Aug. 13, 1874] 



NA TURE 



285 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No notice is taken of ano7iymous 

 communications^ 



The Woolwich Aeronautical Experiment 

 I HAVE read, not without some surprise, the accounts given 

 by the daily papers, stating that the recent experiment at Wool- 

 wich had been fruitless. The lessons of the experiment are 

 numerous, although it would have been easy to predict all that 

 happened ; but the impressions relating to balloons and bal- 

 looning are generally so vague and so incorrect, that I may be 

 justified in trying to summarise tlie results which were obtained 

 in connection with the siege of Paris, which might otherwise 

 be lost altogether. 



As I stated in my article on the "Flying Man" (vol. x. 

 p. 230), the principal object to be considered in the theory 

 of aerial motion is the friction of the moving surface against 

 the air ; the friction increasing according to the square of 

 the mte of motion v'-, the force necessary to move the body 

 at a certain rate varies according to f'. Consequently it is easy 

 to impart a small motion to a balloon ; but the difficulty very 

 quickly becomes insuperable except with an almost inexhaustible 

 source of power, such as a powerf^ul steam-engine. Iland-fowcr 

 cannot be made of any avail ; M. Dupuy de Lome's experiments 

 proved this definitively, and tliat question must be considered as 

 settled in favour of steam -fouvr. 



The problem now at issue is to ascertain whether it is possible 

 to construct a saje fire-engine balloon, and to use it successfully 

 for travelling to a distance. But I shall give some calculations 

 on the recent experiments with hand power. 



If we suppose that the motion of tlie directing balloon is uni- 

 form, the friction consumes all the force which is generated ; * 

 consequently, if n = number of men pulling the fan, /// = the 

 real motive power generated by each man, Z- = a coefficient 

 which depends on the nature of the surface of the balloon, / = 

 great axis of balloon, ;■ = radius of equatorial section, v = rate 

 of motion — I suppose that ;- was the same for Dupuy de Lome 

 as for Bowdler, and that M. Dupuy de Lome's great axis was 2r, 

 his number of men twelve instead of two, and his rate of motion 

 9 ft. per second — we shall try to find what ought to be the motion 

 of Bowdler's balloon . 



As according to the principles of mechanics 



' n.m.l 

 it is easy to find 



</' 



\/6 X 2 

 under these circumstances. But m is not the same, as Dupuy de 

 Lome's men were pulling on a large screw acting without 

 any transmission ; Bo^vdler's apparatus was a small screw 

 3 ft. in diameter. I suppose Bowdler's utilisation was only half 

 of Dupuy de Lome's ; consequently the real equation is 



7^ _- I l_ 



t^' "^6 X 4 \/24 

 not far from J. The motion of Bowdler's balloon could not be 

 more than 3 It. a second. 



It was impossible for Major Beaumont to see any difference 

 with the motion of the air being at a distance from the earth. 

 It could be ascertained witli very great difficulty even with an 

 aeronautical compass of the best description. 



But the fact of the balloon having been put into a state of 

 rotation by the rudder is a demonstration of the fact of a dif- 

 ferential motion having been obtained. It is the very i>ressure 

 resulting from the differential motion which is the only force 

 lliat rotates the balloon in acting on the rudder. The rudder is 

 pushed as it is in the sea when the ship is acted on by sails or 

 steam, and in the air the action is very easy, as the balloon is 

 almost symmetrical around its vertical axis. 



It is true the governing power could be imparted very easily 

 by direct action on an excentric helix adjusted for the purpose, 

 as has been suggested, but not tried, so far as my knowledge 

 goes. I will say the same of the vertical motion, which is very 

 important also for ballooning ; but the theory being a little more 

 •complex, I shall keep it in reserve for a future communication. 



The rotatory power is of importance in making observations 



* I speak only of the motion in still air- 



in the air, and it is praiseworthy in Major Beaumont and Mr. 

 Bowdler to have directed their attention to that particular point. 

