the larger peebles in the fame fituation) will be worse than any other fort.-River-fand will be better than it ;-and pit-fand, when quite free of earth, the best of all,

$ 33.

If the fand be hard, and the particles angular, it is perhaps of little importance whether these be very small or of a larger fize. The fand in the lime that formed the extraordinary firm cement, mentioned P. 345. was as fmall as could well be imagined.

Because fea-fand is ufually smaller than any other fort, and is acknowledged to be lefs proper for making mortar than many other kinds of fand, a prejudice has been, in general, adopted against fine fand for this purpose. But this, there is reason to imagine, is only a vulgar prejudice, arising from the peculiar figure of that fort of fand.

§ 34.

§ 34.

There is another and better reafon for not employing fea-fand in mortar, viz. that there is always a chance that fome particles of falt inay be formed among it by the evaporation of the fea-water upon the fhore.-And, as common falt continues always to be a deliquefcent fubftance, it will have a perpetual tendency to attract moisture from a humid. air, and thus render the wall in which this mortar has been employed extremely damp and unwholefome.

It is from the fame caufe that any porous fort of flone, that has been taken from the fea-fhore, continues at all times to be wet in damp weather. For, while the ftone remained on the shore, its pores would be from time to time filled with falt-water, upon the evaporation of which, the falt it contained would remain behind within the pores of

the

the ftone, which would thus become endowed with the quality of attracting moisture from a damp air, fufficient to diffolve the salt, and make the watery folution ooze out through all its pores.

This is a phaenomenon for which it is more easy to account, than to prescribe an effectual cure.-Perhaps no art can render the stone fufficiently dry after it is once put into the wall.-To let it lie for a confiderable time in a ftream of running fresh-water before it was employed, might obviate the disease.

$.35.

For the fame reafon, lime that has been flaked with fea-water is always unfit for being used as a mortar. For, as it is impoffible ever to extract that falt from the mortar, it continually attracts moisture from the air in damp weather, and oozes through the

pores

pores of the wall in form of drops of fweat, which again difappear when the weather becomes dry.

This is an inconvenience often felt: Dut, as the real caufe of it is ieldom known, few perfons are at proper pains to guard againft it.-Those who obtain their lime by watercarriage are, in a peculiar manner, liable to be hurt by this circumftance; as the lime is, for the most part, flaked at the fhip's fide by the fea-water, which is more eafily got than any other.

When lime that has been flaked in this manner is employed as a plafter, it is rather worse than when ufed as a mortar, as it has lefs fand added to it, and has fewer pores in the infide, in which the drops of water might be allowed to lodge; fo that the wall becomes alternately covered with a cruft of dry powdery falt, and with damp tears running down its furface.

Too

Too much care, therefore, cannot be taken to avoid using lime that has been flaked with fea-water-as it will be impoffible, or extremely difficult, ever to render these walls perfectly dry.

I have thus enumerated, at much greater length than I originally intended, the feveral circumstances that contribute to render lime-cement more or lefs perfect. In doing this, I have had occafion to explain the nature of many of thofe calcareous matters which have been generally used as a manure, which will confiderably fhorten our labour in what remains of this effay.

If I have reprehended, with fome degree of afperity, thofe who, either through ignorance, or a wilful intention to deceive, have endeavoured, by fpecious pretexts, to miflead the ignorant; I hope the candid will be rather ready to afcribe this to a defire of rectifying