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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[July, 



IV. And this brings us to the consideration of the fourth subject of 

 our inquiry; namely, the power of steam which Mr. Hensou purposes to 

 employ for the occasion. It is not our intention in this analysis to 

 enter into any examination of the particular construction of ma- 

 chinery by means of which Mr. Henson has sought to apply this 

 power to his present object. This has been sufficiently largely done 

 in other publications, as well as, more particularly, in the specification 

 of his plans drawn up under his own authority. Upon this point we 

 would merely observe that the main difficulty, to the removal of which 

 Mr. Henson's efforts have been principally directed, is the reduction 

 of the weight of the engine in relation to its effective force. In all 

 modes of developing power, the nature of the process imposes upon 

 the machinery by which it is elicited a certain degree of weight pro- 

 portioned to the force produced. Wherever there is a tension or 

 pressure, there must be of necessity a counter-pressure or resistance 

 in the parts of the generating apparatus ; resistance cannot be expe- 

 rienced without strength of material, and strength of material cannot 

 be had indefinitely without weight; and it is in the relative reduction 

 of this weight that Mr. Henson's ingenuity is presumed chiefly to 

 display itself. l Whether this has been done successfully, that is to 

 the extent claimed, consistently with the retention of the necessary 

 degree of strength, it is impossible for any one to determine apart 

 from actual experiment. According to the published accounts, Mr. 

 Henson's engine has been constructed to produce a resistance equal 

 to the power of ten (some say twenty) horses, within the limits of 

 6001b. weight. If this be true, he has certainly accomplished a very 

 considerable abatement of that which has hitherto constituted a main 

 obstacle to the establishment of a successful aerial navigation; and 

 taking it for granted that such is the case, our intention here is to 

 deal with the question upon these premises, and endeavour to test 

 the applicability of the power so claimed to the purpose for which it 

 has been designed. 



We have said that a solid body cannot be moved through the air 

 without experiencing a certain amount of resistance. Now this re- 

 sistance has been determined to follow the ratio of the squares of the 

 velocities under which it is generated ; and a table has been con- 

 structed upon experimental data, in which the effects of this force 

 upon a given extent of plane surface perpendicularly impinged upon, 

 are set forth in terms corresponding to different assigned rates of 

 motion, varying from one to one hundred miles an hour, with, of 

 course, a power of extending the scale indefinitely, subject, however, 

 to a consideration which we shall disregard at present, but which if 

 taken into account would only magnify the exigencies of the case 

 which we are endeavouring to define; namely, that at high rates of 

 motion, this resistance increases even more rapidly than the ratio of 

 the squares of the velocities otherwise assignable. Without regard 

 to this qualification, however, we are enabled to ascertain that, at the 

 rate of 240 miles an hour, prescribed to the machine as necessary to 

 its support at an angle of 3" of inclination in the suspending plane, 

 every square foot of plane surface perpendicularly directed, or equi- 

 valent amount of oblique surface, would experience an opposition 

 equal to a force of 28Slb. avoirdupois; so that if we can conceive 

 such a machine constructed so as to present to the impact of the air 

 no larger extent of surface than what would be equal to 10 square 

 feet perpendicularly exposed, the aggregate resistance it would ex- 

 perience to its progress at the rate in question, would amount to 

 2S80 Ib.+the horizontal resistance belonging to the plane as such, 

 and which we have stated in round numbers to be, for that rate and 

 degree of obliquity, about 1501b. 2 Unless therefore the power at 



1 The improvement of the machine in this particular does not merely 

 regard the reduction of the weight of the engine itself, but also of the 

 means whereby its energy is maintained — the water and fuel necessary for 

 its use. In the modifications by which a saving of these may be effected, a 

 further step is gained in the same direction, which, however, more properly 

 concerns the question of the duration of the flight when it shall have been 

 accomplished, than its abstract possibility. 



