Geometrical Details.--Calculation of the Capacity of the Balloon.--The Double Receptacle.--The Covering.--The Car.--The Mysterious Apparatus1. --The Provisions and Stores.--The Final Summing up.
Dr. Ferguson had long been engaged upon the details of his expedition. It is easy to comprehend that the balloon --that marvellous vehicle which was to convey him through the air--was the constant object of his solicitude2.
At the outset, in order not to give the balloon too ponderous3 dimensions, he had decided4 to fill it with hydrogen gas, which is fourteen and a half times lighter5 than common air. The production of this gas is easy, and it has given the greatest satisfaction hitherto in aerostatic experiments.
The doctor, according to very accurate calculations, found that, including the articles indispensable to his journey and his apparatus, he should have to carry a weight of 4,000 pounds; therefore he had to find out what would be the ascensional force of a balloon capable of raising such a weight, and, consequently, what would be its capacity.
A weight of four thousand pounds is represented by a displacement6 of the air amounting to forty-four thousand eight hundred and forty-seven cubic feet; or, in other words, forty-four thousand eight hundred and forty-seven cubic feet of air weigh about four thousand pounds.
By giving the balloon these cubic dimensions, and filling it with hydrogen gas, instead of common air--the former being fourteen and a half times lighter and weighing therefore only two hundred and seventy-six pounds--a difference of three thousand seven hundred and twenty-four pounds in equilibrium7 is produced; and it is this difference between the weight of the gas contained in the balloon and the weight of the surrounding atmosphere that constitutes the ascensional force of the former.
However, were the forty-four thousand eight hundred and forty-seven cubic feet of gas of which we speak, all introduced into the balloon, it would be entirely8 filled; but that would not do, because, as the balloon continued to mount into the more rarefied layers of the atmosphere, the gas within would dilate9, and soon burst the cover containing it. Balloons, then, are usually only two-thirds filled.
But the doctor, in carrying out a project known only to himself, resolved to fill his balloon only one-half; and, since he had to carry forty-four thousand eight hundred and forty-seven cubic feet of gas, to give his balloon nearly double capacity he arranged it in that elongated10, oval shape which has come to be preferred. The horizontal diameter was fifty feet, and the vertical11 diameter seventy-five feet. He thus obtained a spheroid, the capacity of which amounted, in round numbers, to ninety thousand cubic feet.
Could Dr. Ferguson have used two balloons, his chances of success would have been increased; for, should one burst in the air, he could, by throwing out ballast, keep himself up with the other. But the management of two balloons would, necessarily, be very difficult, in view of the problem how to keep them both at an equal ascensional force.
After having pondered the matter carefully, Dr. Ferguson, by an ingenious arrangement, combined the advantages of two balloons, without incurring12 their inconveniences. He constructed two of different sizes, and inclosed the smaller in the larger one. His external balloon, which had the dimensions given above, contained a less one of the same shape, which was only forty-five feet in horizontal, and sixty-eight feet in vertical diameter. The capacity of this interior balloon was only sixty-seven thousand cubic feet: it was to float in the fluid surrounding it. A valve opened from one balloon into the other, and thus enabled the aeronaut to communicate with both.
This arrangement offered the advantage, that if gas had to be let off, so as to descend13, that which was in the outer balloon would go first; and, were it completely emptied, the smaller one would still remain intact. The outer envelope might then be cast off as a useless encumbrance14; and the second balloon, left free to itself, would not offer the same hold to the currents of air as a half-inflated one must needs present.
Moreover, in case of an accident happening to the outside balloon, such as getting torn, for instance, the other would remain intact.
The balloons were made of a strong but light Lyons silk, coated with gutta percha. This gummy, resinous15 substance is absolutely water-proof, and also resists acids and gas perfectly16. The silk was doubled, at the upper extremity17 of the oval, where most of the strain would come.
Such an envelope as this could retain the inflating18 fluid for any length of time. It weighed half a pound per nine square feet. Hence the surface of the outside balloon being about eleven thousand six hundred square feet, its envelope weighed six hundred and fifty pounds. The envelope of the second or inner balloon, having nine thousand two hundred square feet of surface, weighed only about five hundred and ten pounds, or say eleven hundred and sixty pounds for both.
