The greatest difficulty in the practical working of the machine consists in maintaining the plate supporting the machinery constantly parallel to the plane of the bottom of the wheels. And when we consider the difficulty of preserving the level even of the axis of a stationary astronomical transit, it seems probable that errors from this want of parallelism must always be apprehended with a plate placed upon a carriage, no matter how carefully constructed. The unequal wearing of the fore and hind axles and tires, and the wear of the running gearing, all would require adjusting screws to the plate, and the constant jarring to which the machine would be subjected must, necessarily, somewhat affect their adjustment. Corrections could, however, be readily deduced by running up and down the same slope, if errors were suspected.

The machine, as a whole, though appearing quite complicated, is really sple; there is no part whose moderate wearing would materially at the accuracy of the work, except the friction cones; and the inventors feel confident that, with due care in hardening the steel, these will not be liable to wear.

Inaccuracies might be occasioned by the temporary enlargement of one of the wheels by the adherence of mud. Scrapers should, therefore, be attached to all the wheels, to remove the dirt as fast as it accumulates, which was not the case with the vehicle to which the instrument examined was attached.

Rapid wear and tear of the machinery would be caused by dust, if it were not excluded; and greater precautions for this purpose should be adopted in a new machine than in that examined by us.

The machine presented was placed on the body of an ordinary fourwheeled carriage, between the fore and hind wheels, and by the springing of the body was liable to be thrown out of its proper position. It should be placed over the hind wheels, resting directly upon their axletree. The three indicators also were all graduated differently and very inconveniently. They consumed much time in reading, and were not a little perplexing, especially as one of them was numbered from the zero around by the left, contrary to the usual method of numbering graduated circles. The machinery was also unnecessarily cumbersome, and susceptible of improvement in many details. These defects are easily remedied, and will probably not appear in any instrument hereafter constructed by the patentees, who have submitted drawings of a greatly improved instrument for our examination. An additional friction cone and wheel are introduced to receive the motion from the perambulator; which will enable the machine, when placed upon a vehicle having wheels of any size, to be adjusted to it without requiring additional parts or alteration in the graduated indicator circles.

Test of the Instrument.

The instrument was taken by the board on January 24, 1859, the day being sufficiently cold to prevent any mud from adhering to the wheels, and thus vitiating the result. The plan adopted was to drive the carriage over a portion of the Washington aqueduct, where the profile was accurately known, and thus to test the instrument by com

paring its results with an actual survey. The route selected was from the "intersection," via Bridge street, to the "College Pond;" thence over the fields to "Drovers' Rest." The first division was a paved street, and the second undulating ground, much cut up by the aqueduct wagon ruts, which, being frozen hard, furnished the means of thoroughly testing the effect of rough travel upon the instrument. In returning, the same route was resurveyed, and the instrument thus subjected to a double test. Table No. 1. exhibits the readings taken in going from the War Department to the office of the Washington aqueduct in Georgetown, and returning over the same route; the two results serving, in some sort, to test each other. Table No. 2. shows the results of the experiment where the work was tested by the aqueduct surveys. The wagon was drawn at a walk, and exactly over the aqueduct line of survey; but as there were often no more definite benches than the intersection of streets, errors of one or two feet may be disregarded.

Time of actual travel going, 22 minutes; time of actual travel returning, 19 minutes.

A scrutiny of the above tables will show that the instrument worked admirably before coming to the rough ground; that in going over this portion of the line, about 3,750 feet in length, an error in altitude of twenty-two feet, and in returning an additional error of six feet, was made; that the jarring seemed to throw the instrument out of adjustment, and to introduce a variable error into the subsequent readings even when passing over smooth ground. These variations were accounted for by Mr. Adams by supposing that the position of the level plate was slightly changed. It was badly secured by a combination of screws and wedges, and he stated that he discovered the wedges to be loose on taking the instrument from the wagon after our return to the War Department. is evident that the slightest disarrangement of this plate must induce great errors; and it is surprising that the succession of severe jars to which the instrumer.t was subjected did not produce a greater effect, considering the faulty means of securing the permanence of this adjustment.

Concluding Remarks, Recommendations, &c.

It

The patentees desire to make a machine for the government upon the improved plan already referred to, and Mr. Hyde, their agent, has offered to furnish it at a cost of $5,000. In a subsequent letter, he states that he can find no builder willing to undertake its construction for less than $2,000.

It is thought by the board that a price securing to the patentee such profit on the mechanical labor and material as is usually made upon other instruments of equally difficult construction should be a sufficient compensation, without any large allowance for the right of the patent. This view is held from the fact that the Topographical Bureau of the War Department already possess an instrument, purchased in 1853, called an "orograph," which, involving many of the principles of Clarke and Adams' "grade delineator," is designed to give the same results, and which, on trial, has done at least as good work as the one presented for our examination.

The main points of resemblance and difference between these two machines will be briefly stated. In the orograph, the motion of the axis which revolves with the change of slope is effected by means of a float in a cistern of mercury, instead of a pendulum. This axis has two equal arms at right angles to it, lying in the same plane. The ends of these arms move in slots at right angles to each other, giving them motions proportional to the cosine and sine of the slope. The motion, proportional to the cosine, is transmitted to the roller carrying the paper by means of a friction wheel pressing against a circular disc (instead of a cone) at a varying distance from its centre, the disc being made to revolve by the perambulator. The motion proportional to the sine is given to the pencil by two friction wheels pressing against two circular dics, (instead of cones,) touching them at the same distance from the centers, when the machine is on level ground, but so adjusted that one friction wheel approaches, and the other recedes from, the center of its disc as the slope is changed. The axis of one of these