

 of the force already accumulated in the body, need be but 

 ^\ as great in the Lever as in the screw pr^ss ; or | as great, 

 if but half a form is worked at once with the latter.* It 



* The force actually exerted at the last point of the revolution of the bar 

 in the Lever press, was found by measui'ement to be on an average, for the 

 lightest kinds of work 30 lbs. ; for the heaviest, 45. 