The network that supported the car was made of very strong hempen19 cord, and the two valves were the object of the most minute and careful attention, as the rudder of a ship would be.
The car, which was of a circular form and fifteen feet in diameter, was made of wicker-work, strengthened with a slight covering of iron, and protected below by a system of elastic20 springs, to deaden the shock of collision. Its weight, along with that of the network, did not exceed two hundred and fifty pounds.
In addition to the above, the doctor caused to be constructed two sheet-iron chests two lines in thickness. These were connected by means of pipes furnished with stopcocks. He joined to these a spiral, two inches in diameter, which terminated in two branch pieces of unequal length, the longer of which, however, was twenty-five feet in height and the shorter only fifteen feet.
These sheet-iron chests were embedded21 in the car in such a way as to take up the least possible amount of space. The spiral, which was not to be adjusted until some future moment, was packed up, separately, along with a very strong Buntzen electric battery. This apparatus had been so ingeniously combined that it did not weigh more than seven hundred pounds, even including twenty-five gallons of water in another receptacle.
The instruments provided for the journey consisted of two barometers22, two thermometers, two compasses, a sextant, two chronometers23, an artificial horizon, and an altazimuth, to throw out the height of distant and inaccessible24 objects.
The Greenwich Observatory25 had placed itself at the doctor's disposal. The latter, however, did not intend to make experiments in physics; he merely wanted to be able to know in what direction he was passing, and to determine the position of the principal rivers, mountains, and towns.
He also provided himself with three thoroughly26 tested iron anchors, and a light but strong silk ladder fifty feet in length.
He at the same time carefully weighed his stores of provision, which consisted of tea, coffee, biscuit, salted meat, and pemmican, a preparation which comprises many nutritive elements in a small space. Besides a sufficient stock of pure brandy, he arranged two water-tanks, each of which contained twenty-two gallons.
The consumption of these articles would necessarily, little by little, diminish the weight to be sustained, for it must be remembered that the equilibrium of a balloon floating in the atmosphere is extremely sensitive. The loss of an almost insignificant27 weight suffices to produce a very noticeable displacement.
Nor did the doctor forget an awning28 to shelter the car, nor the coverings and blankets that were to be the bedding of the journey, nor some fowling29 pieces and rifles, with their requisite30 supply of powder and ball.
Here is the summing up of his various items, and their weight, as he computed31 it:
Ferguson........................... 135 pounds. Kennedy............................ 153 " Joe................................ 120 " Weight of the outside balloon...... 650 " Weight of the second balloon....... 510 " Car and network.................... 280 " Anchors, instruments, awnings32, and sundry33 utensils34, guns, coverings, etc................... 190 " Meat, pemmican, biscuits, tea, coffee, brandy................... 386 " Water.............................. 400 " Apparatus.......................... 700 " Weight of the hydrogen............. 276 " Ballast............................ 200 " ----- 4,000 pounds.
Such were the items of the four thousand pounds that Dr. Ferguson proposed to carry up with him. He took only two hundred pounds of ballast for "unforeseen emergencies," as he remarked, since otherwise he did not expect to use any, thanks to the peculiarity35 of his apparatus.
