1831.] 
On the Longitude of Dihli. 
57 
From the description of the cancars employed as limestones in the interior of 
India, I was led to suspect, that they might readily be converted into hydraulic ce- 
ments. Like the Sheppey tufa, they are found in hard nodules, and consist of sand 
or clay, intimately united by calcareous cement. Such cancars, especially as will 
not slake after calcination, of which many examples are given in Mr. J. Prinsep’s 
tables, and which contain from 60 to 70 per cent, of carbonate of lime, seemed pe- 
culiarly adapted for the purpose, as their composition would obviate the cost of 
pounding and mixing the ingredients before calcination. 
My own experiments have hitherto been confined to the following specimens : — 
1. A species of small nodular cancar, of a dark colour, is found in the Salt Water 
lake, consisting of about equal parts of lime and ferruginous clay, with probably a 
good deal of decomposed vegetable matter. This I found, on trial, to constitute, of 
itself, a very perfect puzzolana, hardening rapidly under water. 
2. A mixture of one-fourth of chalk, well pounded up with the above cancar , 
also yielded a very good cement, but not quite so hard. 
3. The Burdwan cancar contains a much larger proportion of lime, from 65 to 
75 per cent. : it is, therefore, too rich, by itself, to form a good cement, but with 
the addition of one-fourth of blue clay, it answers perfectly well. 
4. The slaked lime of Sylhet, as brought to the Calcutta market, worked up 
with clay, in the proportions of 2 to 1, made into balls, and kilned, answers perfectly, 
though it does not become quite so hard as the preceding : it seems probable, that 
this lime may contain a little alkali, from the wood with which it was originally 
burned. 
The clay used in the above experiments, was dug up from a few feet under the 
ground; it is of a blueish colour; and seems well adapted for the purpose : it is im- 
possible for a clay to be too plastic ; and when it contains much sand, it is needless 
to expect a good result; perhaps, therefore, the up-country cancars, called red nodu- 
lar, and stony, in Mr. Prinsep’s list, the alloy of which is of a much more sandy na- 
ture, may not be suited for hydraulic purposes. But I am informed, that there is 
another species of flat cancar , formed at the bottom of jhlls, which is more clayey, 
and may, probably, succeed well ; I am told also, that the latter is generally preferred 
by native builders, for under-water work, either alone, or mixed up with a surkt 
made of half burned lumps of clay, pounded. 
I have not yet been able to procure specimens of these cancars , but hope in a 
short time to obtain them : at any rate the process I have described is so simple, 
that it will enable any one engaged in building to experimentalize on such materials, 
as may be within his reach, and to provide himself, at pleasure, with a mortar fit 
for any under- water masonry. 
VI . — On the Longitude of Dihli. By Major T. Oliver, 3 d Regt.. 
Ben. Nat. Inf. 
The method of lunar transits is now generally considered to be one of the best 
means of determining terrestrial longitudes. Its principal advantage is the fre- 
quency of occurrence of the phenomenon on which it depends ; thereby allowing a 
multiplication of observations to any desirable extent, so as to reduce the una- 
voidable errors of observation to a mean value. The chief disadvantage underwhich 
it labours, is the moon’s slowness of motion, which is such, that any error in the 
time of observation is multiplied 24 fold in the resulting longitude. To ensure the 
utmost accuracy of which itis susceptible, the observations should be compared with 
