Report on Implements at Preston. 
733 
compress the spiral spring, open the internal valves, and allow 
steam to be discharged until the over-weight is removed. It is 
also impossible for pressure to increase on the internal valve, 
as the lower of the two valves, having the larger area, is at 
once depressed with the least increase of pressure. As compared 
with an ordinary lever, this valve gives very great security ; but 
its superiority to the usual lock-up spring valve adopted by 
many of our exhibitors is not perhaps sufficient to cause it to 
be much adopted in agricultural engines. For marine and 
Fig. 30. — Messrs. W. N. Nicholson and Son's Absolute Safety Valve, 
No. 166. 
other large engines a very slight increase of security is cheaply 
purchased by giving nearly double the usual price of a lock-up 
safety-valve. For ordinary farm work a good lock-up spring 
safety-valve will probably suffice ; for even this amount of 
security is not universally provided. The powerful engines 
sent out by 31essrs. John Fowler and Co. are ordinarily fitted 
with the lever-valve alone ; probably because they are satisfied 
that their boilers have an ample) margin of strength to stand 
occasional tampering with the safety-valve. 
No. 414, Messrs. Davei/, Sleep, and Co.'s " Matchless " Turn- 
wrest Plough, for turning the land upside down, has been already 