几何学般的详细——气球容量的计算——双层气球——气球外壳——吊篮——神秘的仪器——食物——最后的增添
弗格森博士长时间来已经把他的探险计划考虑得滴水不漏。大家明白，博士用来空运的这个绝妙飞行器，是他操心最多的。
首先，为了不让气球的体积太大，他决定往气球里灌氢气。因为，这种气体比空气轻，重量只是空气的十四分之一多点。生产氢气很容易。在飞行试验中，正是用这种气体取得了最佳效果。
博士根据十分精确的计算发现， 连同旅行中的必需物品和他的仪器， 得携带4000斤的重量。因此，必须算出能够吊起这个重量的升力，以及气球的容量。移动4000斤的重量需要44847立方尺①的空气。反过来说，44847立方尺的空气重4000斤左右。
①法国古长度单位，1法尺相当于325毫米。这里44847立方尺约合成661立方米。
如果给气球44847立方尺的容积， 里面不装空气，而是装上比空气轻十四倍半的氢气， 那么气球只重276斤。这样就剩下了平衡问题，因为气球和周围空气之间的平衡被打破了。气球中所装气体的重量与周围空气的重量，两者之间相差3724斤。就是这种差异构成了气球的升力。
不过， 假如把我们说的44847立方尺气体都充入气球中，气球就完全装满了；然而，这样做不行，因为，随着气球的升高，大气层中的空气密度逐渐变小，气球里装的气体逐渐膨胀，随时可能胀破气球外壳，所以，气球里一般只能充三分之二的气体。
但是，博士根据只有他才知道的计划，决定只往气球里灌一半气体。不过，既然必须携带44847立方尺的氢气，他只好让气球具有几乎双倍的容积。
他把气球设计成这种最可取的椭圆形状。气球的水平直径为50尺，垂直直径为75尺①。这样，就得到了一个容积增加到9 立方尺的球体。
①这个尺寸没什么可称奇的。 1784年在里昂，蒙戈菲埃造了一个容量为34 立方尺，即2 立方米的气球。它可以吊起20顿的重量。——原注。
如果弗格森博士能用两个气球的话，他成功的机会就会加大。的确如此，万一其中一个气球在空中破了，就可以丢卒保车，用另外一个维持。但是，当涉及到必须使两只气球保持相等的升力时，操作就变得异常困难了。
经过长期思考，弗格森终于有了主意。他通过一种巧妙的安排，扬长弃短，结果，既集中了两只气球的优点，又克服了它们的不足之处。他让人制造了两只大小不等的气球，并且把小的装在大的里面。外层气球网的尺寸就是我们前面说过的。形状相同，体积略小的气球，水平直径为45尺，垂直直径为68尺。因此，里面这只小气球的容积只有67000立方尺。 小气球必须游浮在包围着它的气体中。两只气球之间有一个阀门，需要时能使它们相通。
这种安排有一个好处：假如需要释放一些气体使气球下降，那就先放大气球中的氢气；必要时，甚至可以把大气球的气全部排空，小气球的气留着不动；这时，就可以像扔掉多余的重量一样，扔掉外面这层外壳。因为剩下来的这只气球小，又只是一个，尽管装满氢气，也没有两只装一半氢气的气球招风。
此外，万一出现事故，万一外层气球不巧被撕破，小气球可以作后备用。
两只气球都是用里昂的斜纹塔夫绸做成的。塔夫绸上面涂了马来树胶。这种树胶脂物质具有极强的不透水性，而且，一点不怕酸和气体的侵蚀。气球的上部用了两层塔夫绸，因为，整个压力几乎都作用在那儿。
这种经过特别加固的外壳可以把气体长期贮存在里面。 外壳的重量为每9平方尺半斤。外层大气球的面积为11600平方尺左右，重量为650斤；里面小气球外壳的面积为920平方尺，重量只有510斤。这样，两只气球自身的重量相加，共1160斤。
用来承受吊篮的高空气球网是用一种强度非常大的麻绳编制而成。两个气门更是被下足了功夫，做工就象船上的舵一样精细。
吊篮的形状是圆的，直径为15尺。吊篮用柳条做成，起加固作用的是一个很轻的铁骨架。吊篮下部配有一些弹性很大的弹簧，为的是气球降落时，缓和吊篮对地面的冲撞。吊篮和气球网的重量合起来不超过280斤。
除此以外， 博士还让人做了四个2分①厚的铁皮箱子。箱子之间由一些带有开关的管子连接。 博士在每只箱子上装上一根约2寸②粗的蛇形管，蛇形管的头叉出两条长短不一的直管。不过，长的才25尺，短的只有15尺。
①法国古长度单位，1法分约合2.25毫米。等于1/12法寸。
②法国古长度单位，1法寸约合27.07毫米，等于1/12法尺。
铁皮箱子嵌放在吊篮里，尽可能地少占些地方。蛇形管应该后来装上，因此它暂时另外装箱。一个电力极强的本生电池也是单独装箱的。这套器械组合得十分巧妙，甚至加上盛在一个特制水箱中的25加仑①水，全部重量还不超过700斤。
①每加仑等于4，453升。
旅行中用的仪器包括两个气压表、 两个温度表、两个罗盘、一个6分仪、两个测时仪、一个水平仪、一个人工地平仪和一个用来测定远方物体位置的地平经纬仪。格林威治天文台早已宣布愿为博士服务。但博士并不打算做物理试验。他只是想用这些器械辨认一下方向，确定一下主要河流、山川和城镇的方位。
弗格森博士还配备了三个经过可靠试验的铁锚，以及50尺长的一条轻便、结实的软梯。
他同样对随身携带的食物重量做了精确计算。这些食物包括茶叶、咖啡、饼干、咸肉和干肉饼。这全是些体积小，营养成分高的物品。除了备有足够的烈酒，另外准备了两个水箱。每箱可容纳22加仑水。
随着这些各种不同食品的消耗，气球提升的重量将逐渐减轻。因为，要知道，气球在大气中的平衡问题是极其敏感的。哪怕失去一点几乎微不足道的负担，也足以使气球的升降产生非常显著的变化。
博士没有忘记带上顶帐篷把吊篮遮住一部分，也没忘记旅行用的整套卧具和猎人肯尼迪的枪支及火药、铅弹。以下是气球升空要承受的重量统计：
弗格森……………………135斤
肯尼迪……………………153斤
乔…………………………120斤
大气球的重量……………………650斤
小气球的重量……………………510斤
吊篮和气球网……………………280斤
锚、仪器、枪支、卧具、帐篷、各种用具…………190斤
罐头肉、干肉、饼干、茶叶、咖啡、白酒…………386斤
水………………………………400斤
全套设备…………………………700斤
氢气的重量…………………………276斤
气球压载物…………………………200斤
合计4000斤
这就是弗格森博士打算带走的4000斤重量的明细录。他只带了200斤的压载物。照他的话说，“仅仅为了以防万一”。他的全套设备即可以满足需要，所以，他算定用不着压载物。
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apparatus
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| n.装置，器械；器具，设备 | |
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solicitude
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| n.焦虑 | |
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ponderous
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| adj.沉重的，笨重的，(文章)冗长的 | |
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decided
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| adj.决定了的，坚决的；明显的，明确的 | |
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lighter
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| n.打火机，点火器；驳船；v.用驳船运送；light的比较级 | |
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displacement
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| n.移置，取代，位移，排水量 | |
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equilibrium
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| n.平衡，均衡，相称，均势，平静 | |
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entirely
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| ad.全部地，完整地；完全地，彻底地 | |
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dilate
