Portable Agricultural Steam Engines at Neiccastle. 699 
From the foregoing it is clear that it must not, however, be 
hastily assumed that an indefinite amount of economy is to be 
deriyed from the use of higher pressures. The increase in the 
temperature of steam does not correspond to the increase in 
pressure, but rises more slowly than the pressure increases ; 
thus : — 
From boilins-point to 50 lbs. temperature rises 6S° 
„ 50 lbs. 100 „ 
100 „ 150 „ 
150 „ 200 „ 
200 „ 250 „ 
and consequently the fall of temperature in working bears a 
smaller proportion to the fall of pressure, and all the mechanical 
difficulties connected with high-pressure steam have to be 
grappled with for, it may be, inadequate gain. These trials 
appear to point to the conclusion that, with our present state 
of knowledge, it is probable that pressures between 150 and 
200 lbs. per square inch will give the best practical results. 
One of the heads of information needed in investigating: the 
performance of engines under trial is the quantity of air passing 
through the boilers, and arrangements were made for measuring 
this directly by closing the ash-pans of the boilers, and arranging 
two rectangular orifices through which alone the air was suffered 
to enter the furnaces, and in passing through which its velocity 
could be measured by means of wind gauges. It was found, 
however, that combustion did not take place as efficiently when 
the current of air was restricted to two streams as when it was 
allowed to enter throughout the whole width of the ash-pan ; 
but fortunately we were spared the necessity of making other 
arrangements by reason of the ease and rapidity with which 
Mr. Stead was enabled to calculate, from the analysis of the 
gases escaping from the boilers, the quantity of air. His results 
are the more valuable in that they represent what took place 
during a prolonged trial in the actual running conditions of 
the engines. It must, however, be borne in mind that this 
mode of computing the air measures not only that which passes 
through the fuel, but also that which enters by the fire-door 
when it is opened for stoking. 
It had long been the desire of the Royal Agricultural Society 
to obtain analyses of the products of combustion, and at Cardiff 
an attempt was made to collect the gases. But the apparatus 
devised proved unsuitable, and the time required to perform the 
analyses, and the cost involved, prevented anv practical results 
from being obtained. Fortunately for the Society, and for the 
scientific completeness of the ^.ewcastle experiments, the assist- 
ance of Messrs. Pattinson and Stead was invoked, with the 
„ 38° 
„ 29° 
„ 20° 
„ 18° 
