102 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



back, serves to guide the machine in the desired direction, and to 

 change this direction at will. 



An experiment was made on the 2nd of February, 1872. The 

 results appeared satisfactory, inasmuch as the aerostat, in spite of a 

 tolerably strong wind, received a velocity from the screw equal to 

 about 10 kilometres and a quarter (about f> miles 440 yards) per 

 hour. With this velocity, the balloon was able to deviate, when the 

 screw was put into motion, from 10 to 12 from the course followed 

 when the screw was stopped, that is to say, when the balloon floated 

 along under the influence of the wind alone. These results, although 

 not so brilliant as those which have been announced by many in- 

 ventors of the direction of aerostats, constitute a real and steady 

 progress which cannot but serve as a starting-point to subsequent 

 improvement. 



This is probably all which we can reasonably hope for in the 

 present state of physical and mechanical research. The substitution 

 of a powerful movement such as the steam-engine, to the muscular 

 force of man, is the principal desideratum of the problem of aerial 

 navigation with hydrogen balloons. The whole question would be to 

 protect it from the inflammability of the gas. 



A word now on the application of aerostation to the study of 

 meteorology. Captive balloons would be able to furnish to this 

 science statements of the highest importance. By placing at different 

 heights a certain number of these machines furnished with registering 

 instruments, data would be obtained which could only be had for a 

 very short ihte'rval of time by aeronautic voyages. 



Gay-Liissac and Biot, during an ascent they made together on the 

 24th of August, 1804, reached a height of 4,000 metres, and procured 

 a series of experiments on the oscillations of the magnetic needle, in 

 order to determine the variations of the magnetic intensity with 

 altitude. The first of these savants made an ascent alone, which 

 carried him to about 7 kilometres (23,000 feet) in vertical height. 

 He was able to recognise that the composition of the atmosphere at 

 this altitude was chemically the same as on the surface of the earth. 



The illustrious physicist, who at the moment of starting read a 

 temperature of + 27 -75 centigrade, found at the greatest elevation 

 a temperature of 9*5 ; more than 37 difference. 



Among contemporary scientific ascents we must mention those of 



