1. In carrying chronometers, the gimbals should be secured by the stay to prevent the chronometer swinging in the gimbals. Aboard ship the gimbals should be unstayed, that the chronometer bowl and gimbal ring may find their level, and the stay should be firmly secured back in its place by its thumb-screw to prevent its being accidentally shoved under, over, or into the slot in the gimbal ring.
2. In all cases when carrying or handling chronometers, it is vitally important to avoid giving them a quick circular motion in a horizontal plane, unless it be to start them after having run down. A chronometer should not be given a circular motion on its vertical axis, whilst running, of 360 degrees in less than ten seconds, or a less number of degrees in less than a proportional time.
The balance of a properly constructed chronometer vibrates, when the instrument is clean and fresh oiled, one and a quarter turns (450°), or five-eighths of a turn (225°) on each side of its point of rest. It receives an impulse from the main spring acting through the train of wheels from the escape wheel at each double vibration, occurring each half second, which is the change of time shown on the dial for the forward and backward swing of the balance. If the chronometer should receive a quick circular motion of a trifle more than three eighths of a turn to the right as the balance was about completing its vibration to the left, or vice versa, the effect would be that the escape wheel would be relieved by the lifting of the spring detent by the discharging pallet on the balance staff, at each end of the forward vibration, as well as at its centre, the balance thus receiving three impulses from the escape wheel for each double vibration, instead of one as designed. The dial would therefore show a change of one and one-half seconds instead of one-half second, as the double vibrations of two turns or more are each performed in the same time as if of one and a quarter turns each. The immediate effect on the chronometer would be to increase its error from one to fifty seconds, according to the violence and extent of the circular motion, the balance; not dropping immediately to a vibration of less than two turns, on account of the additional impulse it receives from the escape wheel.
The general after-effect is for the chronometer to lose on its previous rate from a few tenths to one second per day, presumably on account of the extra strain on the balance spring having destroyed or temporarily changed its elasticity. Chronometers generally regain their previous rates in one or two weeks, provided the balance spring is well tempered.
These results rarely occur, and not at all if persons handling chronometers use ordinary care and avoid giving them a quick circular motion on their vertical axes. A swift motion forward or sideways will not affect the error or rate of a chronometer if unaccompanied by concussions or jars. We believe that a chronometer balance is more frequently overturned by lifting the instrument from a high level, such as a shelf or table, by the strap and twisting it suddenly to a natural position at the side for carrying, or in lifting it from the side to a higher level, than in any other way.
3. Aboard ship the chronometer should always be kept in its outside case, which should be firmly screwed down amidships in a well ventilated apartment, yet free from currents of air, and should not on any account be placed near wood which is in contact with salt water. The outside case should be kept closed except in marking time and winding, in order to protect the chronometer, so far as possible, from sudden changes of temperature and from the injurious influence of salt air. It is advisable in a damp or moist climate to further protect chronometers by wrapping a heavy dry woolen blanket around the outside case, changing it as often as may be necessary to preserve its dryness. On no account should chronometers be taken from their outside cases for deck observations; for such a purpose a hack chronometer or comparing watch should be used.
The reason for these precautions is found in the fact that rust on the steel works of the chronometer is always injurious, and on the balance spring is immediately fatal to its rate. A small spot of rust on the balance spring, barely discernible, will cause a chronometer to lose from one to five seconds per day, and the error will increase as the rust eats into the spring. It is therefore necessary to protect chronometers in every possible way from the injurious effects of salt air and from dampness in every form.
If the chronometer is cold and it should suddenly be exposed to a warm damp current of air by leaving the lids of the case open, moisture will be deposited on the outside of the bowl. If the chronometer is warm and a cold current of air strikes it, moisture will be deposited on the inside of the chronometer if the dew point is reached, which, of course, depends on the hygrometric state of the air inside the bowl. If near saturation, a sudden fall of a few degrees in temperature would cause a deposit of moisture.
