uracy.

1

Ity still the theoretic principle of their combination for deducing ellipticity supposes that the density of our globe, though variable 1 arbitrary law from the surface to the centre, continues identically ich layer concentric with the superficial one; an hypothesis which ne degree from the reality of nature, and which on that account o results of absolute certainty. After these considerations, it will r of surprise that the values of the terrestrial ellipticity, deduced from made in the present era by Borda first, and afterwards by Biot, erent points of the French meridian, by Kater in England, by the reycinet, Duperry, Sabine, Foster and others, under very different atitudes, should sensibly vary from one another, and likewise to the final number, deduced from the examination of all of them, when ith that which results from the sum of the principal geodesic labors. at most does the difference amount? From the esperiments made adulum, there results as the value of the earth's polar compression 29, somewhat greater than the fraction and less than 270; the f these two extreme fractions is equal to ; so that the difference Its obtained by help of the pendulum and by the ordinary processcs will be found to be represented by a number still less than the last. then, that the value of the equatorial radius is in metres 6,377,397, d remain in the length of the polar radius an uncertainty of 2.362; must not be forgotten, on the supposition, really more unfavorable rranted, that the doubt respecting the polar compression of the earth sent to us as equally uncertain the two fractions and But n of the number 2.362 to the value of the equatorial or the polar Lower than that of 1 to 1000: thus in the appreciation of a quantity of a thousand equal parts, it would be at last doubted whether we ed one part more or less than was proper! Instead of being surprised stence of such an uncertainty as this, it might well cause astonishment, Airy has remarked in reference to this subject, that man should have t a knowledge so precise in a matter so difficult and obscure; while till room for confidence that further advances are in his power, and encouragement to persist in the pursuit of the truth.

his confidence and encouragement exist is shown by a simple reference rojects of new geodesical operations and experiments, suggested by he most celebrated of cotemporary astronomers. In 1857, for example, posed to the Academy of Sciences of Paris that a new determination e undertaken, by methods and with instruments more delicate than ore known, of the whole extent of the arc of Peru as well as of the ares of parallel measured in Europe; that experiments with the penduld be multiplied in those localities where considerable anomalies have ed in the direction of the vertical, or where their existence is suspected, iew to ascertain their cause or causes; in a word, that no means should d of discovering all the accidents of form and density which distinguish aqueous globe from the theoretical ellipsoid defined by Bessel and the aticians who, with a degree of precision difficult to surpass, have either or followed him in this enterprise. The ideas of Biot, de'iberately red and digested eventually into the colossal project of measuring a new

. VIEW TO THE ADVANCEMENT OF SCIENCE.

he Smithsonian Institution from the works of Francis Arago, late secretary of the French Academy of Sciences, &c.

1. THE INVENTION OF BALLOONS.

reason of his weight, and the weakness of his muscular power, ed to creep on the surface of the earth, and to have been disqualiing the physical properties of the higher regions of our atmosphere, gh the toilsome ascent to mountain summits. But what difficulties er which genius, united with perseverance, will not eventually triom the most remote times the idea of soaring into the air, far above 1 objects, by means of machines which the imagination endowed ties unfortunately of impossible attainment, has never ceased to ocman mind. Who has not heard of the attempts of Dedalus and he projects of Roger Bacon, and of Fathers Lara and Galen? But, it had been granted to no one to realize the dream of so many ages. chel Montgolfier, who was born in 1740, at Annonay, in the departe Ardéche, and who died, a member of the Academy of Sciences, in calculated that through the rarefaction, by means of heat, of the air in a paper balloon of a certain extent, an ascensional force might be fficent for elevating men, animals, and any desired instruments. So idence had he in his theory that he did not hesitate to undertake, June public and formal exhibition before the deputies of the provincial Vivarais, assembled at Annonay. Montgolfier has himself described, in ing terms, this first experiment, which forms an epoch in the history of important discoveries: "The aerostatic machine was constructed of ned with paper, and covered by a network of twine attached to the It was nearly of a spherical form, and of a circumference of 110 feet, ) a frame of wood, 16 feet square, steadied it on its base. Its capacity t 22,000 cubic feet. It, therefore, displaced, supposing the mean weight equal to of the weight of water, a mass of air equivalent to 1,980 (969 kilograms.)

800

weight of the gas (heated air) was nearly half that of the air, for it equalpounds, and the machine, with the frame, weighed 500 pounds. For the of equilibrium there remained, therefore, 490 pounds, as was found cone to the experiment. The different pieces of the balloon were fastened simply by means of button-holes and buttons. Two men sufficed to fill it with gas, but it required eight to retain it. When released, at a igual, it mounted with an accelerated velocity, though less rapid towards of the ascension, to the height of 1,000 toises, (upwards of 6,000 feet.) 1, scarcely perceptible at the surface of the earth, bore it to the distance 0 toises from the place of departure. It remained ten minutes in the air. ss of gas by the button-holes, needle punctures, and other imperfections, ted any longer suspension. The wind was, at the time, southerly

filled with hyd iron filings. This balloon ascended from 83, at five o'clock in the afternoon, in It remained eralded by salvos of cannon.

