262 OF THE REGULATION AND [SECT. vni. 



547. FIELD'S VALVE. An ingenious mode of cutting off the steam at any period 

 of the stroke has however been discovered by Mr. Joshua Field. 1 It consists of a 

 valve placed in the situation usually assigned to the throttle valve ; that is, near to 

 the place where the steam is admitted to the cylinder. This valve is to be opened 

 at once, at the commencement of the stroke, so as to afford full passage to the 

 steam, and shut at once after a certain part of the stroke is made, that the rest of 

 it may be completed by the expansive power of the steam. This may be done by 

 causing the valve to open by a tooth or cam on a cylinder, on one of the revolving 

 shafts formed to raise the valve, and keep it open till the shaft has made part of its 

 revolution, and then shut it. If the toothed cylinder be made to slide on the shaft, 

 and the form of the tooth be such that as the cylinder is moved in one direction 

 the valve will shut sooner, and in the other direction later, there is then the means 

 of regulating the period the valve shall be open, and consequently of regulating 

 the power of the engine. This may either be done by hand, or by causing the 

 cylinder having the tooth to slide by the governor. Its application to Maudslay's 

 portable engine, where it is moved by the governor, is shown in Plate xv. It was 

 there first applied by way of experiment, which will account for the indirect 

 passages for the steam, and for retaining the throttle valve. The saving of power, 

 according to the experiment, amounted to about 10 per cent. 



548. When atmospheric engines condensing in the cylinder have to work 

 under loads inferior to their whole power, they are regulated by lessening the 

 quantity of injection, or by shutting the injection cock sooner ; but in almost all 

 engines employed for raising water which are regulated by hand, it is necessary to 

 provide the means of warning the attendant of the power being in excess. 



549. SPRING BEAMS. In engines with fly wheels no precaution is necessary to 

 limit the motion of the beam, because this is most effectually done by the length of 

 the crank, while the fly continues the rotary motion so as to prevent strain on the 

 crank shaft ; but in engines where a crank is not used, as in engines for pumping, 

 a very strong piece of timber is bolted across the top of the beam at each end, as 

 shown in Plate xn. each of which strikes against two wooden springs, one placed 

 on each side of the beam, on the two longitudinal beams which support the axis of 

 the engine beam, and which are on this account called the spring beams of the 

 engine. To prevent noise, the springs are covered with cork at the place where 



1 This is an ingenious adaptation of what Mr. Watt practised in the reciprocating engine, viz. 

 shutting off the steam at any required period of the stroke. In the rotative engine the same object 

 has been accomplished since the application of Murdoch's slide valve, in 1799, by increasing the 

 depth of its face to the required expansion in the cylinder, and properly adjusting the motion of 

 the excentric. ED. 



