Fuly 8, 1886] 
NATURE 
231 
of one of the boilers, and exposing this to the direct impact of a 
powerful burner during the time the water is being boiled, and 
you will see that it comes out perfectly clean and uncoloured. 
‘Now it is well known that paper becomes charred at a tem- 
‘perature of about 400° F., and the fact that my test-paper is not 
‘charred proves that it has not been exposed to this temperature, 
‘the flame being, in fact, extinguished by the cooling power of 
the water in the vessel. I need hardly remind you that the 
Speed with which convected or conducted heat is absorbed by 
any body is in direct ratio to the difference between its own 
temperature and that of the source of heat in absolute contact with 
it ; and therefore, as the source of the heat taken up by the 
vessel is nothing but unburnt gases, at a temperature below 
402° F., the rate of absorption cannot, under any circumstances, 
be great, and the usual practice is to compensate for this ineffi- 
iency by an enormous extension of surface in contact with the 
ater, which extension I will prove to you is quite unnecessary. 
;ou will see I have here a copper vessel with a number of solid 
“copper rods depending from the lower surface ; each rod passes 
through into the water space and is flattened into a broad head, 
“which gives up its heat rapidly to the water. My theory can be 
‘stated in a few words: The lower ends of the rods, not being 
in close communication with the water, can, and do attain, a 
emperature sufficiently high to admit of direct flame contact, 
and as their efficiency, like that of the water surface, depends 
on the difference between their own temperature and that of the 
‘source of heat in absolute contact with them, we must, if my 
theory is correct, obtain a far greater duty from them. TI do not 
wish you to take anything for granted, and although the surface 
of the rods, being vertical, can only be calculated for evaporat- 
‘ing power at one-half that of a horizontal surface, as is usual in 
oiler practice, my margin of increased duty is so great that I 
¢an afford to ignore this, and to take the whole at what its value 
“would be as horizontal surface, and still obtain a duty 50 per 
sent. greater from a surface which is the same in area as the flat- 
bottomed vessel on the fire side, but having only one-third the 
surface area in contact with the water. I do not, of course, pro- 
fess to obtain more heat from the fuel than it contains, but 
‘simply to utilise that heat to the fullest possible extent by the 
use of heating surfaces, beyond comparison. smaller than what 
have been considered necessary, and to prove not only that the 
heating surfaces can be concentrated in a very small area, but 
also that its efficiency can be greatly increased by preventing 
close water contact, and so permitting combustion in complete 
contact with a part of the heating surface. I will now boil 4o 
ounces of water in this flat-bottomed copper vessel, and, as you 
will see, sharp boiling begins in 3 minutes 15 seconds from the 
‘time the gas is lighted. The* small quantity of steam evolved 
"before this time is of no importance, being caused partly by the 
driven off from the water and partly from local boiling at the 
ges of the vessel owing to imperfect circulation. On the 
tom of this vessel is pasted a paper label which you will see 
is untouched by the flame owing to the fact that no flame can 
“exist in contact with a cold surface. 
_ It may be thought that, owing to the rapid conducting power 
‘of copper, the paper cannot get hot enough to char. This is 
“quite a mistake, as I will show you by a very curious experi- 
‘ment. I will hold a small plate of copper in the flame for a few 
‘seconds, and will then hold it against the paper. You will see 
‘that, although the copper must of necessity be at a temperature 
‘not exceeding that of the flame, it readily chars the paper. We 
can, by a modification of this experiment, measure the depth of 
‘the flameless space, as the copper, if placed against the paper 
‘before it has time to be previously heated, will, if not thicker 
than 1/40 inch, never become hot enough to discolour the 
aper, showing that the flame and source of heat must be below 
‘the level of a plate of metal this thickness. 
Tn repeating this experiment I must caution you to use flour 
4 aste, not gum, which is liable to swell and force the paper 
ast the limit of the flameless space, and also to allow the paste 
)dry before applying the flame, as the steam formed by the 
wet paste is liable also to lift the paper away and force it into 
‘the flame. I will now take this vessel, which has only one-half 
the surface in contact with the water, the lower half being 
eovered with copper rods, 3/16 inch diameter, 34-inch centres 
‘apart, and 13 inch long, and you will see that with the same 
mer as before, under precisely the same conditions, sharp 
boiling takes place in 1 minute 50 seconds, being only 13 seconds 
nore than half the time required to produce the same result with 
i€ same quantity of water as in the previous experiment. 
