Art. 24. An Account of the Measurement of a Bafe on Hounflow Heath. By Major General William Roy, F. R. S.

This long and laborious business appears to have been undertaken in confequence of a paper tranfmitted in October 1783, by Comte d'Adhemar the French Ambaffador, to Mr. Fox, then Secretary of State, being a Memoir of M. Caffini de Thury, in which he fets forth the great advantage that would accrue to aftronomy by measuring a series of triangles from London to Dover, there to be connected with thofe already executed in France, by which combined operations the relative fituations of the two moft famous Obfervatories in Europe, Greenwich and Paris, would be more accurately afcertained than they are at prefent; for, according to him, the uncertainty hitherto, all cir cumftances confidered, has been amazingly great indeed.

In order to this, it was neceffary to have an extensive baseline measured with all poffible accuracy, as a beginning to fuch 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-weft extremity, beween Cranford Bridge and Longford, to Hampton Poor-Houfe, near Bufhy Park, being upwards of five miles. This was done, firft with a chain of 100 feet long, but of a new and very proper conftruc 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 fame with deal rods, they found that the interfection on the tripod terminating the 27400 feet, only overfhot 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 itfelf, they preferred the measurement with glafs rods. Were our opinion of any weight, we should prefer the ufe of fuch 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 Excurfion from Chefter, 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: Occafional Enquiries into the State of the Atmosphere, favouring their Direction: With various philofophical Obfervations and Conjectures. To which is fubjoined, Menfuration of Heights by the Barometer, made plain, with extenfive Tables. The whole ferving as an Introduction to Aereal Navigation. By Thomas Baldwin, Efq. A. M. 8vo. 7s. 6d. Boards. Chester, printed: Sold by Lowndes, London. 1786.

HIS gentleman appears to have made a more fuccessful voyage than any of his predeceffors, and his work is by

far

far the most valuable of any that we have yet met with on the fubject: it presents to us a great number of interefting facts and obfervations, and fully answers its ample title-page.

The Author firft gives a minute detail of the prepararions for, and circumstances attending, his aereal excurfion; he defcribes the path of the balloon over a tract of about 30 miles, and the appearances of terrestrial objects as feen from it at different heights, and in different pofitions; he dwells with rapture on these enchanting profpects, 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 fame places. Many directions and cautions to the aeronaut are interspersed, incidentally resulting from the obfervations, in regard to the things he ought to be provided with, his attitude and employment in the air, the management of the balloon, the figns of rifing, finking, currents, &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 obfervations on different fubjects relative to it, both in the practical and philofophic line. As we cannot venture, in our confined fituation, to follow the flights or manoeuvres of the aeronaut, we shall endeavour to entertain our Readers with fome of the remarks of the philofopher.

At a certain height in the atmosphere (between 500 and 1000 -yards) a kind of chilliness was perceived, not afcertainable by the thermometer, The fenfation was fuddenly impressed four times, in afcending and defcending to the fame height. The lower ftratum of clouds is on the fame level (1000 yards in fine weather and 500 in changeable), and it appears to have been in paffing through them that this fenfation was felt, though no cloud was vifible to the eye at the time of his tranfit.

At the fame height, likewife, the appearances of the earth and clouds were very remarkable. In afcending, the circular profpects of the fubjacent earth instantly contracted; and in defcending, they inftantly enlarged themfelves to the eye. The clouds appeared on the fame horizontal plain with the eye, but at the diftance of a mile they were plainly composed of three or more tiers, failing at great intervals one above another; all which regularly vanished on approaching their respective levels, as if intantly thrown into the circumference of a circle whose radius was a mile. During the afcent, on paffing their supposed level, the clouds inftantly appeared far below him, and during the defcent as far above. A cloud which was thought by the aeronaut to be four miles below him, touching the city of Chefter, was thought by fpectators there to be a mile above them enveloping the balloon; fo that a cloud seems to be, not an accumulation of

vapour in one place, but the effect of looking through a certain extent of vapor, different according to its denfity.

The circular opening in the clouds, through which the earth's furface was prefented to the eye, difcovered a smooth level plain; a fort of fhining carpet, enriched with an endless variety of figures depicted without fhadow, as on a map; what was really fhadow forming a feparate colour, and not being confidered at the time as fhadow. All was colouring; no outline; yet each appearance curiously defined by a ftriking contraft of fimple colours, which ferved to diftinguish the respective boundaries with moft exact precifion, and inconceivable elegance. Red waters, yellow roads, inclofures yellow and light green, woods and hedges dark green, were the only objects clearly diftinguishable, and their colouring was extremely vivid : the fun's rays, reflected from the fea and other waters (which appeared all red) dazzled the fight.

It is known, that the apparent magnitudes of objects decrease as the diftance increases; and that the diminution of magnitude is in the proportion of the fquare of the distance. This, however, obtains only where the medium they are feen through continues nearly the fame; for an object, feen along the ground, will appear lefs as it rifes above the ground, and leaft of all in the zenith, though its diftance from the eye continues the fame, and hence the Author deduces the large and oval appearance of the fun and moon in the horizon. It follows, that the height of a balloon in the air is not really fo great as it appears to be to a fpectator on the ground, being feen from a part of the atmofphere lefs impregnated with vapours. It fhould follow likewife, conversely, that to a spectator in the balloon, objects on the earth fhould appear larger than when feen along the ground at an equal diftance; but the fact was directly the contrary, for known objects feen from the balloon at the height of a mile and a half, by one who had been in the habit of viewing and efti mating diftances and magnitudes on the ground, continued, with unvaried uniformity, to fuggeft the diftance of at leaft feven miles: objects whofe magnitudes at the distance of a mile and a half on the ground were familiar to the eye, appeared from the balloon, at the fame diftance, full five times less.

