‘ 
400 REPORTS ON THE STATE OF SCIENCE, ETC. 
Are there any objections, other than that of scarcity, to the use of 
helium only, as a complete substitute for hydrogen ? I only know of one, 
but I think it is fatal: heliwm has less lifting power. As a rough working 
figure, applicable to hydrogen of ordinary purity and in normal barometric 
conditions, one may say that every 1,000 cubic feet of hydrogen will keep 
68 lb. in the air at ground level: a corresponding figure for helium is 
63 lb. 
Now this at first sight is not a big difference,—but then a mere per- 
centage does not convey the right idea: so much of the 68 (or 63) lbs. is 
employed on unavoidable duty,—in lifting the airship structure, the 
engines, and the crew. Let us see what the difference would have meant 
to R.33 in her recent flight. She is a ship of 2,000,000 cubic feet capacity, 
so the difference of 5 lbs. per 1,000 cubic feet means that, inflated with 
helium instead of hydrogen, she would have had 10,000 Ibs. less of ‘ useful 
lift’. Other things being equal (and for the most part they leave no 
choice), she would have had less petrol in her tanks to the amount of 
1,430 gallons. Now the published accounts of R.33’s flight state that 
two hours after she broke away Lieut. Booth reported 1,600 gallons of 
petrol on board, and that this as a matter of fact was sufficient, but only 
just sufficient, to bring her home. Inflated with helium, she must have 
been wrecked ‘somewhere in Europe’: hardly a triumphant vindication 
of helium from the standpoint of ‘ safety first’! 
Even in time of war, it is arguable whether we should unquestionably 
use helium if we could. Airships filled with hydrogen can be set on fire 
by incendiary bullets, and undoubtedly some will, if they become the prey 
of hostile aeroplanes. But in war we have to choose between risks of 
different kinds,—to strike a balance between the danger and the importance 
of the objective. A five-million ship (such as we are building now) could 
carry 11 tons more bombs with hydrogen than with helium; with the 
same weight of bombs it could carry additional petrol, giving it about 
24 hours more endurance at cruising speed; with the same weight of 
bombs and petrol it could fly 2,000 feet higher (and the official altitude 
record for airships is only slightly over 10,000 feet).4 Is it really certain 
that these advantages will be forfeited in order to make the airship non- 
inflammable? I personally will venture the prediction that America 
herself, if she uses them in war, will inflate her airships with hydrogen, 
and take special measures for their protection. 
The Helicopter. 
8. I pass on now to the Helicopter, that hardy perennial of the ‘ aero- 
nautical column’; and in case any one in this hall is unacquainted with 
the nature of the beast, I have brought a young one with me. 
(A helicopter model was flown at this povnt.) 
The full-sized helicopter is not as simple as that, of course, but you 
will readily grasp the basic idea: the Helicopter is a ‘form of aircraft 
whose support in the air is derived from airscrews with their axes vertical ’. 
That is the definition given in the British Standard Glossary of — 
_ *C£. Flight, January 29, 1925, 
