CAI 



CAI 



carefuly computed all the eclipses cfthe 

 sun and moon tiua had happened since 

 the cimstiaii sera, whicli were printed in 

 the work, entitled " L'Art de verifier les 

 Dates," &c. Paris, 175U, in 4to. He also 

 compiled a volume ot astronomical ephe- 

 mendcs tor the years 1745 to 1755 ; an- 

 other for the years 1755 to 1765 ; and a 

 third for the years 1765 to 1775; as also 

 the most correct solar tables of any ; and 

 an excellent work, entitled " Astronomic 

 Fundamenta novissimis> Solis et Stellarum 

 Observatiombus stabilita." 



Having gone through a seven year's 

 series of astronomical oDserv ations in his 

 own observatory in the Mazarine College, 

 lie formed the project of going to observe 

 the southern stars at the Cape of Good 

 Hope ; being countenanced by the court, 

 he set out upon this expedition in 1750, 

 and in the space of two years he observed 

 there the places of about 10,000 stars in 

 the southern hemisphere, that are not vi- 

 sible in our latitudes, as well as many 

 other important elements, viz. the paral- 

 laxes of the sun, moon, and some of the 

 planets, the obliquity of the ecliptic, the 

 refractions, &.c. Having thus executed 

 the purpose of his voyage, and no present 

 opportunity ottering for his return, he 

 thought of employing the vacant time in 

 another arduous attempt; no less than 

 that of taking the measure of the earth, 

 as he had already done that of the heavens, 

 whence he discovered, that the radii of 

 the parallels in south latitude are not the 

 same length as those of the corresponding 

 parallels in north latitude. About the 

 23d degree of south latitude he found a 

 degree on the meridian to contain 342222 

 Paris feet. The court of Versailles also 

 sent him an order to go and fix the situa- 

 ation of the isles of France and of Bour- 

 bon. 



M. de la Caille returned to France in the 

 autumn of 1754, after an absence of about 

 four years ; loaded, not indeed with the 

 spoils of the East, but with those of the 

 southern heavens, before then almost un- 

 known to astronomers. Upon his return, 

 he first drew up a reply to some strictures 

 which the celebrated Euler had published 

 relative to the meridian; after which he 

 settled the results of the comparison o 

 his observations for the parallaxes, with 

 those of other astronomers : that of the 

 sun he fixed at 9$" ; of the moon at 56' 

 56" ; of Mars in his opposition, 36" ; of 

 Venus 38". He also settled the laws by 

 which astronomical refractions are varied 

 by the different density or rarity of the 

 air, by heat or cold, and by dryness or 



moisture. And, lastly, he shewed an 

 easy and practicable method of finding 

 the longitude at sea by means of the 

 moon. His fame being now celebrated 

 every where, M. de la Caille was soon 

 elected a member of most of the acade- 

 mies and Societies of Europe, as London, 

 Bologna, Petersburg!!, Berlin, Stockholm, 

 and Gottingen. He died in 1762, aged 49. 



CAISSON, in the military art, a wooden 

 chest, into which several bombs are put, 

 and sometimes only filled with gunpow- 

 der; this is buried under some work, 

 whereof the enemy intend to possess 

 themselves, and when they are masters 

 of it, is fired, in order to blow them up. 



CAISSON is also used for a wooden 

 frame, or chest, used in laying the foun- 

 dations of the piers of a bridge. 



The practice in building in caissons is 

 a method sometimes adopted in laying 

 the foundation of bridges in very deep or 

 rapid rivers. There are large hollow- 

 vessels, framed of strong timbers, and 

 made water tight, which being launched 

 and floated to a proper position in the 

 river, where the ground has been previ- 

 ously excavated and levelled, are there 

 sunk. The piers of the bridge are then 

 built within them, and carried up above, 

 or nearly to the level of the water, when 

 the sides of the caisson are detached from 

 the bottom, and removed ; the bottom, 

 composed of a strong grating of timber, 

 remaining and serving for a foundation 

 to the pier. The most considerable 

 work, where caissons have been used, 

 was in the building of Westminster- 

 bridge ; of these, therefore, a particular 

 account may be acceptable. Each of the 

 caissons contained 150 loads of fir timber, 

 and was of more tonnage than a man of 

 war of 40 guns ; their size was nearly 80 

 feet from point to point, and 30 feet in 

 breadth ; the sides, which were 10 feet 

 in height, were formed of timbers laid 

 horizontally' over one another, pinned 

 with oak trunnels, and .framed together 

 at all the corners, except the salient an- 

 gles, where they were secured by proper 

 iron- work, which, being unscrewed, would 

 permit the sides of the caisson, had it 

 been found necessary, to divide into two 

 parts. These sides were planked across 

 the timbers, inside and outside, with 3 inch 

 planks, in a vertical position. The thick- 

 ness of the sides was 18 inches at bottom, 

 and 15 inches at top ; and in order to 

 strengthen them the more, every angle, 

 except the two points, had three oaken 

 knee timbers,properly bolted and secured. 

 These sides, when finished, were fasten- 