We would here remark that, in our experience, chronometers in use in the government service are much less liable to become rusty than are those in our mercantile marine. This is probably due to better general care, and to their being used in larger and better ventilated vessels, which are freer from bilge air and do not carry cargoes that sweat or throw off corrosive vapors. We have known cases in which vessels invariably rusted every chronometer that was used aboard in a short time, but the evil was readily cured by wrapping a dry blanket around the chronometer and changing it each day.
We believe the general custom in United States naval vessels is to place the chronometers in an outside case provided by the Navy Department, lined and padded with curled hair to protect against jars, and made to contain two or more chronometers. The outside cases provided by the makers are made in this way, and are used in the merchant service for protecting the chronometers from salt or damp air, but in the government service they are used as transporting cases.
4. Chronometers that are constructed to run fifty-four or fifty-six hours should be wound daily, at or about the same hour. Chronometers constructed to run eight days should not be wound daily, but only once a week. In winding, turn the chronometer bowl over in the gimbal slowly with the left hand, slide the valve by pressing the forefingers of the left hand against the nail-piece on the valve until the keyhole is uncovered, insert the winding key with the right hand, and wind to the left until a decided stop is felt. After removing the key, do not let the chronometer bowl swing of its own accord to its level, but let it down carefully until horizontal.
5. Chronometers should never be placed near enough to magnets, compasses or electro-magnets to be under magnetic influence.
6. To start a chronometer running after it has run down, or after it has been wedged for transportation, avoid altering the position of the hands, if possible, by waiting until they indicate the proper time, and then start it by a circular motion of about one-half a circle in the plane of the dial.
7. In transportation of chronometers overland by rail, when they are not at all times in care of a special trustworthy messenger, who understands the character of the instrument and the necessity of carrying it as described in § 2, the balance should be stayed. This can be done as follows: Stay the bowl and ring, unscrew the bezel, place the thumb and fingers of the left hand over and around the edge of the dial, but avoid touching the hands; turn the chronometer over with the right hand, when the movement will drop out of the brass bowl into the fingers of the left hand. Bring the balance to rest by a piece of light paper gently applied to the outer diameter of the balance. Then stay the balance with two thin cork wedges made of new cork slid gently underneath the rim of the balance, on opposite sides, to the right of the horizontal bar of the balance, but close to it. Avoid touching the timing screws with the wedges.
Chronometers are frequently seriously damaged by the balance having been corked for transportation by inexperienced persons. Damage may result by their using old cork impregnated with acid, thereby causing rust on the steel part of the balance, or by shoving thick lumps of cork underneath the free end of the rims of laminae of the balance with such force as to bend the laminae and sometimes the balance staff pivots. We therefore advise that whenever possible chronometers be prepared for transportation by chronometer makers. In cases where their services cannot be obtained, it can be done by others than experts by carefully observing the proper precautions.
It is important that the thin cork wedges should not be placed under the rim of laminae of the balance at the free end, but close to the horizontal bar, and that they should be shoved under with only force enough to stay in place and hold the balance steady without exerting a strong pressure against the rim of the balance or the balance staff pivots, which are only 0.0045 of an inch in diameter.
For transportation by express, chronometers should never be packed in a box as ordinary merchandise, but should be sent in their own outside or transporting cases, marked "chronometers, handle by strap with care." In this way they are treated with some degree of care by express messengers, as the character of the instrument is shown. Our experience has been that, when transported overland packed as merchandise, they are invariably damaged from rough handling received.
8. Chronometers that have been transported on land, either with the balance corked or otherwise, should be re-rated, if possible, before being sent on sea service. In cases where there is not time to reestablish rates and errors before going to sea, they can be transported, running, with but slight chance of change of rate or error, as follows: Remove the bowl from the gimbal ring by unscrewing the front pivot screw of the ring, wrap the bowl in large sheets of soft paper, and place it in a basket of rectangular shape, with soft cotton or hair packing around it, with dial upwards and as far removed from the vertical axis of the basket as possible. Send the basket by special messenger, to be carried by the handle. The boxes and gimbals of the chronometer can be forwarded otherwise. Chronometers transported running in this way will not be liable to the jars of transportation, owing to the lightness and elasticity of the basket and packing, and the balances will not be liable to overturn if the basket is given a circular motion, as they are not situated in the vertical axis of the basket.
