Observations on the experiments given in the above table.

It is to be regretted that such discrepancies are to be noted in the table. They are ascribable, in the first place, as suggested above, to the difficulty of bringing the mixture always to the same condition as regards the dissemination of the ingredients, when worked in so dry a state; but, probably, chiefly to the difficulty of filling the moulds always with equal accuracy, and ramming every part with equal force, when using so incoherent a mortar, united with so large a proportion of very coarse ingredients.

Notwithstanding these discrepancies, however, several deductions may be fairly drawn from the table, which, if confirmed by future trials, will be useful. 1st. When the mortar was made of cement, sand, and lime, or of cement and sand without lime, the concrete was the stronger as the sand was less in quantity In 50 comparisons 19 exceptions. But there may be 0.50 of sand and 0.25 of lime without sensible deterioration; and as much as 1.00 of sand and 0.25 of lime, without great loss of strength.

2d. A mortar of cement and sand does not seem to be improved by the addition of lime, while the bulk of sand is only equal to, or is less than, the bulk of cement; but as the quantity of sand is further increased, the mortar appears to be more and more benefitted by the addition of a small quantity of lime.

3d. Two measures of mortar, in concrete, are better than one measure; that is to say, a quantity of mortar equal to the bulk of the void space does not give as strong a concrete as twice that quantity of mortar. In 30 comparisons, 7 exceptions. Nevertheless, the strongest example was with one measure of mortar, and it is not unlikely that the deficiency of strength in the other cases resulted from the difficulty of causing all the voids to be accurately filled, when the mortar was a minimum, and the space into which it was forced so small. It is not improbable that the voids may be perfectly occupied, even with one measure of mortar, when the mass of concrete is large enough to permit the full effect of the rammer.

4th. The results of the experiments recommend the several compositions of the table, in the following order, namely:

2.

1. Brick gravel,
do.

with 2 measures of mortar,

No. 8.

5th. It appears that the best material to mix with mortar to form concrete, is quite small, angular, fragments of bricks: and that the worst is small, rounded, stone-gravel.

6th. Grout, poured amongst stone, or brick fragments, gave concretes inferior to all, but one, of those obtained from mortars.

A piece of sound and strong red sand-stone, 12 inches by 4 inches by 4 inches, required a weight of 3673 pounds to break it-there being 9 inches

abs

between the supports. According to the formula P=R.- * prisms of

C

In this formula P is the weight causing fracture, e the distance between the sup ports, a the breadth, and 6 the depth of the prisms.

this stone of the size of our prisms of concrete, would require the weight of 12,596 lbs. to break them; whence it appears that the strongest prism under trial, was, after eight months exposure, half as strong as this sand

stone.

CHAPTER XXV.

Some recent experiments with Mortars made of Lime and Sand.

There will be presented, in conclusion, some experiments, made very recently at Fort Adams, with lime mortars without cement; they were instituted in reference to the best proportions of lime and sand, and also to a comparison of coarse and fine sand, and salt and fresh water.

In making these, a cask of fresh Smithfield lime, of the best quality, was taken, and the lumps broken into pieces of about the size of a pigeon's egg. These being carefully screened, in order to get rid of all dust and fine lime, and carefully intermixed, in order to obtain uniformity of quality throughout, were slaked by the affusion of water to the amount of one third the bulk of lime. When cold, the slaked lime was returned to the barrel, which was carefully headed and put in a dry place; and on all occasions of withdrawing a portion of this lime for use, the cask was carefully re-headed. The sands used were those described in page 4, as sand No. 1, sand No. 2, sand No. 3, and sand No. 4.

In making the mortars, just enough water was added to the slaked lime taken from the cask, to make a stiff paste. This paste being passed through a hand paint mill, which ground it very fine, was mixed, by careful manipulation, with the due proportions of sand. Much care was bestowed upon the operation of filling the prism-moulds with mortar; and each prism was submitted to a pressure of 600 lbs. for a few minutes, that is to say while the succeeding prism was being formed.

