17,550 17,828 17,194 17,238 17,191 17,168 17,464 17,160 17,115 17,717 16,900

69

454

60

564

43

48

70 8

72 8

72 8

74 8

66 8

526

70 8

64 8

64 7 52 6

68 8

63 7

74 8 75 10 75 10 80 10 68 8 72 9 72 9 60 8 68 8 70 7 72 9 60 8 17,160 16,673 17,403 17,316 17,400 15,600 12,480

50 444 524 48 60

60

60

60

60

60 441

If we divide double the product of all the four wheels by the product of all the three pinions, the quotient will be the number of beats, as given in any of the trains contained in this table; also, if we take the second and third wheels, and their pinions respectively, as a compound fraction of an hour, they will give the seconds in which the contrate-wheel, attached to the latter pinion, will revolve; thus, of 3 of 3 of 60m=1" or 60′, which numbers

are consequently proper for a watch that indicates the seconds; and if the beats be 18,000, or 14,400, there will be five or four beats respectively in a second, which are the best trains for measuring fractional parts of a second.

CHRONOMETERS.

CHRONOMETERS differ from an ordinary watch principally in the escapement and balance. These machines deserve more than usual attention, as well from their practical utility in navigation, as from the principles on which they are constructed, in which the irregular forces both of impulse and resistance are greatly diminished by the exactness of form and dimension.

In the reign of queen Anne, the British parliament passed an act, offering a reward of 10,000l. for any method of determining the longitude within the accuracy of one degree of a great circle; of 15,000l. within the limit of forty geographical miles; and of 20,000l. within the limit of thirty such miles, or half of a degree; provided such method should extend more than eighty miles from the coast. The hope of obtaining this reward stimulated a watch-maker named Harrison to be indefatigable in his endeavours to effect the required improvement, which eventually led him to apply the principle of the apposite expansions of different metals to a watch to effect a self-regulating curb, for limiting the effective length of the spiral pendulum-spring to correspond to the successive changes of heat and cold, which changes were now known to alter the force of this spring, and the momentum of the balance.

After Harrison had by his industry and perseverance obtained the large reward, the act was repealed, and another substituted, offering separate rewards to any person who should invent a practicable method of determining, within circumscribed limits, the longitude of a ship at sea; for a time-keeper, the reward held forth to the public is 5,0007. for determining the longitude to or within one degree; 7,5007. for determining the same to forty geographical miles; and 10,000l. for a determination at or within half a degree. This act, notwithstanding its abridged limits and diminished reward, has produced several candidates; of whom Mudge, the two Arnolds, and Earnshaw, have had their labours crowned with partial success.

Although, in respect to Mudge's time-keeper, great expectations were at first raised, it has, from the complexity of the machinery, and consequent expense attendant upon making it, gradually fallen into disrepute, and is now seldom ever made. Such of our readers who wish to see its

or

manner of construction and performance, we must refer to "The Description of Mr. Mudge's Time-keeper," published in 1799, by Thomas Mudge, jun.

The chronometer we purpose to lay before our readers is that constructed by Mr. Earnshaw, as we are strongly disposed to conclude, from various documents we have seen, and from the similarity so evident in the construction of the escapement, that Mr. Arnold derived the knowledge of his principle from Mr. Earnshaw.

In Mr. Earnshaw's chronometer the escapement is detached, which is the best for the equal measurement of time, because the vibrations of the balance are free from the friction of the wheels, excepting about one-twelfth part of the circle, while the scape-wheel is acting on the pallet to keep up the motion of the balance, which is done with considerably more power and less friction than by any other escapement, as it receives but one blow from the wheel, whilst other escapements receive two; it has also an equal advantage of the same quickness of train, and when the impulse is given to the balance by the wheel, it is given in a similar direction, and not in opposition, as most escapements are which produce a recoil.

The pivots of the balance-axis should be the size of the verge-pivots of a good sized pocketwatch, and of the annexed shape, which will greatly add to their strength, the extreme end, or acting part, only being straight; the jewel-hole should be as shallow as possible, so as not to endanger cutting the pivot, and the part of the action of the hole made quite back, with only a very shallow chamber behind to retain the oil; deep holes are very bad, for when the oil becomes glutinous, it will make the pivots stick, so as to prevent the balance from its usual vibration. The pallet should be half the diameter of the wheel, or a little larger, for if smaller, or one-fourth the diameter, as is the case in Arnold's, the wheel will have too much action on it, which will increase friction most considerably, and likewise cause the balance to swing so much farther to clear the wheel; consequently, a check in the motion of the balance may stop the watch, and cause time-keepers so constructed to stop. The face of the pallet should run in a line of equal distance between the centre of the pallet and its extremity, and not in a right line to its centre, as this causes an increase of friction, and a loss of that power which is obtained by the wheel, acting on the extremity of the pallet. The scape-wheel teeth should form the same direction as the face of the pallet, under-cut for

the purpose of avoiding friction, and maintaining the power, and for safe unlocking. The points of the wheel-teeth must not be rounded off, but left as sharp as possible. The pivots of the scape-wheel are to be a very little larger than the balance-pivots.

The wheel is locked by a spring, instead of a detent with pivots, as the French have made them; for those pivots must have oil, and when the oil thickens, the spring of the pivotdetents become so affected by it, as to prevent the detent from falling into the wheel quick enough, which causes irregular time, and ultimately a stoppage of the watch.

When the spring is planted on the side of the wheel, the part on which the wheel rests should be a little short of a right angle, so that the wheel may have a tendency to draw the spring into it; for if sloped the other way, or beyond a right angle, it will have a tendency to push the spring out, in which case the wheel will have liberty to run. The wheel should take no more hold on the spring than just sufficient to stop it, otherwise the friction will be increased. The small return-spring should be as thin as possible at the end fastened to the other spring, but at the outer end a little thicker; the spring should be planted down as close to the wheel as to be just free of it: the discharging pallet about one-third, or near one-half the size of the large or main pallet, the face of it in a right line to the centre, the back of it a little rounding off from the centre. Great care must be used, in taking off the edges of this discharging piece to make it round, to prevent cutting the spring, nor can it be made too thin, provided it does not cut; the end of it nearest the balance should be a little more out from the centre of the balance-axis than the lower part of it towards the potence, for counteracting the natural tendency of the spring downwards from the pressure of the scape-wheel; and that part of the spring on which the wheel rests should be sloped a little down, to give the wheel a tendency to force it up, to counteract the natural inclination which the wheel has to draw it down by its pressure on it.

The balance is to be made of the best steel, and turned from its own centre to the proper size, and then put into a crucible with as much of the best brass as will, when melted, cover it. The brass will adhere to the steel, and when set, is to be turned to its proper thickness, and hollowed out, so as to leave the steel rim about the thickness of a repeating-spring to a small sized repeating-watch. The brass is to be turned to near twice or three times the thickness of the steel; cross