1909-10.] Illuminating Power of Groups of Pin-hole Burners. 61 
ruddy, while the three- and four-burner flames were decidedly smoky at small 
distances apart. The two burners would not be deprived, for any very 
appreciable section of their flames, of the streams of air coming from the 
surrounding space, but the tops of the flames would suffer from the currents 
of C0 2 and other combustion products formed at the lower parts, and hence 
the tops should show smokiness to a slight extent. This applies much more 
to the three and four flames, since all these products formed inside the space 
would, owing to the draught, be confined inside the space throughout the 
whole length of the flames. An increased heat and draught, due to increased 
number of flames, would tend to make the flames longer, while an increase 
in the confined space would conduce to smokiness as above. It was ob- 
served that the four flames were both much longer and much more smoky 
than any of the others, for small distances apart. 
A puzzling phenomenon occurred with the two flames at small distances. 
They increased in size without being smoky, but showed no tendency to 
coalesce. Instead they vibrated rapidly at their tips, say several times per 
second, and in the same phase, in planes perpendicular to the common plane 
of the flames. Persistence of vision made the appearence of the flames, 
viewed in their common plane, very like that of a fish-tail. 
A very rough estimate of the distances for which the second set of causes 
becomes fairly important, or at least attains the same relative importance 
in the various cases, may be got by examining the inflexion points. As on 
page 59 the values of x for the inflexion points rise as we increase the 
number of burners, so that the diminishing cause, whatever it is, comes 
into prominence sooner and sooner as we increase the number of burners. 
This is what we would expect if the cause is the twofold one mentioned 
above — the sharing of the total oxygen supply, and the presence of the 
products of combustion. 
Both for maximum values and inflexion values the figures for the 
two burners differ more from those of the three burners than those of the 
three burners differ from those of the four. This might correspond to the 
big difference between the volume outlined by the two burners and that 
outlined by the three and four. 
The results of the above series of experiments are therefore as follows : — - 
(1) As the distance of adjacent burners of a symmetrical group is 
decreased, the illuminating power of the group increases to a maximum of 
from 35 per cent, to 40 per cent., and then decreases rapidly and is still 
decreasing when the burners ('8 cm. external diameter) touch. The maxi- 
mum points and inflexion points are well marked for the cases of three and 
four burners, but only suggested for the two burners. 
