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SEPTEMBER I I , 1909.

determine the correct value forthe pitch, bysubstitutingthe numerical values for the corresponding symbols inthe equation.No value has yet been assigned to thesymbol W\ inthe equation, but aswe are dealing with air in units ofcubic feer, the value ofW\ is therefore, the weight ofon ecubic foot of air, which can be taken approximatelyas -073 pound, although in practice this may vary con-siderably, depending on barometric and thermometricdifferences, and thevelocity with which the air isbeinghandled. The numerical value of g is, as usual,32-16.Now having numerical values for all of thevariantsinvolved except the pitch, the equation can be stated. , 50 xP 3 x8ooo x-07-?arithmetically thus : -. , which solved3 64-32 x 11000 'for P s = 24-2 , makes P =2*893 a n o - completes the ful l setof correct proport ional values for the e l e m e n t s of thes t andard propel l e rs thus : H . P . = 20, R . P . S . = 20,P . =2-893, A. =50.

T o get comple te da ta forc o m p a r i s o n , lett he th rus t beca lcu la t ed inac tua l quan t i ty also, a n d , r e m e m b e r i n g th a tth e propel l e r can be d e s i g n e d to give the effect of airb lowing agains t a n o r m a l disc , t he fo rmula P =SV 2-oc>3c a n be u s e d by a l t e r ing the co-efficient to c o r r e s p o n dwith V in feet per se cond , and sub st i tu t in g thes y m b o l sw e are us ing here fort h e s a k e ofuniformity , an d us ing Tas the symbol fort h rus t , it b e c o m e s T = Az> 2*ooi39.App ly ing th i s fo rmula to the s t anda rd p rope l l e r g ivesSox (2*893 x 2 o ) 3 x "00139 =232-67 lbs . , or 1 1 6 l b s . perh.p . As the propos i t ion under cons idera t ion requ i restwo p ropel l e rs of vary ing th rus t s per h.p.,a n o t h e rpropel ler having a d e c r e a s e d t h r u s t perh .p . can now bep r e p a r e d for c o m p a r i s o n .Not ing f rom the t ab le (Var i a t ion 7 in ouri ssue ofJ u l y3rd) that ani ncrease of p i t ch reduces the th rus t per un i tof p o w e r , let t he same h .p . and speed of revo lu t ion bemain ta ined a n d it isevident that the area wi l l have to ber e d u c e d .A s d o u b l i n g thepi tch without al ter ing the p o w e r ors p e e d of revo lu t ion wou ld reduce thea r e a toone-e igh thof its pre viou s size (this effect is not s t a t ed injus t th isway int he t ab le , but can be deduced f rom Var ia t ion7),it will do as well to i ncrease the p i t ch any amoun t tha twill not vary the work ing cond i t ions toogrea t ly ,andaccord ing ly let the p i t ch be m a d e 3 ft. and so lve theA x 27 x8 0 0 0 x - 0 7 3 , . . ,equat ion for A,t h u s 7 a n d A is foundH ' 11000x64-32to be44-87 sq. ft., or adiameter ofapproximatelyand calculating for thrust 44*87 x(3 X 2o)2x-00139 =224-18, or I I * 2 lbs. per h.p.With theoretically perfect efficiency, we now have theequivalent of two normal surfaces, one having anareaof50 sq. ft.and being moved at avelocity of 57 "86 ft. persecond with an expenditure of 20h.p, and onehavingan area of 44-87 sq. ft.and being moved at avelocityof60 ft. per second utilizing the same h.p.Now, presuming that thetotal head resistance of themachine (whether composed of area and drift in adynamically sustained machine or area alone in abuoyancy-sustained machine) to which thepropeller isattached, isequal tothe resistance caused by a surface of20 sq. ft. area, normally presented, this resistance isproportional tothe square ofthe velocity of flight v2an dproduces slip inthe propeller.Representing the factors ofthe initial propulsive forcein thepropeller bythe symbols A for thearea againstwhich the force is exerted, andv for the velocity at

which A isbeing moved, it isapparent that this velocitywill bedecreased by any additional area moved in pro-portion to the added resistance, which, as we know,varies with 7>-, consequently symbolising theadded areaby a,the resultant velocity of the combined resistance isexpressed bythe formula :rwin which Arepresents the surface against which thepro-pulsive force isexerted, and A+ a the surfaces presentingresistance topropulsion.Stating arithmetically the resultant velocity of themachine with the standard propeller attached, thus:

r 5 o x ( 2 ~ 8 9 3 X 2 o ) 2 _ ^ , . ; - , . . -50 x 20we find it to beapproximately 49 ft. per second, and forthe same machine with the second propeller744-87 x(3 X2o )3_ / . [ . . : \ / = 1 ,.---...:. V 44.-87 + 2O -..-- :.:a velocity of approximately 50 ft per second, thus show-

ing that it is possible for a propeller having a lowerthrust per h.p. than another when themachine towhichit is attached is held stationary, to drive themachinefaster through the air in flight, than the propeller havingthe higher thrust per h.p.I wish tospecially comment on thecommon error ofusing thewind pressure coefficient in propeller thrustcalculations. I have ,only used it so far forthe sake ofsimplicity in laying down com parative effects. Beforeclosing this series of lessons I will give it differentiatingvalues under varying conditions. As noted above, itsvalue notonly changes materially with barometric andtherm om etric differences an d the velocity with which theair is being handled by thepropeller, butalso withthespeed of the machine, towhich the propeller is attached,through the air, its greatest variation being most notice-able inpropellers designed for sustentation only, due tothe increase in its value caused bythe inertia oftheairon the intake side of the propeller, andagain in theopposite condition, of full flight, decreasing its value.This is true only ofpropellers revolving atconstant speed,different values obtaining again when propeller andmotor are so designed as topermit of therotating speedincreasing as the machine containing them acquiresvelocity up tothe full flight maximum. ;

ft.; TH E " DAILY M AIL" AIR SHIP GAR AGE.T HE last day of August saw the garage which hasbeen built at Wormwood Scrubbs tohouse the Clement-Bayard airship, when it flies from Paris toLondon, com-pleted. Building operations comm enced on July 15th,and although the weather has considerably interferedwith thework, every available moment hasbeen takenadvantage of. On an average 150 men have been keptat work and the wages bill amounted to ^ 2 , 0 0 0 . Thebuilding is365 ft. longmore than half aslong again asthe nave ofSt. Paul's Cathedral65 ft. broad, and 98 ft.high, while the wind-resisting capacity of the structure is40 lbs. per sq. ft. According to theDaily Mail, thefollowing materials were used in itsconstruction :

17,000 pieces of iron sheeting, 6 ft. or 10 ft. by 2 ft. ; totalweight , 85tons.500 tons ofsteel girders.Thirty-eight windows, 13 ft. by 7 ft., containing 266sheets offrosted glass, 7 ft. by 2 ft. ; total weight, 30tons.Sailcloth curtain atentrance, 100 ft. by75 ft. ; weight, 2 tons.Ashes tothe depth of 6 ins. are laid onthe 23,725 sq. ft. offloor.156