430 
Abstract Report of Agricultural Discussions. 
again. The smallest qiiantit}- of air that can be introduced will cause it to boil 
again." 
Now, what frequently Lappens, I will not say in connection witli 
agricultm-c, bnt in connection witli manufactm-es generally, is this : 
when the men stop for a meal, for instance at dinner-time — they are 
perhajis doing a job by mcasiu-e — being very anxious to get the steam 
up as soon as ijossible after dinner is over, they take the i)Vccaution 
to keep everything as hot as they possibly can ; and although perhaps 
they imagine that no boiling can take place, and that the engine 
being at rest is not likely to be in a dangerous state, yet explosions 
have taken jjlace while the engine has been in that supposed state of 
rest, from the formation of vapour dm-ing a time when the water was 
actually hotter than when boiling, and yet not boiling. 
Accidents of this kind are of very rare occmTcnce in agiiculture, 
because, as engines are now tm-ned out, something is always made to 
give way ; and if the excellent precautionary rules for the management 
of steam-engines, given by Mr. Eansome,* and approved by Messrs. 
Clayton, Shuttle worth, and Co., and other great machine-makers, were 
observed in practice, we should, I believe, have fewer accidents. One 
of these rules is very often evaded. Mr. Eansome says : — 
" As soon as the voater begins to boil, the safety-valve should be opened by 
hand and examined, to make sure that it is not obstructed in any way ; the 
spring-balance may then be screwed down to about 10 lbs., and when the steam 
blows off at that point it may be gradually screwed down to 45 or 50 lbs., as 
the steam rises. The spring-halance should on no account he left always 
scretoed down to the full ^'ressurc when the engine is not at work, and the steam 
not iip^ 
It is to be feared that the spring-balance is too often left screwed 
while the men are at dinner. On more than one occasion I have 
myself had to interfere, in consequence of the state in which I have 
found it ; and it has always been a source of anxiety to me that there 
should be so much ignorance of the effects of confining a large body of 
steam in a small space. Dr. Carpenter says : — ■ 
" The expansion of liquids under the influence of heat increases very rapidly 
as the temperature is raised, and it is particularly great when the liquid is 
heated nearly to its boiling-point. The change of bulk is then very great and 
sudden ; for all vapours have many times the bulk of the liquids iTrom which 
they rose. Thus a pint of water would produce 1694 pints of steam at the 
ordinary pressure. Though the vaporization of fluids takes place chiefly under 
the influence of heat, yet the quantity of heat required to jiroduce it is very 
different under different degrees of pressure. Thus, if we take water at the 
ordinary pressure as the standard, we should find that any additional pressure 
(such as would be produced if the vessel were tightly closed) would render an 
additional quantity of heat necessary to convert it into steam ; whilst, on the 
other hand, the removal of the ordinary pressure of air will cause water to boil 
at a much lower temperature, as happens on the tops of high mountains, or 
may easily be shown by the air-pump. Under pressure of the most powerful 
kind, water has been heated to such a degree that the iron Vessel which con- 
tained it was red-liot throughout ; and if the pressure had been withdrawn in 
* 'Journal Koyal Agricultural Society,' vol. xix. p. 430. 
