152 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[Mat, 



satisfy tbe exigencies of the case. In the first place, the data upon 

 which the calculations of the resistance of the atmosphere have been 

 grounded, are deduced from a consideration of the density under little 

 more than mean pressure, equivalent to the support of a column of mer- 

 cury 30 inches in height ; whereas it is well known that it frequently, 

 or at all events occasionally, falls to an extent of nearly two inches, 

 implying a reduction in the density equal to Jjth of its whole amount. 

 In respect of this condition alone, therefore, 25 miles per hour 

 would have to be added to the above to enable the machine to main- 

 tain its pristine elevation, but slightly removed above the surface of the 

 earth. But the mere retention of this elevation, without the power of 

 augmenting it at will, within certain limits, would be evidently very 

 insufficient for the purposes of a perfect aerial navigation. It is unne- 

 cessary to dwell upon the many occasions which the inequalities of the 

 earth's surface would present, in the extancy of the objects upon it, 

 both natural and artificial, when it would be necessary to assume a 

 higher course in order to escape a collision. Some of these, it is truei 

 could be gone round, and thus avoided, instead of being surmounted. 

 But there is still a great variation in the prominence or altitude of the 

 plains and large ranges of mountain country, which could not be so 

 dealt with; and in respect of these, a considerable allowance must be 

 made in the powers of the machine. No part, for instance, of the 

 Alps or Pyrenees, extending entirely across Europe, could be traversed 

 under an elevation of 8000 feet, and without the power of accomplish- 

 ing this, the use of the machine would be restricted to a very small 

 portion of our own continent, or forced to make a detour equal in some 

 cases to perhaps the entire extent of its intended route. Now for 

 every 1200 feet of elevation, in the beginning of the scale, a reduction 

 of one inch may be counted upon in the height of the barometrical 

 column ; so that if we only presume an elevation of 8000 feet, we shall 

 have imposed upon the aerial machine an obligation of conducting her 

 operations in an atmosphere the density of which will have been re- 

 duced to an extent indicated by a fall of the barometer to 23 inches; 

 from which, if we further subtract the amount of the variation from 

 mean pressure previously mentioned, we shall have a density of the 

 medium only (hree-fourths of that, upon the hypothesis of which her 

 former velocity had been assigned. Now the pressure, according to 

 the first law of resistances, being directly as the densities of the 

 media, by this condition, therefore, we shall have incurred a fresh 

 obligation of speed, following the ratio of the square roots of the den- 

 sities, the total amount of which will be determined by the rule of 

 three proportion ; thus, as V3 : V 4 '• '• lln O = the number of miles 

 per hour necessary to accomplish the required resistance under mean 

 pressure : 124G miles (nearly), = the rate at which the machine must 

 be impelled to enable it to effectuate the same amount of resistance 

 under the reduced pressure to which, in its elevation, we have seen 

 it will have to be exposed. And this augmented velocity, be it ob- 

 served, arising solely out of the reduction in the density of the 

 medium, will be maintainable by the application of the same effectual 

 force, namely 1501b., by which the first assigned velocity was shown 

 to be determined. 



Hitherto we have regarded the qualifications of the machine in 

 respect of rate and force apart from any considerations but the neces- 

 sity of securing an elevation in the air. But a velocity of 50 miles an 

 hour, however it might satisfy the conditions of support, would 

 evidently be insufficient to realize that progress independent of the 

 action of the winds which is necessary to the constitution of a certain 

 and serviceable mode of transport. For this purpose, something more 

 than the actual rate of the winds is the least amount of speed which 

 could be considered sufficient to meet the exigencies of the case. 

 >>ow, the average rate of the winds, in this climate at least, may be 

 stated to be about 25 miles an hour. This we are enabled to 

 determine, not from the observations of the meteorologist alone, but 

 also, (and what is more to the point because founded upon expe- 

 rience in that part of the atmosphere with which we have more 

 especially to do) from a consideration of the average rate of Mr. 

