AERONAUTIC VOYAGES. 333 



state of the air, the plienomena of the magnetic needle, the proportions of po- 

 larized light contained in the light of the atmosphere, the diaphaneity, the 

 color more or less blue of the different strata of air, &c., do not exist at all, or 

 else require important modifications before being applied to the research of the 

 laws by which the phenomena vary with the height, which is itself not deter- 

 mined with entire precision by barometrical observations. For half a century 

 many learned bodies — the French Academy of Sciences, that of St. Pctcrsburgh, 

 the British Association for the advancemimt of science, the Academy of Dijon, 

 &c. — have directed inquiry to the means of supplying the defect of which I 

 speak, and of furnishing aeronauts with adequate instruments of investigation. 

 But the problem has been by no means considered under all its aspects, and is 

 very far from having received a complete solution; at all events, the suggestions 

 which have been derived from the voyages of Biot and Gay Lussac, and especially 

 from those of Barral and Bixio, should be taken into serious consideration by 

 those whose zeal shall hereafter prompt them to encounter the perils of such en- 

 terprises, in the view, particularly, of reaching the most highly rarefied aerial 

 regions, atid traversing the atmosphere under its most variable conditions. The 

 principal questions on which the attention of such explorers should be fixed 

 are the following : 



1. The law of the decrease of atmospheric temperature with the elevation. 



2. Influence of the solar radiation in the different regions of the atmosphere, 

 deduced from observations made npon thermometers whose bulbs are coated 

 with very different absorbing substances. 



3. Determination of the hygrometric state of the air in the several atmos- 

 pheric strata, and comparison of the indications of the psychrometer with the 

 dew-point at very low temperatures. 



4. Analysis of the air from different heights. 



5. Determination of the quantity of carbonic acid contained in the higher 

 regions of the atmosphere. 



6. Examination of the polarization of light by cloitds. 



7. Observation of different optical phenomena produced by the clouds. 



8. Observation of the diaphaneity, and of the intensity of the blue color of 

 different strata of air. 



9. Observation of the declination and inclination of the magnetic needle, and 

 of the intensity of magnetism. 



10. Study of the electric state of different atmospheric strata. 



11. Experiments on the transmission and reflection of sound in different 

 strata of air in a serene state of the sky, and in a sky containing clouds. 



12. Physiological observations on the effects produced by the rarefaction of 

 the air, "very low temperatures, extreme dryness, &c. 



The instruments at the disposal of the voyagers should be the sai?ie as those 

 •which, by my ov.-n advice, and that of my illustrious colleague, M. Regnault, were 

 carried by MM. Barral and Bixio in their expeditions, and which they would 

 have continued to use had they been able to make other ascensions, to wit : 



1. Two siphon barometers, graduated on glass, of which the aeronaut need 

 observe only the upper meniscus, the position of the lower meniscus being given 

 by a table constructed after direct observations made in the laboratory. Each of 

 these barometers shov;ld be provided Avith a thermometer divided in centigrade 

 degrees, so as to present a scale extending from +35° to — 39^. It is now 

 known that the aeronaut may encounter strata of air having a temperature 

 lower than that of the congeation of mercury ; hence the ordinary barometer 

 will not answer, and an instrument should therefore be furnished, founded 

 on the pressure exerted by the atmosphere on an elastic spring, and tested at 

 very low temperatures under feeble pressures obtained by the pneumatic machine. 



2. A vertical thermometer, of arbitrary graduation, the cylindrical reservoir 

 of which is placed in the axis of several concentric envelopes of bright tin, open 



