L. MECHANICS AND ENGINEERING. 501 



duration and the second of great altitude. The latter 

 voyage and its disastrous consequences have already been 

 noticed, and, in fact, so preoccupied the attention of the 

 world that we are apt to lose sight of the importance of its 

 predecessor, whose duration of twenty-two hours and forty 

 minutes places it among the most remarkable voyages on 

 record. Throughout the whole of this voyage the members 

 of the expedition carried on, without interruption, a series 

 of observations, and executed numerous experiments. De- 

 parting from Villette on the 23d of March, at 6.20 P.M., the 

 balloon carried five scientists, 1100 kilogrammes of ballast, 

 and the instruments for observation. The determination of 

 the altitude and the direction of the route was specially al- 

 lotted to Sivel, who, by means of the plumb-line and a cord 

 of 800 meters' length, which extended to the earth and kept 

 the balloon always in a fixed direction, was able to observe 

 their course satisfactorily by the compass. As is well known 

 to aeronauts, the course of the balloon was a continued se- 

 ries of slight ascents and descents, the highest elevation 

 reached being about 1800 meters, and the average altitude 

 being about 1000 meters, except during the last six hours, 

 when the altitude averaged about 500 meters. The course 

 described by the balloon was very nearly toward the south- 

 west, the entire path being some 573 kilometers. By means 

 of an apparatus invented by Penaud, they were able, from 

 their height in the air, to determine barometrically and con- 

 tinuously the velocity of their horizontal movements. This 

 instrument is formed of a graded arc, around the centre of 

 which an alidade moves. The observer sights, under an an- 

 gle of 30, some object visible on the earth in the direction 

 of the march of the balloon. When this object has passed 

 under the line of the alidade, the latter is moved to 60, and 

 the same object is observed until it is exactly past the ali- 

 dade the second time. Another observer has meanwhile 

 noticed the time elapsed between the two readings. By 

 the aid of the two angles thus observed, and knowing, in 

 addition, the altitude by the barometric readings, it is pos- 

 sible to compute by a simple trigonometric formula the ve- 

 locity of the balloon. These observations, executed many 

 times, gave very precise figures, which could be verified 

 subsequently. In the morning, as soon as the sun had risen 