ir, and fell at Gonesse, near Econen, five s demonstrated the possibility of making y impermeable by hydrogen, the lightest advantages over the heated air. Yet this ascensional force with, balloons of limited pted, and sundry experiments were suctats inflated with air heated by a fire of was with such a balloon, having an oval eter of 15, and a capacity of 2,056 cubic d'Arlandes made the first aerial voyage xe in balloons wholly detached and unconu de la Muette, November 21, 1783, they t an elevation of about 1,000 meters, having, or 20 or 25 minutes. The 1st of December led from the Tuilleries in a spherical balloon, elastic, and having a diameter of only 8.50 ydrogen. After a passage of about nine ch at Nesles, where Robert left the car, while elevation of about 2,000 meters, alighting having experienced a cold of -5°, or + licated on the ground +7°, 443 Fah. From the practical possibility of balloon voyageshich have become, at a later period, a pastime ot speak here of the attempts which have been ostats in military expeditions, nor of the nucourse through the air, nor of the unfortunate f fire with the employment of hydrogen, for ith his life, nor of the substitution of illumiitution which renders these enterprises less scensional force of apparatus of a determinate f to aeronautic voyages, performed with a view

emy of Sciences if we would find an account ence was benefited through the employment as was used as an agent. The expeditions of in 1804, were preceded by the ascensions of , which yielded some interesting results; but ry were the remarkable voyages of MM. Barral hortly afterwards by those of Mr. John Welsh.

E MADE IN AEROSTATIC ASCENSIONS.

ake aerial voyages form, in general, no idea of esolved, nor of the difficulties to be surmounted

certain elements of discussion. The instru, as well the temperature as the hydrometric

ke-have directed inquir

peak, and of furnishing aeronau
But the problem has been by no
ery far from having received a c
which have been derived from the
from those of Barral and Bixio,
those whose zeal shall hereafter
erprises, in the view, particular
regions, and traversing the atmo
principal questions on which the
are the following:

1. The law of the decrease of
2. Influence of the solar radi
deduced from observations ma
with very different absorbing s

3. Determination of the hyg
pheric strata, and comparison o
dew-point at very low tempera

4. Analysis of the air from
5. Determination of the qua
regions of the atmosphere.

6. Examination of the polar
7. Observation of different o
8. Observation of the diaph
different strata of air.

9. Observation of the declin of the intensity of magnetism. 10. Study of the electric sta 11. Experiments on the tr strata of air in a serene state o 12. Physiological observati the air, very low temperatures

The instruments at the disp which, by my own advice, and carried by MM. Barral and B have continued to use had the

1. Two siphon barometers, observe only the upper menisc by a table constructed after di these barometers should be p degrees, so as to present as known that the aeronaut ma lower than that of the cong will not answer, and an in on the pressure exerted by t very low temperatures under

2. A vertical thermometer of which is placed in the ax

ssociation for the advancement of

ected inquiry to the means of supplying the defect of which I furnishing aeronauts with adequate instruments of investigation. em has been by no means considered under all its aspects, and is having received a complete solution; at all events, the suggestions een derived from the voyages of Biot and Gay Lussac, and especially 7 Barral and Bixio, should be taken into serious consideration by zeal shall hereafter prompt them to encounter the perils of such enthe view, particularly, of reaching the most highly rarefied aerial traversing the atmosphere under its most variable conditions. The estions on which the attention of such explorers should be fixed wing:

w of the decrease of atmospheric temperature with the elevation. nce of the solar radiation in the different regions of the atmosphere, om observations made upon thermometers whose bulbs are coated lifferent absorbing substances.

mination of the hygrometric state of the air in the several atmosa, and comparison of the indications of the psychrometer with the at very low temperatures.

ysis of the air from different heights.

mination of the quantity of carbonic acid contained in the higher the atmosphere.

mination of the polarization of light by clouds.

ervation of different optical phenomena produced by the clouds.

ervation of the diaphaneity, and of the intensity of the blue color of trata of air.

ervation of the declination and inclination of the magnetic needle, and ensity of magnetism.

udy of the electric state of different atmospheric strata.

xperiments on the transmission and reflection of sound in different air in a serene state of the sky, and in a sky containing clouds. hysiological observations on the effects produced by the rarefaction of ery low temperatures, extreme dryness, &c.

nstruments at the disposal of the voyagers should be the same as those y my own advice, and that of my illustrious colleague, M. Regnault, were by MM. Barral and Bixio in their expeditions, and which they would ntinued to use had they been able to make other ascensions, to wit: wo siphon barometers, graduated on glass, of which the aeronaut need only the upper meniscus, the position of the lower meniscus being given le constructed after direct observations made in the laboratory. Each of arometers should be provided with a thermometer divided in centigrade 5, so as to present a scale extending from +35° to -39°. It is now that the aeronaut may encounter strata of air having a temperature than that of the congeation of mercury; hence the ordinary barometer ot answer, and an instrument should therefore be furnished, founded pressure exerted by the atmosphere on an elastic spring, and tested at ow temperatures under feeble pressures obtained by the pneumatic machine. A vertical thermometer, of arbitrary graduation, the cylindrical reservoir ich is placed in the axis of several concentric envelopes of bright tin, open