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Art. 24. An Account of the Measurement of a Base on Hounslow
Heath, By Major General William Roy, F. R. S. This long and laborious business appears to have been under. taken in consequence of a paper transmitted in October 1783, by Comte d'Adhemar the French Ambassador, to Mr. Fox, then Secretary of State, being a Memoir of M. Callini de Thury, in which he sets forth the great advantage that would accrue to astronomy by measuring a series of triangles from London to Dover, there to be connected with those already executed in France, by which combined operations the relative situations of the two most famous Observatories in Europe, Greenwich and Paris, would be more accurately ascertained than they are at prefent; for, according to him, the uncertainty hitherio, all cir. cumstances considered, has been amazingly great indeed.
In order to this, it was necessary to have an extensive base. line measured with all possible accuracy, as a beginning to such series of triangles, and we have here a very full account of the performance of this on Hounslow-Heath, from a place called King's Arbour, at the north-west extremity, beween Cranford Bridge and Longford, to Hampton Poor-House, near Bushy Park, being upwards of five miles. This was done, first with a chain of 100 feet long, but of a new and very proper construce tion for the purpose; then with deal rods, and 3dly with glass rods, Having first measured 274 chains, and marked it with a picket, and afterwards done, the same with deal rods, they found that the intersection on the tripod terminating the 27400 feet, only overlot the picket answering to the 274th chain by two inches and nine tenths. Yet, for reasons that they have given at large, but for which we must refer to the Paper itself, they preferred the measurement with glass rods. Were our opinion of any weight, we should prefer the use of such a chain, with plenty of help, as they had, to all other methods, both for fimplicity and accuracy.
Art. XI. Airopaidia : Containing the Narrative of a Balloon Ex
cursion from Chester, the 8th of September 1785, taken from Minutes made during the Voyage : Hints on the Improvement of Balloons, and Mode of Inflation by Steam : Means to prevent their Descent over Water : Occasional Enquiries into the State of the Atmosphere, favouring their Direction : With various philosophical Observations and Conjectures. To which is subjoined, Mensuration of Heights by the Barometer, made. plain, with extensive Tables. The whole serving as an Introduction to Aereal Naviga. tion. By Thomas Baldwin, Eig. A. M. 8vo. 75. 6d. Boards, Chester, printed : Sold by Lowndes, London. 1786. THIS gentleman appears to have made a more successful voyage than any of his predecessors, and his work is by
far the moft valuable of any that we have yet met with on the fubject : it presents to us a great number of interesting facts and observations, and fully answers its ample title-page.
The Author first gives a minute detail of the prepararions for and circumftances attending, his aereal excursion; he describes the path of the balloon over a tract of about 30 miles, and the appearances of terrestrial objects as seen from it at different heights, and in different positions; he dwells with rapture on these enchanting prospects, and endeavours to convey fome idea of them to his readers, by a print of the balloon in the air, and two coloured views from the balloon, accompanied with an explanatory print or map of the same places. Many directions and cautions to the aeronaut are interspersed, incidentally result. ing from the observations, in regard to the things he ought to be provided with, his aititude and employment in the air, the management of the balloon, the signs of rising, finking, cure rents, &c,
This narrative of the voyage, divided into thirty-fix chapters, makes nearly one half of the work; the remainder, in forty-four chapters more, contains observations on different subjects relative to it, both in the practical and philosophic line. As we cannot Venture, in our confined fituation, to follow the fights or manoeuvres of the aeronaut, we thall endeavour to entertain our Readers with some of the remarks of the philosopher.
At a certain height in the atmosphere (between soo and 1000 yards) a kind of chilliness was perceived, not ascertainable by the thermometer, . The sensation was suddenly impressed four times, in ascending and descending to the same height. The lower ftratum of clouds is on the fame level (1000 yards in fine wea. ther and soo in changeable), and it appears to have been in pafling through them that this sensation was felt, though no cloud was visible to the eye at the time of his tranfit.
At the same height, likewise, the appearances of the earth and clouds were very remarkable. In ascending, the circular-pro. fpects of the subjacent earth instantly contracted; and in defcending, they instantly enlarged themselves to the eye. The clouds appeared on the same horizontal plain with the eye, but at the diftance of a mile: they were plainly composed of three or more · riers, failing at great intervals one above another; all which regularly vanished on approaching their respective levels, as if inftantly thrown into the circumference of a circle whose radius was a mile. During the ascent, on passing their supposed level, the clouds instantly appeared far below him, and during the descent as far above. A cloud which was thought by the aeronaut to be four miles below him, touching the city of Chester, was thought by fpcctators'there to be a mile above them enveloping the balloon; so that a cloud seems to be, not an accumulation of
carpet, madow, as and not beinesi
vapour in one place, but the effect of looking through a certain extent of vapor, different according to its density.
The circular opening in the clouds, through which the earth's surface was presented to the eye, discovered a smooth level plain; a sort of shining carpet, enriched with an endless variety of figures depicted without shadow, as on a map; what was really shadow forming a separate colour, and not being considered at the time as fhadow. All was colouring; no outline; yet eacha appearance curiously defined by a striking contrast of fimple colours, which served to diftinguith the respective boundaries with most exact precision, and inconceivable elegance. Red waters, yellow roads, inclosures yellow and light green, woods and hedges dark green, were the only objects clearly distinguishable, and their colouring was extremely vivid : the sun's rays, reflected from the sea and other waters (which appeared all red) dazzled the fight.