- These numbers are not strictly correct. They should have been stated 

 at 223 miles an hour for the rate, with an horizontal resistance equal to 

 15/ lb., as subsequently represented in table. The difference arises, partly 

 from the omission of the cosine of the lesser angle in determining its pro- 



command be adequate to the development of a reaction equal to this 

 resistance, the machine could not be impelled at the rate with the 

 angle prescribed ; and we should be driven to adopt either a slower 

 rate of motion at the expense of an increased angle of inclination, 

 and of the increased horizontal resistance by which the plane itself, 

 at that increased angle would be encountered, or else a more rapid 

 rate of motion due to a reduced angle of inclination, at the cost of 

 the increased resistance due to the solid proportions of the machine 

 with this additional impulse — whichever should be found to be pro- 

 ductive of the least amount, in the aggregate of this resistance. 



To apply this with mathematical accuracy to the illustration of the 

 case before us, is clearly impossible, because we are entirely unpro- 

 vided with anything like a proximate definition of the main force to 

 be overcome; namely, the capacity of the solid parts of the machine 

 for effecting resistance. In assigning 10 square feet we have no 

 doubt that we are keeping far within the limits of the truth; and 

 were we to assign a higher figure we might be met by a denial of our 

 premises which no reasoning, apart from actual admeasurement, and 

 that of the most complicated description, could enable us to dispose 

 of. Fortunately, however, for the purpose of our investigations, we 

 are not put to such an alternative, as we shall readily see that the ex- 

 igencies of the case, even upon the mildest hypothesis, are so far 

 beyond the means appointed to satisfy them, that there will be no oc- 

 casion to suspend our reasoning upon so disputable a point as the 

 correct estimate of the resisting surface of the machine. 



In order, however, to be able more satisfactorily to elucidate this 

 position, it will be necessary to take into account the elements of the 

 force we are considering in relation to the process by which it is pur- 

 posed to be developed. In estimating the actual amount of any force 

 generated or required, it is well-known that, equally with the weight 

 to be moved or the resistance to be overcome, 3 is comprised in the 

 constitution of the force the velocity at or under which this result is 

 to be accomplished. Without regard to this condition it is as im- 

 possible to give an intelligible definition of any force as it is for the 

 force itself to exist or be exercised. A body cannot be moved unless 

 it is moved through some space; and as every operation occupies 

 some time, the time in which through a given space, or the space 

 through which in a given time, the body is to be moved, are as essen- 

 tial ingredients in the constitution and definition of the force em- 

 ployed as the weight or resistance of the body which is the object of 

 its application. Thus it is clear that before anything can be pre- 

 dicted with distinctness as to the actual value of any force in action, 

 or the adequacy of any contemplated power, the rate under which it 

 will have to be exercised must be included in the terms of the propo- 

 sition. In taking therefore the resistance which a given plane is cal- 

 culated to develop in opposition to its course at a given rate, as the 

 measure of the force which is required to produce it, the rate must 

 be specified, not of the body so moved, but of the instruments of its 

 propulsion, by the operation of which that resistance is to be called 

 forth ; and the reduction of this rate becomes exactly as important an 

 object in the economy of the forces employed, as the reduction of the 

 actual resistance, to the constitution of which it contributes. Now the 

 minimum rate of action that could be assignable to the instruments of 

 a progressive motion is obviously that required to be conferred upon 

 the machine itself, as the ultimate result of their operation ; for, it is 

 clear, by no appointment of surface, or other expedient, could the 

 machine be propelled at a greater speed than that at which the agents 

 of its propulsion were themselves accomplishing their reaction. 

 Whether the conditions of surface, by which alone this conclusion 



portion of vertical resistance, and partly from the adoption of a rate in round 

 numbers, 12 miles an hour, as that at which a plane surface generates a re- 

 sistance by perpendicular impact equal to two-thirds of a pound to the 

 square foot, instead of 11-666, which is its true denomination. The re- 

 sults in the table further on are all based upon the actual quantities, and 

 are mathematically exact. 



8 It seems scarcely necessary to observe that in mechanical operations a 

 resistance to be encountered is the same thing with a weight to be raised ; 

 the rate of development in the one case being exactly analogous to the rate 

 of elevation in the other. 