About one week was consumed in preparing the prisms-namely, from the 7th to the 15th of May, 1838. And they were broken on the 1st of July, 1838, making the average duration of the experiment, 50 days.

Three prisms were made of each composition. But, on the principle that there are several causes which tend to make a prism weaker than it should be, and few or none that tend to make it stronger, only the maximum result of each experiment is given in the following table.

It may, however, be well to state that precisely the same inferences are deduceable, if the mean of the results be taken instead of the maximum.

Table No. LXX.

Trials made on the 1st of July, 1838 of the strength of the mortars made between the 7th and 15th of May, 1838 (50 days.) The results show the weights, in pounds, required to break prisms of mortar 6 inches long, by 2 inches by 2 inches: the distance between the supports being 4 inches, and the power acting midway between the supports.

Observations on the experiments of table No. LXX.

1st. Within the limits of the experiments, the mortar was the stronger as the quantity of sand was the less-in 96 comparisons, 12 exceptions.

2nd. Although the above inference is derived from the whole range of the table, still, when the quantity of sand was less than the quantity of lime, the weakening effect of the sand on the mortar was not very sensible. And it would seem from table No. LXV. that from one-fourth to one-half of sand may be slightly beneficial.

3rd. It appears that coarse sand, or, rather, sand composed of coarse and fine particles, (sands No. 1 and 2,) is a little inferior to sand that is all fine (sands No. 3 and 4;) in 36 comparisons, 16 exceptions; and also that sand reduced by pounding to a fine powder (No. 4,) afforded some of the best results of the table. It is to be regretted that no experiments were instituted in order to compare sand all coarse, with sand all fine.

4th. It appears that the mortars made with salt water-that is to say, the water of the ocean, was decidedly weaker than those made with fresh water; 1 exception in 12 comparisons. The aggregate strength of all the prisms made of coarse sand and salt water was 2674 lbs.; while the ag gregate strength of the corresponding prisms of coarse sand and fresh water was 3174 lbs. And the aggregate strength of all the prisms of fine sand and salt water was 2800lbs. while the aggregate strength of the corresponding prism of fine sand and fresh water was 3346 lbs.

DESCRIPTION OF THE PLATES.

PLATE I.

Fig. 1. a, a, Prism of mortar under trial. b, b, Iron stirrups, supporting the prism.

c, c, Iron collar, embracing the prism.

d, d, Iron link, to which the ropes of the scale-pan are fastened. e, e, check, against which the collar rests when on the middle of the prism. f, f. Timber, to which the stirrups are attached..

g, Scale pan, in which the weights to break the prism are put.

Fig. 2. h, Interior of the furnace.

i, Door of the furnace.

k, k, Chimney.

1, Register.

m, m. Arches, under the hearth, in which the fuel is placed,

n, n, Conduits, to lead the flame and a current of air into the furnace. Fig. 3. o, Plan of lime kiln.

P, p, Nut of the kiln.

q, q, Steps descending to the doors of the kiln.

r, Steps, up which the materials are carried to the top of the kiln. 8, s, Doors of the kiln.

t, t, Portions of spherical arches leading to the doors of the kiln.

PLATE II.

Figs. 4, 5, 6, 7 and 8, represent Mr. Petot's "curves of energy" of fat lime, hydraulic lime-plaster-cements-calcareous puzzolanas, and clay. Fig. 9. a, b, Half staples, driven into the floor.

f, g, A pair of bricks united by mortar.

c, c, Iron piece, embracing the ends of the upper brick, and suspended from the steelyard.

d, Steelyard.

e, Bucket, into which sand flowed from the trough.

h, Trough.

i, Floor.

Fig. 10. a, b, c, Iron lever, with a steel point at a to impress the mortar f, on the brick g.

d, Steelyard, connected with the lever a, b, c, at c.

e, Iron rod, from which the steelyard is suspended.

h, h, Uprights, supporting the rod e.

i, Uprights of iron, supporting the fulcrum of the lever a, b, c.

FINIS.