334 AERONAUTIC VOYAGES. 



at tLcir bases to admit the circulation of air. This arrangement has been devised 

 in order to obtain, at least approximately, the temperature which a thermometer 

 would indicate in the shade. 



3. Three thermometers, having arbitrary scales, attached to a metallic plate 5 

 centimeters apart. The reservoir of the first of these thermometers should have 

 a vitreous surface ; the surfiice of the second should be coated with lamp- 

 black ; and the reservoir of the third should be covered with a cylinder of 

 polished silver, which must also envelop a portion of the stem. The reservoirs 

 should be narrow cylinders, much elongated. Immediately below the reservoirs 

 the metallic plate should support another plate brightly coated with silver. The 

 plate bearing these thermometers should be arranged horizontally on one of the 

 Bides of the car, with a view to its remaining constantly exposed to the solar 

 radiation. 



4. A psychrometer formed by two thermeters of an arbitrary scale. 



5. One of Regnault's condensing hygrometers. 



6. Tubes of caustic potash, alid also of pumice, wet with sulphuric acid, for 

 the determination of the carbonic acid of the air. The air should be drawn in 

 by means of a pump of the capacity of one litre (1.760 pint) accurately gauged. 



7. Tavo balloons of one litre capacity, furnished with stop-cocks of steel, for 

 collecting the air of the higher regions. These balloons, enclosed in tin bexes, 

 ■should be scrupulously exhausted of air before the ascent. 



8. A minimum thermometer of M. AValferdin, which should be enclosed in a 

 tin cylinder, pierced with holes. It is best that this instrument should be placed 

 under seal, as was done by MM. Barral and Bixio, since the control of the per- 

 sonal observations by means of a mute instrument imparts considerable value 

 when they come to be verified, and affords a triumphant reply to objections 

 which, through a natural tendency of the human mind, always ojjpose themselves 

 'to results which cannot be immediately verified by new experiments made under 

 the same conditions. In the event, moreover, of the ascension of the balloon to 

 heights where the temperature falls below — 40°, the point of congelation for 

 mercury, it will be necessary to have thermometers of alcohol or sulphuret of 

 carbon, graduated below that point of the thermometric scale, so that the obser- 

 vations may not be interrupted by a circumstance Avhich has ceased to be con- 

 sidered as of impossible occurrence. 



9. It is from the considerations just stated, that I would recommend also the 

 use of the apparatus devised by M. Regnault, and intended to indicate the 

 minimum of barometric pressure, and consequently the maximum of elevation to 

 which the balloon has attained. This apparatus should be enclosed in a tin case 

 pierced with numerous small openings. The lid of this case should be secured 

 with a seal like the minimum thermometer. 



10. Polariscopic telescopes, such as I have described, Astronomie populaire, 

 ii, p. 101. 



11. Instruments for showing the declination, inclination, and intensity of 

 magnetism, suspended in such a manner as not to be aflFected by the movements 

 of rotation of the balloon in its ascent, as has been observed by MM. Biot, Gay 

 Lussac, Barral, and Bixio. 



12. Electrometers so constructed as to be capable of indicating at once the 

 kind and the intensity of the electricity of difiereut atmospheric strata. 



It is scarcely probable that in an ascension, observers will be able to embrace 

 at one time so many subjects of study, or use successively and opportunely so 

 many instruments. The aeronaut should, on each occasion, limit himself to a 

 small number of important inquiries. It is only in a series of aeronautic expe- 

 ditions that a collection of records can be made corresponding to the great number 

 of questions which the constitution of the ten-estrial atmosphere presents for 

 solution. 



