CAI 
studied the classics, philosophy, and mathe- 
matics. He afterwards studied divinity in 
the College de Navarre, with the view of 
embracing the ecclesiastical life, but never 
entered into priest’s orders. His turn for 
astronomy soon connected him with the ce- 
lebrated Cassini, who procured him an 
apartment in the observatory ; where, as- 
sisted by the counsels of this master, he soon 
acquired a name among the astronomers. 
In 1739 he was joined with M. Cassini de 
Thury, son to M. Cassini, in verifying the 
meridian through the whole extent of 
France ; and in the same year he was nam- 
ed professor of mathematics in the College 
of Mazarine. In 1741 he was admitted 
into the Academy of Sciences, and had 
many excellent papers inserted in their me- 
moirs ; beside which, he published several 
useful treatises, viz. Elements of Geometry, 
Astronomy, Mechanics, and Optics. He 
also carefully computed all the eclipses of 
the sun and moon that had happened since 
the Christian sera, which were printed in 
the work entitled, “ L’Art de verifier les 
Dates,” &c. Paris, 1750, in 4to. He also 
compiled a volume of astronomical ephe- 
merides for 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, “ Astronomiae 
Fundamenti novissimis Solis et Stellarum 
Observationibus stabilita.” 
Having gone through a seven years’ series 
of astronomical observations in his own ob- 
servatory in the Mazarine College, he formed 
the project of going to observe the soutliern 
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 he- 
misphere that are not visible in our lati- 
tudes, as well as many other important fele- 
ments, viz. the parallaxes 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 offering for his 
return, he thought of employing the vacant 
time in another arduous attempt ; no less 
than that of taking tlie 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 corres- 
ponding parallels iu north latitude. About 
the 23d degree of south latitude he found a 
CAI 
degree on the meridian to contain 342222 
Paris feet. The court of Versailles also sent 
him an order to go and fix the situation of 
the Isles of France and of Bourbon. 
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, 
lie 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 of his 
observations for the parallaxes, with those 
of other astronomers : that of the sun he 
fixed at 91'; of the moon at 56' 56'' ; of 
Mars in his opposition, 36" ; of Venus 38”. 
He also settled the laws by which astrono- 
mical refractions are varied by the different 
density or rarity of the air, by heat or cold, 
and by diyness or moisture. And, lastly, 
he sliewed an easy and practicable method 
of finding the longitude at sea by means of 
the moon. His fame being now celebrated 
every where, M. dela Caille was soon elect- 
ed a member of most of the academies and 
societies of Europe, as London, Bologna, 
Petersburg!), Berlin, Stockholm, and Gottin- 
gen. 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 gunpowder; 
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 foundations of 
the piers of a bridge. 
The practice of building in caissons is a 
method sometimes adopted in laying the 
foundation of bridges in very deep or ra- 
pid rivers. These 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 previously excavated and 
levelled, are there sunk. The piers of 
the bridge are then built witliin them, and 
carried up above, or nearly to the level of 
the water, when the sides of the caisson are 
detached fi-om the bottom, and removed : 
the bottom, composed of a strong grating 
of timber remaining, and serving for a 
foundation to the pier. The most conside- 
rable work, where caissons have been used, 
was in the building of Westminster-bridge ; 
of these, therefore, a particular account may 
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