202 EIGHTH REPORT — 1838. 



As it is necessary to observe with some precision the pressure 

 of steam in the boiler, during experiments made according to 

 this method, the writer of this report constructed a self regis- 

 tei'ing steam-guage for the purpose. 



A smaller cylinder in which a piston is accurately fitted, simi- 

 lar to the cylinder and piston of common " indicators," is let 

 into the boiler at a place near the position of the engineer. The 

 piston-rod is carried through a tube outside the boiler, in which 

 it is made to act on a spiral spring, the force of which is op- 

 posed to the motion of the piston, when driven upwards by the 

 pressure of the steam. The position of the piston being deter- 

 mined by this spring becomes an indication of the pressure of 

 steam, and so far the instrument is a mere steam-guage. 



Attached to a part of the piston-rod is a pencil, the point of 

 which is lightly pressed against the surface of a drum or cylin- 

 der which stands over the boiler and near the steam-guage. 

 This drum is covered with paper rolled repeatedly round it, and 

 gradually discharged from it to a small roller placed beside it, and 

 pressed by a spring against it. On the axis of the drum is fixed 

 a worm-wheel, which is driven by an endless screw. The latter 

 receives its motion from a ratchet-wheel, in which a claw or 

 catch acts. This claw is alternately raised and drawn down by 

 some part of the machinery which has a reciprocating motion, 

 so that for each stroke of either piston the ratchet-wheel is 

 pulled through a space equal to one, two, three, or more of its 

 teeth, according to adjustments which are provided in the appa- 

 ratus. In this manner the drum receives a slow motion of ro- 

 tation, bearing a known relation to the revolution of the driving- 

 wheel, and therefore to the speed of the engine. By such means 

 the drum may be made to revolve once in a quarter of a mile, or 

 any other given distance. 



If the pressure of steam in the boiler remain unvaried, the 

 pencil will continue in the same position, and the paper moving 

 under it will receive the mark of a straight and horizontal line 

 at a certain height, which, by a scale previously adjusted, will 

 express the pi*essure of steam in lbs. per square inch. If, how- 

 ever, the pressure of the steam vary, the pencil will have a cor- 

 responding variation of height, and a curve will be traced the 

 ordinate of which will express the pressure ; and since the 

 absciss will represent the motion of the pistons, it will repre- 

 sent according to a known scale the motion of the engine along 

 the road, and therefore the absciss corresponding to any ordi- 

 nate will register the exact part of the road where the pressure 

 of the steam was expressed by that ordinate. 



The method of investigating the amount of resistance from 

 friction above explained is attended with the further advan- 



