This we shall do by a comparison with the power of steam, according to the usual mode of estimating it; that is, reckoning a one-horse power to be equal to the traction or draught of 32,000 lbs. through the space of one foot in a minute. According to this scale, observing the corresponding conditions of the spring--namely, the weight it balances on the barrel, (answering to the force of traction) = 45 lbs., the circumference of the barrel (answering to the space traversed) = one foot, and the time of uncoiling for each turn, (answering to the rate of the operation) say, three seconds and a half--we find the power of the spring employed in the propulsion of the model, to be as nearly as possible the forty-second part of the power of one horse; from whence it is easy to deduce the conditions of flight assignable to the same, and to different sized Balloons of the same shape, at any other degree of speed. Assuming, for instance, a Balloon of 100 feet in length and 50 feet in height, and proposing a rate of motion equal to 20 miles an hour, we have, in the first instance, the power required to propel the model at that rate, compared with that already ascertained for a velocity of six miles an hour, in the ratio of the _squares of the two velocities_, as nearly ten to one; that is, ten forty-seconds, or about one-fourth of a horse power. To apply this to the larger Balloon, we must take the squares of their respective diameters; which being nearly in the ratio of 56 to 1, gives an amount of 56 times one-fourth or about 14 horses, as the sum of the power required. From what particular source the power to be employed in the propulsion of the Balloon should be deduced, is not indeed a question without some difficulties and doubts in the determination. To all the moving powers at present before the world some objections apply which disparage their application, or altogether exclude them from our consideration. The power of the coiled spring is too limited to be employed upon a larger scale. The use of the steam-engine is accompanied with a gradual consumption of the resources of the Balloon in ballast, and consequently in gas, the one being exactly answerable to the other, and is therefore not calculated for voyages of long duration. Human strength appears to be too feeble for great results, and moreover, requires repose; which reduces the amount assignable to each man to a fraction of its nominal value. Of electro-magnetism as yet we know too little to enable us to pronounce upon it with certainty. Of the remaining powers known only one is worth mentioning in connexion with this subject, namely, the elastic force of air; and this I only mention because it has been taken up by one whose authority in these matters is deservedly entitled to much weight, and who entertains great hopes of making it ultimately subservient to the purpose in view.