or $0.087 per cubic foot. It appears that the expense of making the mortar was $12.08, being about of a cent for a cubic foot.

The proportions in the above mortar are about 1 of lime in paste to 21 of sand-should the proportion of lime be greater, the mortar will, of course,

cost inore.

The above statement refers to mortar made without addition of any hy. draulic substance. But such mortars are now never used at Fort Adams. Hydraulic cement, or burnt clay, or brick dust, or some other similar matter is added to every kind of mortar made at the work, in proportions varying with the purpose to which the mortar is to be applied. The poorest mortar we make contains 1 barrel of hydraulic cement to 3 barrels of unslaked lime and about 15 barrels of sand; the cement being added before the sand, and while the lime is being reduced under the wheel.

All the mortars used in the experiments in the tables, were made by hand with the trowel, with such exceptions, only, as are noticed.

Trials of the Strength of Mortars.

The strength of mortars as regards tenacity, was determined by measuring the force required to separate bricks that, having been joined by the mortar, had been left, for the desired length of time, in some place safe from frost or accident.

The bricks were joined in pairs, being crossed at right angles thus, so that, supposing each brick to be 4 inches wide, the surface of contact would be 16 square inches. The real surface, or surface of effectual contact, was, in every case, found by actual measurement. The mortar joint separating the bricks was made about of an inch thick: and, in order that this mortar should in all cases be equally consolidated, each pair of bricks was submitted to the pressure of 600 lbs. for 5 minutes, immediately after being joined.

An idea of the mode of separating the bricks may be got from fig. 9, Pl. II, where a and b represent two strong half-staples fastened to the floor: under these the ends of the lower brick are passed, while the ends of the upper brick are embraced by the piece of iron c, c, suspended from the steelyard d. The force needed to separate the bricks, is applied by pouring sand, at a uniform rate, into the bucket e. The weight of the sand and bucket, the mark on the beam where the weight was applied, and the weight of the poise, enable us to ascertain the force necessary to tear the bricks asunder. In the tables, the force required to separate the bricks is reduced to the proportional force required to tear up a surface of one square inch: so that if there were 16 square inches of actual contact, and the force used in separating the bricks was 1000 pounds, the table would represent the tenacity of the mortar by 621lbs.—equal to 100°.

The hardness of the mortars was determined by ascertaining the weight, applied on a circular plane surface of 0.16 of an inch in diameter, (or .02008 of an inch area,) which the mortar would support. This mode of trial is represented in fig. 10, Pl. II. The circular surface at the extremity a, presses upon mortar still adhering to one of the bricks. The arms of the lever b, are of equal length, so that the upward force at c is equal to the pressure at a. The force is applied by means of a steelyard and sand, as

in the preceding case. The experiments were generally made with several pairs of bricks, and a mean was taken of the results; unless it had obviously been subjected to some accident or disturbance, being made to contribute to the mean. Very few results were rejected. There could be only as many trials of tenacity, in each particular experiment, as there were pairs of bricks. But for hardness, it was often possible to make a considerable number of distinct trials on the same surface of mortar: on the other hand, it would sometimes happen that the surface would be left too ragged and uneven for this trial: and in several instances this test seemed to be entirely inapplicable-the mortar beginning to yield with light weights, and continuing to yield more and more as the weight was increased, the whole effect being a gradual crumbling. In a great majority of cases, however, the effects were sufficiently decided to leave no doubt as to the moment when the power prevailed over the resistance and sufficiently consistent to afford useful comparisons.

The method, just described, of trying the strength of mortars, was adopted in the Fort Adams experiments, on account of the facility of application. There was, in the first instance, no purpose of extending the experiments beyond what was deemed indispensable to a proper choice, and judicious application of materials, in the construction of a work of some magnitude, then being begun. One series of experiments, however, involved another and another, until the series became extended and the experiments too numerous and valuable, not to make it desirable that subsequent ones should be comparable with them, and, consequently, the same mode of test was continued.

It is probable that the method followed by Genl. Treussart, of making rectangular prisms of mortar, and subjecting them to fracture by weights suspended from the middle, is the best mode. It, at any rate, has the advantage of allowing mortars made in different places, and at distant times to be compared. This mode was adopted in some of the later trials at Fort Adams.

The following table exhibits the mean results of all the experiments made from 1825 to 1832; comprising seven series. The time of exposure of the 1st series was 5 months; of the 2nd. series, 10 months; of the 3rd, 10 months; of the 4th, 5 months; of the 5th, 10 months; of the 6th, 25 months; and of the 7th, 11months. In the 1st series, there were 2 pairs of bricks to each experiment; in the 2nd, 3 pairs; in the 3rd, 3 pairs; in the 4th, 1 pair; in the 5th, 4 pairs; in the 6th, 2 pairs; and in 7th, 3 pairs.

The first column prefixes a number to each kind of mortar, for convenient reference; the 2nd column expresses the nature, or composition of the mortar; the 3rd column, whether the bricks were wet or dry when joined together; the 4th, the number of series of which the results are a mean as to tenacity; the 5th, the tenacity, as expressed by the number of pounds required to tear open a joint of one inch square; the 6th, the number of series of which the results are a mean as to hardness; and the 7th, the number of pounds required to force into the mortar a circular plane surface of 0.16 of an inch in diameter.

Table No. LXV.

Observations on the Experiments of Table No. LXV.

1st. Generally, within the limits of the experiments, a mortar made of lime and sand, or of hydraulic cement and sand, or of hydraulic cement, lime and sand-whether it was cement A, or cement B, or Roman cement, was the stronger, as the quantity of sand was the less. In 24 comparisons, 3 exceptions.

In 13 comparisons of tenacity, 2 exceptions.

In 11 comparisons of hardness, 1 exception.

2nd. It appears that with cement A, or cement B, any addition of sand weakens the mortar. In all the cement experiments, except one, composed of Roman cement 1-sand (No. 26,) the cement alone, was stronger than when mixed with sand in any proportion whatever. Cement A (No. 6,) would seem to be another exception, but it is not; the strength of cement A, alone, as given in No. 2, is the average of five results with different specimens of cement, some of which were of inferior quality; while the result given in No. 6 is of one trial only, and that of a cement proving to be the best used; the particular result of No. 2 which corresponds with No. 6—— that is to say, which was afforded by the same specimen of cement, gave for tenacity 74.7 lbs. and for hardness 1063 lbs., while No. 6 shows a tenacity of 61.9 lbs. and a hardness of 1055 lbs.

3rd. It appears that when cement mortars are not required to be the strongest that can be made-a little lime may be added, without great loss of tenacity, and, of course, with a saving of expense.

4th. Mortar made in the mortar-mill was superior to mortar made by being mixed, in the common mode, with the hoe.

5th. When the bricks were dry and the mortar more fluid than usual, the mortar was better, both as to TENACITY and HARDNESS-in five cases out of seven, than when the bricks, being wet, were put together with mortar of common consistence.

In the next table there is a comparison of the three kinds of lime-of the three modes of slaking, of various proportions of sand-of the effect of wet and of dry bricks on the mortar, &c.

In most cases six pairs of bricks were put together at the same time, and of the same materials; of which three pairs were separated after about 6 months, and the remainder after the lapse of 4 years and 5 months.