all his operations must depend; and in the more 
extensive operations of trigonometry the correct 
measurement of a good base line is of vital am- 
portance to the success of the whole operation.— 
Millington. 
CHALK. A species of carbonate of lime. It 
is inodorous and insipid; and, when applied to 
the tongue, feels slightly adhesive. It has either 
a white or yellowish or greyish-white colour. It 
feels roughish to the touch; is pulverulent and 
not very hard; breaks with an earthy fracture ; 
stains the fingers, and leaves marks on wood or 
stone; and has a specific gravity of from 2°315 
to 2:78. But in hardness, fracture, and lustre, it 
is exceedingly various; sometimes occurring soft 
and dull, almost like a saponaceous powder, and 
at other times occurring hard and semi-lustrous, 
almost like fine limestone or coarse marble. It 
dissolves in water containing an excess of car- 
bonic acid; but is very partially soluble in pure 
water. It readily effervesces with almost any 
acid, throwing off its own carbonic acid, and 
surrendering its calcium into combination with 
the foreign acid to form a salt. Most specimens 
of it contain a portion of alumina; and many 
contain portions of silica and oxide of iron. Its 
principal difference from limestone is its being 
in a large degree a hydrate, or containing, in 
combination with its carbonate of lime, a com- 
paratively large proportion of water. Specimens 
| of limestone analyzed by Thenard and Biot con- 
| tained only 1:63 per cent. of water; and speci- 
mens of chalk, when carted from pits, and laid 
down upon land for manure, are frequently found 
to contain 24 or 25 per cent. 
Chalk occurs in beds, strata, or vast masses in 
Poland, some of the Danish islands, the north of 
France, the north-east of Ireland, and particu- 
larly in the centre and south of England, within 
a range which commences at Flamborough-Head 
| in Yorkshire, and is continued, with irregular 
interruptions, in Lincolnshire, Suffolk, Surrey, 
| Sussex, and Hampshire, into Dorsetshire. While, 
in a geological respect, limestones occur of a great 
variety of age and character, from what are called 
_ the primary or non-fossiliferous rocks, up to a 
high part of the tertiary formations, chalk occurs 
only in the upper regions or among the newest 
members of the secondary formations, and, in its 
_ most characteristic strata, is always accompanied 
| with flints. 
In many of the chalk districts of 
England, the crust of the earth, to a great depth, 
is a continuous and solid mass of chalk, and the 
very soil itself consists of little else than carbon- 
ate of lime and a small admixture of decomposed 
vegetable matter. The chalk of Ireland is very 
much harder than almost any of the chalk of 
England; and, even in spite of containing flints, 
and possessing other most decided geognostic 
indications of being in all respects chalk, it pos- 
sesses so close a resemblance to fine limestone in 
both appearance and properties, as to be almost 
universally called limestone by the natives, and 
as to be extensively imported into Scotland un- 
der the name of Irish lime, for exactly the same 
uses as the richest. calcareous limestones of Bri- 
tain. Some of the chalks of England, particu- 
larly of the extreme south and of the Isle of 
Wight, have an unctuous or saponaceous char- 
acter, and ought to be regarded as akin to cal- 
careous marl; but most are hard and flakey, and 
may be treated, for manurial purposes, as simply 
a kind of hydrated limestones. 
Those soils of the chalk districts of England 
which consist almost wholly of pure chalk, with 
a small intermixture of decomposed vegetable 
matter, generally constitute the softly-rounded 
chains and groups of hills which bear the name 
of downs in the south, and of wolds in the north; 
and, in general, they are far less adapted to til- 
lage than to pasture, and are, in consequence, 
very extensively used as sheep-walks. Yet the 
lower slopes and the valleys and hollows of the 
downs and wolds have received such a large com- 
mixture of the finest particles of the soil, from 
the constant though slow depositions of rills 
from the higher grounds, as to have become very 
productive arable lands ; and they are usually 
farmed upon the common Norfolk system, and are 
found to make excellent returns of barley and of 
other crops. “ There is a plant,” says the Rev. 
Mr. Rham, in reference to these districts, “ which 
seems to delight in chalky soils, and to flourish 
better there than in deeper and richer earth. 
This is the sainfoin, Hedysarum onoirychis ; and | 
its introduction among the cultivated grasses has 
greatly raised the value of chalky land, previously 
considered too poor to repay cultivation. It not 
only produces an excellent fodder, superior to 
meadow hay, and scarcely inferior to clover, but 
by its roots it so divides and enriches the soil, 
that after having covered it with luxuriant crops 
of hay for several years, it leaves it in a fit state 
to produce excellent crops of grain, with very | 
moderate manuring. Chalky soils are not sub- | 
ject to the same perennial weeds which infest 
richer lands; couch grass is seldom found in | 
chalk ; but the annuals, such as may-weed, char- 
lock, poppy, crowfoot, and several others, abound | 
in it. When a chalky soil produces thistles, it is 
an indication of its containing a portion of argil- 
laceous earth, which improves its fertility.” 
Soils which have a large admixture of argillace- 
ous earth, but which are immediately incumbent 
on chalk, also derive great benefit from the growth 
of sainfoin, but require a different course of hus- 
bandry from the slopes and hollows of the downs 
and wolds. Most of these soils consist of separ- 
ate layers of hazel-loam, thin flinty clay, or strong 
red clay, of different degrees of admixture and of 
depth; and wherever they are sufficiently light 
and porous to be suitable for the growth of tur- 
nips, they should be sown at remote intervals 
with sainfoin, and subjected throughout the in- 
tervals to long and diversified rotations. Sain- 
foin cannot be advantageously repeated except 
CHALK. : 