Compensation for Temperature and the Errors of the Ordinary and other Compensation Balances.
In every chronometer with the ordinary balance, there is a temperature at which it has its greatest gaining or least losing rate, or, in other words, a temperature at which it goes the fastest.
The chronometers of our make, in use in the U.S. Navy, are compensated to have their greatest gaining rate at or about a temperature of 70° F. It probably would not vary five degrees from that point in any chronometer in good working order. In a few cases the fastest rate might be between 65° and 70°, but seldom or never above 72° F.
The average temperature correction is 0.0025 sec. multiplied by the square of the number of the degrees from the temperature at which they have the fastest rate, or calling t the number of the degrees from 70°, correction = — 0.0025 t2 seconds.
The ordinary balance is the compensation balance in general use for correcting the chronometer for changes of temperature, preventing errors from arising from the contraction or expansion of the balance spring. Without compensation the change of rate would be about: six minutes a day for a range of 60° in temperature. With but few exceptions the chronometers in use in the U. S. Navy have the ordinary balance. Many balances of other forms have been designed and made, for the purpose of obtaining a more accurate compensation for a greater range of temperature, but, thus far, all such can be considered experimental only, and have not been introduced by the makers, ourselves amongst the number, into their chronometers of commerce. Some of these modifications have shown more or less favorable results for limited periods of time in observatories, or where stationary and not exposed to the hardships and rough usage incidental to sea service or transportation.
Probably in no other direction has there been so much time and thought given in the last thirty years, and so little accomplished, as in the effort to improve on the ordinary compensation balance. Many have been invented for which great claims were made, but most of them have passed into oblivion, having been used by the maker to establish a reputation from short trials in observatories under favorable circumstances and with no thought of adopting them in the chronometers of trade.
The ordinary balance has stood the test of time, and when properly made and adjusted is more serviceable than any of the auxiliary compensations, or any of the improved forms yet invented. The former are uncertain in their action, unstable, and subject to disarrangement from various causes; the latter show sea rates widely different from the shore rates, the error being occasioned by tremors produced by the action of machinery or waves, or both, and varying in accordance with the amount of tremor thus produced. When chronometers with properly made ordinary balances fail to give satisfactory results under ordinary ranges of temperature, it is from their having been left by the maker with their fastest rate at too high or too low a temperature, either from carelessness, ignorance, or for lack of facilities for obtaining artificial temperatures for adjusting them. We frequently find chronometers, otherwise well made, that have their fastest rate as high as 120° F. or higher, or as low as 32° or lower. We adjust our chronometers to have their fastest rate as near 70° as possible, on the theory that they are not liable to be exposed to an average temperature of over twelve or fifteen degrees above that point for any great length of time, and that they can and should be protected from a temperature of more than fifteen degrees below it. In special cases, such as a voyage to the Arctic regions, they should either be specially compensated for low temperature, or have their temperature correction applied.
For ordinary cruising in the temperate or the torrid zones we think it as well to neglect the temperature corrections, except in unusual cases, where the chronometer had been exposed to great extremes of heat or cold for a considerable length of time between ratings. In all cases chronometers should have their errors corrected, and their rates re-established at every convenient opportunity.
Of the chronometers in service at sea throughout the world, probably 99.99 per cent, have the ordinary compensation balance. But few of the many thousand chronometers of various makes that have passed through our hands in the course of forty years' business have been of different form, or with auxiliary attachments, and have in all cases been the most unreliable, showing errors greater than those due to the ordinary balance.
On Cleaning.
Chronometers should be cleaned and fresh-oiled every three and a half years, and sooner if they show unsteadiness in their rates, having- previously been regular. This would prove, either that the oil had dried or gummed, and that the pivots were cutting, or that there was rust on the steel works, and in such cases the chronometer should have the rust removed, and be cleaned and oiled as soon as possible.