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| vt.使膨胀，使扩大 | |
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elongated
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| v.延长，加长( elongate的过去式和过去分词 ) | |
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vertical
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| adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置 | |
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incurring
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| 遭受，招致，引起( incur的现在分词 ) | |
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descend
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| vt./vi.传下来，下来，下降 | |
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encumbrance
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| n.妨碍物，累赘 | |
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resinous
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| adj.树脂的，树脂质的，树脂制的 | |
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perfectly
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| adv.完美地，无可非议地，彻底地 | |
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extremity
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| n.末端，尽头；尽力；终极；极度 | |
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inflating
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| v.使充气（于轮胎、气球等）( inflate的现在分词 )；（使）膨胀；（使）通货膨胀；物价上涨 | |
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hempen
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| adj. 大麻制的, 大麻的 | |
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elastic
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| n.橡皮圈，松紧带；adj.有弹性的；灵活的 | |
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embedded
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| a.扎牢的 | |
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barometers
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| 气压计，晴雨表( barometer的名词复数 ) | |
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chronometers
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| n.精密计时器，航行表( chronometer的名词复数 ) | |
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inaccessible
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| adj.达不到的，难接近的 | |
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observatory
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| n.天文台，气象台，瞭望台，观测台 | |
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thoroughly
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| adv.完全地，彻底地，十足地 | |
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insignificant
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| adj.无关紧要的，可忽略的，无意义的 | |
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awning
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| n.遮阳篷；雨篷 | |
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fowling
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| 捕鸟，打鸟 | |
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requisite
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| adj.需要的，必不可少的；n.必需品 | |
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computed
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| adj.[医]计算的，使用计算机的v.计算，估算( compute的过去式和过去分词 ) | |
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awnings
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| 篷帐布 | |
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sundry
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| adj.各式各样的，种种的 | |
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utensils
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| 器具，用具，器皿( utensil的名词复数 )； 器物 | |
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peculiarity
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| n.独特性，特色；特殊的东西；怪癖 | |
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