 The abstract principles of aeronautics have been pretty well 

 ascertained, but the practice is a very difficult thing, and can only 

 be tested by a series of experiments. With such an experienced 

 balloonist as Mr. Coxwell, and the resources of an enlightened 

 Government like that of linyland, it seems likely that such experi- 

 ments will be tried more easily than in France. Under the present 

 circumstances, I think it is our duty to assist you so that you 

 may derive benefit from the knowledge we bought so dearly 

 amidst our great national calamities. 



W. DE FONVIELLE 



Fogs, Field-ice, and Icebergs in the Atlantic 



■Three unv.-elcome phenomena have this year, in more than an 

 ordinary degree, vexed the coasts of the United States and the 

 navigation of tlie Atlantic ; I allude to fogs, field-ice, and ice- 

 bergs. The first have so much interfered with the success of 

 the Nantucket fishermen that but few mackerel have been 

 caught by the seine, the schools cannot be followed, and the 

 boats have frequently remained idle for days. No one who has 

 not met with these fogs can form an idea of their density. With 

 a bright sun shining over head, objects cannot be discerned at the 

 distance of 100 ft Collisions have been numerous in all the gre.at 

 American ports and rivers. On one occasion hundreds of tons 

 of cargo remained two days in New York before it could be 

 transported across the Hudson to Jersey city, although the 

 distance was frequently under .-. mile from wharf to wliarf 



At sea these fogs have extended almost without a bre.ak for 

 1,600 miles, the wind being from east, through south, to west. 

 When sounding the steam whistle I noticed, what has probably 

 been noted before, tliat tlie denser the fog the greater were the 

 reverberations, and that llie echo was always heard to windward 

 as plainly as if it were deflected from a cliff in that direction. I 

 presume that this arose from the resistance the waves of sound 

 encountered in travelling against the wind, none being heard to 

 leeward. These fogs are attributed to the great difference which 

 exists in spring and summer between the temperatures of the air 

 and water. Having, however, often remarked that they come 

 when these conditions are not found, I am induced to believe 

 the cause must often be looked for in the atmosphere alone, by 

 the mixture and condens-ation of the diffeient strata of air there. 

 At times these fogs are in streaks, and the alternations of heat 

 and cold, as they sweep by, are very noticeable. Now, if the 

 sole cause were due to a simple difference of temperature between 

 the air and water, I cannot understand why this should be, unless 

 the sei was composed of similar streaks of hot and cold water, 

 wliich here is not the case. 



In the Atlantic, seamen were astonished to find that early in 

 February field-ice and bergs had reached the parallel of Cape 

 Race, and have since been seen as far south as 43" N. lat., 

 drifting to the north-east in the heated waters of the Gulf 

 Stream. Two steamers and an equal number of sailing vessels 

 are known to have been seriously ilamaged by colliding with 

 them ; and the wonder is that so few accidents have taken place 

 when it is borne in mind tliat for hundreds of square miles the 

 steam and sailing tracks between America and this country are 

 dotted with them. A few of the bergs have been supposed to 

 be three miles in length, and on two occasions steamers passed 

 through or around ice-fields 100 miles in length. It is also 

 alleged that another was stopped five hours by field-ice so far 

 south as the forty-third parallel. 



There is a general belief that the vicinity of ice may be readily 

 detected by the fall in the temperature of the water. Unless it 

 be in very large masses, and the ship close to, this test is not of 

 mucli value, owing to the natural law which causes a cold surface 

 fluid to sink until equilibrium is restored. A better test is the 

 cold, damp fesling of the air, but this is only noticeable when to 

 leeward ol^the berg or field, and is piactically of no value, as the 

 wind p.assing over the sea-water at 2S" will cause a similar sen- 

 sation. In some states of the atmosphere the clouds near the 

 horizon assume a peculiar grey tint when the ice-field is of large 

 dimensions. 



Unless the weather be veiy foggy, an iceberg is easily dis- 

 tinguished on the darkest night at a considerable distance by the 

 light reflected from it, and to this cause I attribute the great 

 immunity of ships from accidents. Ordinary islets dropped in 

 the Atlantic would cause an infinity of wrecks, owing to the 

 absence of this useful property. When an iceberg reaches a low 

 latitude it loses much of its beauty ; the brilliant white and pris- 