 Green's aerial excursions, " deduced from a series of 240 voyages, 



•* " I he total distance which Mr. Green accomplished in the course of his 



executed generally, of course, in the most favourable periods of the 

 year. From this we learn that 25 miles an hour is the mean rate at 

 which a body floating in the air may expect to be transported; to this 

 we must add. considerably more to secure a balance in favour of the 

 machine, for which an additional amount of force must be provided 

 over the amount assigned for its support. 



The determination of this new quantity however depends entirely 

 upon data with which we have not been furnished. In the calculations 

 which we have hitherto made, we have regarded the machine as a 

 mathematical plane, altogether unproductive of any resistance except 

 what contributes to the counteraction of its descent. This, however, 

 it is scarcely necessary to observe, is an hypothesis which is not cor- 

 rect in fact; and the distinction will be seen to be of serious impor- 

 tance, when we consider that, fur every square foot of plane surface 

 made up of all parts and projections that receive the direct impact 

 of the air, the machine experiences, at the rate of 100 miles an hour, 

 a resistance equal to about 501b.; so that if we only suppose it to 

 present an additional extent of resisting surface equivalent to a plane 

 of 10 square feet, it will encounter an opposition equal to four times 

 what has already been assigned to it, and consequently involve a ne- 

 cessity for four times the amount of force which was previously re- 

 quired. 



Now the conditions of force in the first instance prescribed might 

 have been considerably (almost indeed, indefinitely) reduced by in- 

 creasing the superficial contents of the suspending plane. For in- 

 stance, if instead of an area of 1500 square feet, it had presented a 

 surface of double the extent, (which it could have been made to do, 

 by doubling the breadth or adopting another form, without augmenting 

 the difficulties of construction,) the same degree of elevation would 

 have been obtained with less than half the force ; as will be evident 

 if we consider that while the resistance follows the ratio of the di- 

 mensions, the dimensions observe the ratio of the squares of the ve- 

 locities: so that in this point of view the suspending plane of Mr. 

 Henson's machine may be shown to be contrived without due regard 

 to the economy of the forces by which it is upheld. 



But there is another purpose which the suspending plane is re- 

 quired to fulfil, and which must he taken into consideration in adjudi- 

 cating upon the plan before us. It is to be recollected that this eleva- 

 tion is maintained only by the exercise of a progressive motion, aud 

 that in order to effectuate a safe descent this motion must be suspended. 

 It is scarcely necessary to observe that a projectile force, however mo- 

 dified it might be, is quite incompatible with the preservation of hu- 

 man life under any mode of accomplishing a descent which could be 

 proposed; and that unless it can be accomplished by the independent 

 resistance of the suspending plane, it can never be safely accomplished 

 at all. There are two modes by which the actual rate of descent may 

 indifferently be ascertained. To one of these modes, which is an 

 inference from the pressure exerted by the air in motion at different 

 degrees of velocity, we have already adverted in determining the 

 speed at which, under perpendicular pressure, the plane in question 

 would generate a resistance equal to its weight. From this we have 

 seen that a rate of 12 miles an hour is exactly answerable to the con- 

 ditions of the case ; being that at which a plane surface develops the 

 resistance assignable to Mr. Henson's machine, namely, two thirds 

 of a pound to the square foot; and accordingly that number expresses 

 the rate at which the machine in question would descend through 

 the air if left of itself to fall. 



In addition to this method, Dr. Huttou has left us a formula 

 by which the same may be calcjlated with sufficient accuracy 

 for practical purposes, and which it will be seen leads to a 

 result confirmative of the approximate correctness of our conclu- 

 sions. Taking W to represent the weight and D the diameter 



of a circular plane of the given area, rrV w gives the number of 



first 200 aerial excursions, a verj potation enables him a 



GO00 miles; and t he time consumed in the performance at240houi 

 former of these two quantitise divided by the latter gives the result 

 Areanoutiai. page 298. 