Although the water surface in contact with the source of heat 
is only one-half that of the first vessel, and the burner is the 
same, we Can see the difference not only in the time required to 
boil the 40 ounces of water, brt also inthe much greater force and 
volume of steam evolved when boiling does occur. With refer- 
ence to the form and proportions of the conducting rods, these 
can only be obtained by direct experiment in each case for each 
distinct purpose. The conducting power of a metallic rod is 
limited, and the higher the temperature of the source of heat, 
the shorter will the rods need to be, soas to insure the free ends 
being below a red heat, and so prevent oxidation and wasting. 
There are also other reasons which limit the proportions of the 
rods, such as liability to choke with dirt and difficulty of clean- 
ing, and also risk of mechanical injury in such cases as ordinary 
kettles or pans ; all these requirements need to be met by dif- 
ferent forms and strengths of rods to insure permanent service, 
and, as you will see further on, by substituting in some cases a 
different form and type of heat conductor. To prove my theory 
as to the greater efficiency of the surface of the rods in contact 
with the flame as against that in direct contact with the water, I 
have another smaller vessel which, including the rods, has the 
same total surface in contact with the flame, but only one-third 
the water surface as compared with the first experiment. Using 
again the same quantity of water and the same burner we get 
sharp boiling in 2 minutes 10 seconds, being an increase of duty 
of 50 per cent., with the same surface exposed to the flame. The 
rods in the last experiment form two-thirds of the total heating 
surface, and if we take, as I think for some careful experiments 
we may safely do, one-half the length of the rods to be at a 
temperature which will admit of direct flame contact, we have 
here the extraordinary result that flame contact with one-third of 
the heating surface increases the total fuel duty on a limited area 
50 percent. This really means that the area in contact with 
flame is something like six times as efficient as the other. In 
laboratory experiments it is necessary not only to get your result, 
but to prove your result is correct, and the proof of the theory 
admits of ready demonstration in your own laboratories, al- 
though it is unfit for ‘a lecture experiment, at all events in the 
only form I have tested it. If you will take two ordinary metal 
ladles for melting lead, cover the lower part of one of these 
with the projecting rods or studs and leave the other plain, you 
will find on melting a specified quantity of metal in each that the 
difference in duty between the two is very small. The slight 
increase may be fully accounted for by the difference in the 
available heating surface reducing the amount of waste heat 
passing away, and this proves that flame-contact, and therefore 
quick absorption of heat, takes place on plain surfaces as soon 
as these are above a certain temperature, which, in a metal 
ladle, very soon occurs. What the temperature is which admits 
of flame-contact I have, as yet, not been able to test thoroughly, 
and it will need some consideration how the determination of 
this is to be correctly made ; at the same time it is a question in 
physics which should be capable of being answered. 
Let us now take the other side of the question. If the effi- 
ciency of a surface depends on flame contact, there must of course 
be flame, or at least gases of an extremely high temperature, and 
we therefore cannot expect this extraordinary increase of efficiency 
in any part of our boiler except where flame exists, and if these pro- 
jectors are placed in a boiler, anywhere except in contact with 
flame, their efficiency must be reduced to that of ordinary heat- 
ing surface. They are, of course, useful, but only in the same 
way as ordinary flue surface. When we come to boilers for 
raising steam, which have to stand high pressures, we come to 
other difficulties of a very serious nature, which require special 
provision to overcome them. To put such rods as I have re- 
ferred to in a boiler-plate necessitates the plate being drilled all 
over with holes, causing a dangerous source of weakness, as the 
rods cannot be used as stays; further than this, they would render 
really efficient examination a matter of extreme difficulty, and 
would be liable to give rise to frequent and almost incurable 
leakages ; but there is, fortunately, a very simple way to over- 
come this difficulty. I have found that rods or points, such as 
I have described, are not necessary, and that the same results 
can be obtained by webs or angle-ribs rolled in the plates. My 
experiments in this direction are not complete, and at present 
they tend to the conclusion that circular webs, which would be 
of the greatest efficiency in strengthening the flues, are not so 
efficient for heating as webs running lengthwise with the flue, 
and in a line with the direction of the flame. This point is one 
which I am at present engaged in testing with experimental 