There are many cases of chronometers performing well for five and six years, or even longer, without cleaning or oiling, but such cases are exceptional and should not be thought to establish a rule. Chronometers ARE THE BEST CARED FOR AND GIVE THE BEST RESULTS WHEN THEY ARE CLEANED AND OILED BEFORE THEY ABSOLUTELY REQUIRE IT, for the following reason: if they are allowed to run until the oil becomes gummed or dried, the arc of vibration of the balance, and consequently the action of the balance spring, becomes reduced from the increased resistance and wear of the pivots. The damage to the mechanical action can be repaired by re-polishing the pivots, re-bouching the holes, inserting new jewels, etc., but the balance spring receives injury, varying with the length of time it has been running below its normal action. This is shown after the chronometer has been cleaned and repaired, and the balance and balance spring restored to their original action, by its not settling to any permanent rate for a long time, and if a largely reduced action of the spring has been allowed to exist for a long time, the rate may never become permanent. In this case the only remedy is a new balance spring, which is expensive, requiring as it does a total readjustment of the chronometer for compensation for heat and cold, isochronism, etc., and with the disadvantage, moreover, that a chronometer with a new balance spring cannot be depended upon for two or three years. All new chronometers or chronometers with new balance springs have a tendency to gain on their rates if the spring is properly tempered.
We find this to be almost invariably the result when chronometers have been allowed to run for a long time with a much decreased vibration. Our theory of the bad defects shown after cleaning is that the elasticity and molecular cohesion of the balance spring had accommodated themselves to the reduced vibration, and that this adjustment was disarranged on the spring being restored to a largely increased action, by the molecules rearranging themselves in the new action suddenly forced upon them. This is more particularly shown in trials in changes of temperature. These defects are not shown equally before cleaning, as the decrease from a large to a small vibration is very gradual, being almost imperceptible between rates and partly compensated by the isochronism of the balance spring.
This injury from the chronometer running too long without cleaning or oiling is, in our opinion, frequently the cause of remark so often heard from shipmasters—"that their chronometer ran well for six years (or more) without cleaning, but since it had been cleaned by Mr. --- it had never run well";—not realizing that it was their own negligence, in allowing it to run so long, that caused its subsequent bad performance.
On Oils.
The question of oil for a chronometer is a serious one, as the performance of the best made and best adjusted chronometer depends finally on the oil used.
The general custom among chronometer makers is to purchase oil from dealers without due regard to its nature and fitness for the purpose intended, discovering its unfitness only by the failure of the chronometer to which it is applied. Realizing thirty years ago the injury that might accrue to the reputation of our chronometers from uncertain oil, we resolved never to use any except such as we prepared ourselves from carefully selected crude material. When it is considered that there is less than one drop of oil divided among the ten least or smallest pivots and bearings of the chronometer, and that the balance, weighing about seven pennyweights, makes 126,144,000 vibrations of 450° each yearly on pivots the diameter of which is only 0.0045 in., to which not more than a thirtieth of a drop of oil can be applied, the importance of cleaning and oiling at least every three and a half years is evident. We dwell on this subject, inasmuch as persons owning or using chronometers are, as a rule, disposed to let them run for an indefinite length of time without cleaning or oiling, seldom giving the subject any consideration, unless their attention is called to it, and frequently allowing them to run until they stop from the pivots having been cut off. This is not fair or just to the chronometer, to the owner, or to the reputation of the maker.
The oil we use is obtained from the jaw of the porpoise (those caught off Cape Horn being the best), but is mixed with other oils to give it body when necessary. We procure this by urgent appeals to our seafaring friends, who catch the porpoise, extract the oil from the jaw only, and forward it to us before it becomes rancid, when we prepare it by methods of our own.
Oil suitable for chronometers should have the following qualities:
First. Freedom from acid or alkali.
Second. Fluidity and lubricating properties to allow freedom of action of the pivots in their bearings, while possessing sufficient consistency to stay where placed without spreading or running away.
Third. The property of not congealing at a very low temperature or of becoming too thin at a high one.
Fourth. The property of retaining its fluidity and lubricating power under the effects of friction, and of not gumming or drying up for a reasonable length of time.
To obtain these requisites for some of the pivot actions, we are frequently compelled to resort to a judicious combination of thick and thin oils.