pansion. It became, therefore, necessary to determine the expansion of the pendulum by direct experiment, instead of adopting the conclusions of others, and for this purpose the following method was used: A trough of deal was made of a length sufficient to receive the bar intended for the pendulum, which was placed edgewise in the middle of the trough, being secured at one end by wedges on both sides. The bar was supported on small pieces of glass tube, serving as rollers to prevent friction, and the trough was of the same depth as the width of the bar. Two transverse lines were drawn near the extremities of the edges of the bar, distant from each other 49,5 inches, and a third line was subsequently drawn one inch beyond. The microscopes were placed over the lines, and left, together with a thermometer, for twenty-four hours previous to the experiment. The temperature being then registered, and the microscopes having been examined, to see that the lines bisected the angles formed by the spiders threads, the trough was filled with hot water to the edge of the bar, and two thermometers were placed in it, one just beneath the surface of the water, and the other at the bottom of the trough. The bar rapidly expanded, and the line on it was followed by the micrometer till it became stationary. The bisection was then perfected, and the mean of the degrees shown by the thermometers registered, together with the number of revolutions and parts made by the micrometer. The whole was now suffered to remain till the temperature had become several degrees lower, when the contraction of the bar, occasioned by such decrease of temperature, was measured; and thus several successive observations were made. The mean, which was ,000009959, may be taken as the the expansion of the pendulum in parts of its length due to a change of temperature of one degree of the ther mometer. Of the Method of deducing the Length of the Pendulum vibrating Seconds. The distance between the knife edges was taken when the standard scale' and the pendulum were both of the same temperature; and as this temperature did not differ considerably from 62°, the difference in the rate of the expansion (if any) between the pendulum and the scale may be neglected as perfectly insensible, and 62° be considered as the temperature of measurement. The number of vibrations made by the pendulum in twenty-four hours raving been determined at a different temperature, the length of the pendulum will be greater or less as the temperature of observation exceeds or falls short of 620; and by applying the expansion due to such difference of temperature, derived from the experiments contained in the preceding article, the distance of the knife edges, or length of the pendulum, will be known for the temperature at which the number of vibrations was determined, whence the length of the pendulum vibrating seconds may be readily deduced, the lengths of pendulums being to each other inversely in the duplicate ratio of the number of their vibrations in twenty-four hours. Of the Correction of the Buoyancy of the Atmosphere. The length of the pendulum thus found, differing from what it would have been had the vibrations been made in vacuo, it is necessary to apply to it a correction for the buoyancy of the atmosphere. For this correction, the weight of the pendulum, com pared pared with that of air, at the time of observation, must be known. The pendulum being composed of different kinds of brass, the specific gravity of each part was carefully determined, and from thence the specific gravity of the whole mass. Part of the Pendulum. Weight in Air. Specific Gravity. lb. 8,417 7,816 .3,30 8,532 Three weights (cast brass)....3,14 From the above data, the specific gravity of the pendulum is 8,469; or the weight of the pendulum compared with water is as 8,469 to 1. It has been determined by Sir George Shuckburgh, (Phil. Trans. for 1777,) that water is 836 times heavier than air, when the thermometer is at 53°, and the barometer at 29,27 inches. But the specific gravity of air varies directly as the height of the barometer, and inversely as its expansion, which is known to be one-four hundred and eightieth of its bulk for each degree of Fahrenheit; consequently, for any other state of the barometer and thermometer, the number 836 will vary inversely as the height of the barometer, and directly onefour hundred and eightieth part of each degree of the thermometer above 53°. Thus the specific gravity of water, compared with that of air, may be known for the temperature and altitude of the barometer at the time of observation; and multiplying this by the specific gravity of the pendulum, the ratio of the weight of the pendulum compared with that of air will be obtained. This ratio will express the diminution of the force of VOL. XXXIV.-SECOND SERIES. X gravity gravity arising from the buoyancy of the atmosphere; and, in order that the number of vibrations may be the same in vacuo as in air, the length of the pendulum must be increased in the proportion of this ratio to 1, the lengths of pendulums vibrating in the same time, varying directly as the force of gravity. TO BE CONCLUDED IN OUR NEXT. Description of a self-adjusting Crane. With a Plate. From the TRANSACTIONS of the SOCIETY for the Encouragement of ARTS, MANUFACTURES, and COMMERCE. The Gold Isis Medal was voted to Mr. JAMES JONES for this Communication. IN Na country so entirely dependant on its commerce as Great Britain, every means of facilitating its operations becomes of the highest importance, and every improvement, however small, that can be effected in the agents necessarily employed, must, I think, merit the attention of those who are most materially interested in its prosperity. Impressed with this belief, and earnestly desirous of contributing my mite to the general stock of knowledge, I have lately directed my attention to a subject of some importance in the removal of goods, namely, Cranework; and the result of my consideration I now respectfully submit to the Society for the Encouragement of Arts, Manufactures, and Commerce, for their inspection. The defect usually attributable to cranes is, that it is necessary to pass through as much space to raise a light load as a heavy one, unless an alteration be made in the relative relative velocities of the power and the load, by manually changing some wheel or pinion. To obviate this objection (which on public wharfs in particular, when almost every succeeding load varies in weight from the one last raised, is a very material one) was the purpose I had in view, and as I conceived that cranes possessing the property of spontaneous regulation would be a near approach to perfection, my attention was more particularly directed to that point, that is, to preserve at all times an equilibrium between the power and the weight, as nearly as possible, without much additional care or trouble to the labourer employed; and to effect this desirable purpose, I have endeavoured so to contrive the parts of a crane as that the attendant should scarcely be aware of the difference of the weights he is raising, but by the greater or less time required in the operation, the intensity of exertion remaining in all cases the same. As the means by which I raise the load are different from those which are usually employed in cranes, that is, by the Universal Lever, it may not be improper (previous to detailing the manner of application) to describe it, as there are doubtless many persons unacquainted with the instrument so called. The Universal Lever consists of a large vertical ratchet-wheel attached to a barrel round which a rope winds, drawing up the weight. Immediately over the wheel in the same vertical plane is fixed a lever of the first order, the end of the shorter arm being directly over the axis of the wheel; there are two rods of iron jointed to the end of the lever, one terminating in a broad flat point, and the other in a broad hook; if the longer arm of the lever is depressed, the hook attached to the short end having taken hold on a tooth on one edge of the wheel, draws that side of the wheel upwards, whilst the broad point passes freely over, the teeth on the |