The greatest height above the clouds conveyed no idea of danger, or of depth below the clouds, the earth's furface being presented to the eye as on a level with the clouds themselves, or at leaft coming up to their under fides, and appearing fo much a part of them, that the aeronaut feems able to defcend from his car upon the clouds, and walk over them, as on a sheet of tranfparent ice across a river, whofe depth is equal to the fmall thicknefs of the clouds. The fhadow of the balloon was diftinctly feen upon them; fmall when he was at the greatest height, about the fize and fhape of an egg, but growing larger as he de

fcended

fcended, and fometimes furrounded, at a little diftance, with a brilliant iris.

The temperature of the upper regions did not feel colder, but rather warmer, than below, except in currents of air coming from colder fituations. The thermometer, which was at 65° on his quitting the earth, funk to 56, in paffing through a fea breeze, and on getting above that current, it rose again to 60. The fun fhone 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 flood with his face from the light.

Sounds, immediately under the balloon, feemed as if they originated near the ear, and louder than they would have been heard at the diftance of fome yards only on a level; augmenting, rather than decreafing, till the balloon had nearly reached the height of the lower ftratum of clouds; but afterwards dying away much fooner than expected. A like obfervation was made in defcending, founds from below rushing suddenly on the ear about the fame height. We find one obfervation of found originating in the higher regions, seemingly when he was nearly at his greatest height: on a view of the delightful fcenes, he tried his voice, and fhouted 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 paffed in his aereal vehicle, gave him opportunities of obferving a remarkable and regular tendency of the balloon towards water, a tendency to defcend when over the water, and to re-afcend upon receding from it, without any apparent caufe that could influence its levity. Our readers will doubtless recollect the perilous fituations of former aeronauts in the vicinity of water, though it was not fufpected that the water as fuch had any influence in producing that vicinity or depreffion of the balloon. Mr. Baldwin endeavours to prove, that there is a depreffion of the atmosphere, or current of air downwards, over all waters, fufficient to act fenfibly upon a balloon juft balanced in the air, though not otherwife perceivable; and that on large bodies of water there is a mediocèanal current, or current downwards in the middle, producing a refilition or current upwards at. the fides, or the reverse according to circumftances.

On this interefting fubject the Author enters into large difcuffions. He adduces many refpectable authorities, and obfervations of his own, in proof of defcending ftreams; and of violent ftorms and hurricanes in general being really defcents, not horizontal propulfions, of air. Thus, the firft effect of wind on trees is an oblique depreffion, fucceeded by a recovery, then a momentary paufe, and return of the depreffing torrent. In inland countries, where lakes are furrounded by mountains, as in thole of North Wales, of Cumberland and Weftmoreland, and

the lake of Geneva, the air rufhes forcibly upon the furface of the water, in defcending torrents, as he has himself frequently feen. He appeals to all mariners who have weathered ftorms off Cape Hatteras (where the wind is probably perpetual), or who have made an Eaft India voyage, whether, if the wind blew in an horizontal direction only, it could produce fuch an inequality of furface; or whether, when the fea runs mountains high, the tremendous furges muft not rife from the violent action of winds repeated at intervals, fometimes defcending perpendicularly, but oftener in torrents of oblique depreffion and inftant refilition. As the firft and flighteft changes in the motion of the air are obfervable 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 depreffion, the Author examines the phenomena of evaporation; and from his obfervations on that subject he concludes, that so much warm and light air (warmed and rarefied by the fun, and united with watery vapour), as is raised up from the furface of any water, fo much heavy and cool air is inftantaneously, conftantly, and forcibly depreffed upon it, to fupply the place of the other, and continue the evaporation. Hence the coolnefs of the air over waters and watery meadows, from the defcent of the cool air of the middle regions of the atmofphere. Thus in large towns, warmer in winter than the country, there is, in that feafon, a conftant breeze from all fides towards the centre of the town, as may be perceived from the motion of the fmoke, and by perfons travelling qut of the town, who in all directions meet the breeze; whereas in fummer the reverse is the cafe, more particularly in hot climates; for the country being then warmeft, a depreffion takes place upon the town, and scatters the fmoke on all fides.

The Author confiders the effect of different fituations, relatively to the fun, in producing, and varying the direction of these currents; and points out, from his theory, the times of the day, and the feafons of the year, in which feas or large pieces of water may be paffed over with the greateft probability of efcaping their dangerous influence; as alfo the circumftances in which the fuperior air may be expected to be warmer or colder than that below. Though we cannot fay that his reasoning is clofe, or his facts very skilfully arranged, yet, taken all together, they appear to us to have great weight, and we recommend them to the serious attention both of the aeronaut and the meteorologift.

For the fecurity of a balloon that may be caught in a depreffing current over water, he propofes to have a ballaft thrown out that fhall float on the water, and which may afterwards be drawn up again. Means are defcribed for fuftaining balloons

ftationary