It is known, that the apparent magnitudes of objects decrease as the diffance increases; and that the diminution of magnitude is in the proportion of the square of the distance. This, however, obtains only where the medium they are seen through continues nearly the same ; for an object, seen along the ground, will appear less as it rises above the ground, and least of all in the zenith, though its diftance from the eye continues the same, and hence the Author deduces the large and oval appearance of the sun and moon in the horizon. It follows, that the height of a balloon in the air is not really so great as it appears to be to a spectator on the ground, being seen from a part of the atmosphere less impregnated with vapours. It should follow likewise, conversely, that to a spectator in the balloon, objects on the earth should appear larger than when seen along the ground at an equal distance; but the fact was directly the contrary, for known objects seen from the balloon at the height of a mile and a half, by one who had been in the habit of viewing and estimating distances and magnitudes on the ground, continued, with unvaried uniformity, to suggest the distance of at least seven miles: objects whose magnitudes at the distance of a mile and a half on the ground were familiar to the eye, appeared from the balloon, at the same distance, full five times less.
The greatest height above the clouds conveyed no idea of danger, or of depth below the clouds, the earth's surface being · presented to the eye as on a level with the clouds themselves, or at least coming up to their under fides, and appearing so much a part of them, that the aeronaut seems able to descend from his çar upon the clouds, and walk over them, as on a sheet of transa parent ice across a river, whose depth is equal to the small thickness of the clouds. The shadow of the balloon was distinctly seen upon them; small when he was at the greatest height, about the size and shape of an egg, but growing larger as he de.
of the upper rete in currenhich was at a lea
fcended, and sometimes surrounded, at a little distance, with a brilliant iris.
The temperature of the upper regions did not feel colder, but racher warmer, than below, except in currents of air coming from coider fituations. The thermometer, which was at 650 on his quitting the earth, funk to 56, in paffing through a fea breeze, and on getting above that current, it role again to 6o. The fun Mone brighter and fiercer when the balloon was at its greateft height; the heat piercing through his clothes (which were of
a dark colour) while he food with his face from the light. . Sounds, immediately under the balloon, seemed as if they ori
ginated near the ear, and louder than they would have been heard at the distance of some yards only on a level ; augmenting, rather than decreasing, till the balloon had nearly reached the height of the lower ftratum of clouds; but afterwards dying away much sooner than expected. A like observation was made in defcending, sounds from below rushing suddenly on the ear about the fame height. We find one observation of found orja ginating in the higher regions, feemingly when he was nearly at his greatest height; on a view of the delightful scenes, he tried his voice, and shouted for joy : his voice was unknown to him. felf, Thrill and feeble; there was no echo.'' • The well-watered and maritime part of the country, over which this gentleman passed in his aereal vehicle, gave him opportunities of observing a remarkable and regular tendency of the balloon towards water,--a tendency to defcend when over the water, and to re-ascend upon receding from it, without any apparent cause that could influence its levity. Our readers will doubiless recollect the perilous situations of former aeronauts in the vicinity of water, though it was not suspected that the water as such had any influence in producing that vicinity or depression of the balloon. Mr. Baldwin endeavours to prove, that there is a depression of the atmosphere, or current of air downwards, over all waters, fufficient to act sensibly upon a balloon just balanced in the air, though not otherwise perceivable ; and that on large bodies of water there is a mediocèanal current, or current downwards in the middle, producing a refilition of current upwards at. the fides, or the reverse according to circumstances.
On this interesting subject the Author enters into large dis. cullions. He adduces many respectable authorities, and observations of his own, in proof of descending streams; and of violent storms and hurricanes in general being really descents, not horizontal propulsions, of air. Thus, the first effect of wind on trees is an oblique depression, succeeded by a recovery, then a momentary pause, and return of the depressing torrent. In inland countries, where lakes are surrounded by mountains, as in bhole of North Wales, of Cumberland and Westmoreland, and
the lake of Geneva, the air rushes forcibly upon the surface of the water, in descending torrents, as he has himself frequently seen. He appeals to all mariners who have weathered storms off Cape Hatteras (where the wind is probably perpetual), or who have made an East India voyage, whether, if the wind blew in an horizontal direction only, it could produce such an inegua. lity of surface; or whether, when the sea runs mountains high, the tremendous surges must not rise from the violent action of winds repeated at intervals, sometimes descending perpendicuJarly, but oftener in torrents of oblique depression and instant refisition. As the first and fighteft changes in the motion of the air are observable on water, the descent of air, even in the finest weather, is familiar to mariners, under the appellation of light airs, playing in eddies upon the surface.
To account for this depression, the Author examines the phenomena of evaporation; and from his observations on that subject he concludes, that so much warm and light air (warmed and rarefied by the sun, and united with watery vapour), as is raised up from the surface of any water, so much heavy and cool air is instantaneously, constantly, and forcibly depressed upon it, to supply the place of the other, and continue the evaporation. Hence the coolness of the air over waters and watery meadows, from the descent of the cool air of the middle regions of the acmosphere. Thus in large towns, warmer in winter than ihe country, there is, in that season, a constant breeze from all fides towards the centre of the town, as may be perceived from the motion of the smoke, and by persons travelling qut of the town, who in all dire&tions meet the breeze; whereas in summer the reverse is the case, more particularly in hot climates; for the country being then warmest, a depresion takes place upon the town, and scatters the smoke on all sides.
The Author considers the effe&t of different situations, relatively to the sun, in producing, and varying the direction of these currents; and points out, from his theory, the times of the day, and the seasons of the year, in which seas or large pieces of water may be passed over with the greatest probability of escaping their dangerous influence; as also the circumstances in which the superior air may be expected to be warmer or colder than that below. Though we cannot say that his reasoning is close, or his facts very silfully arranged, yet, taken all together, they appear to us to have great weight, and we recommend them to the serious attention both of the acronaut and the meteorologist.
For the security of a balloon that may be caught in a depreffing current over water, he proposes to have a ballast thrown out that shall float on the water, and which may afterwards be drawn up again. Means are described for sustaining balloons