as accurately as possible above the surface of the liquid. If the readings in a dark-colored liquid are always made in the same way, the resulting error will not be great, and successive readings will be comparable.

7. Influence of Temperature

When the temperature of a liquid changes, its specific gravity also changes and the indication of a hydrometer in the liquid will therefore be different at different temperatures. All hydrometers should be marked with the temperature at which they are intended to be correct.

In actual practice, however, it is not always necessary that the hydrometer be used at its standard temperature, but if readings are to be compared, they must all be made at the same temperature. For example, in practice it may be found by experience that a certain sirup has the proper consistency when cool, if the reading on the hydrometer is 1.36 at 80° F. The standard temperature of the instrument may be 60° F, but if experience has shown that a certain reading of the hydrometer at some other temperature gives satisfactory results, it is not necessary to wait for the liquid to cool to the standard temperature of the instrument.

XI. TIME

1. Types of Household Clocks

The usual clocks found in the home may be divided into two classes, the mantel or wall clock type, which has a pendulum, and the common alarm-clock type, in which the movement is controlled by the vibrations of a balance wheel, as in a watch. The second variety is quite portable and will usually run in any position, but the pendulum clock must be kept fixed in an upright position and must be adjusted every time it is moved.

2. Moving a Pendulum Clock

The pendulum clock usually has its pendulum suspended by a thin flat spring, and to avoid breaking this spring when the clock is to be moved from one place to another it is best either to unhook the bob from the pendulum rod or to secure the pendulum tightly to the clock works or case so that it can not swing. In setting up such a clock after removal it is necessary to put the clock "in beat"; that is, to make the successive vibrations of the pendulum, or the time between successive ticks of the clock, of equal length. This must be done by carefully leveling the clock on its support (unless the clock is provided with adjusting thumbscrews at the top of the pendulum by the movement of which one way or the other it can be made to beat uniformly).

3. Setting a Clock

The setting of a pendulum clock is usually best done by turning the minute hand forward, several revolutions if necessary, to bring the hour hand to the correct hour. If the clock does not have a striking mechanism, the hour hand, which is usually held on its slightly conical shaft by friction, may be moved forward a sufficient number of hours, and the minute hand adjusted to the correct minute. As the hour hand may have become loosened on its shaft by this procedure, however, it should be pressed tightly into place after it is set correctly. In some clocks with a striking

mechanism the minute hand should not be moved backward across a striking point, although it can, without injury, be moved back short distances in other parts of the dial to set it correctly.

4. Regulating a Clock

Few clocks of either the pendulum type or the alarm-clock type are made with devices to compensate for changes in temperature, and as changes in temperature will change the rate it is desirable to keep the clock in the part of a room where its temperature will be most constant. Even with the best conditions in this respect it will be necessary to regulate the clock's rate frequently on account of the changes of temperature with season or with the conditions of heating or cooling of the room. A rise of temperature will lengthen the pendulum rod and make the clock run more slowly. It will be necessary, therefore, to raise the pendulum bob by turning the supporting nut, unless an adjustment device is provided by which a contact point on the suspension spring at the top of the pendulum can be changed. This is done by turning a key to right or left in a small keyhole in the face of the clock, usually near the upper part of the dial. This has the effect of shortening or lengthening the pendulum.

In the alarm-clock type the regulation is done by moving a small lever, usually at the back of the clock, which engages with the hairspring on the balance wheel, and so decreases or increases the effective length of the spring, thus controlling the time of a vibration of the balance. The lever should be moved toward the letter "S" when one wishes to make the clock run slower and toward "F" when it should run faster. The same rule applies in the regulation of a watch.

When regulating a pendulum clock (see Fig. 58) by the key device, the key should be turned overhand toward the letter "S" or "F" according as one wishes to make the clock run slower or faster. If there are no indicating letters ("F" and "S") provided, the usual rule is to turn the key in the direction the hands move to make it go faster or counterclockwise to make it run slower.

The amount of movement required to correct the rate must generally be found by trial. Thus, if the clock gains five minutes a day, and one turns the key of the regulator two revolutions toward

"S," or moves the lever of an alarm clock two divisions toward "S," and the clock then loses three minutes a day, one can obtain nearly zero rate by turning the key three-quarters of a revolution back toward "F" or by moving the lever three-quarters of a division back toward "F." In some pendulum clocks there may be some motion lost in reversing the regulation, and this should be taken into account in estimating the amount to move the regulator.

5. To Correct the Striking of a Clock

While some clocks of a more recent type have the hour and minute pinions and the striking mechanism so geared together that it is almost impossible for the clock to strike wrongly, this frequently happens with other types of clocks. This difficulty can be easily remedied in the latter case by several methods. One method, which can be used in case the hour hand is held in position on its shaft by friction only, is to move the hour hand backward or forward an hour or more as may be necessary to make the hour indicated by the clock face agree with the striking mechanism, pressing the hour hand tight on its shaft afterwards, as described above. Then set the clock to correct time by moving the minute hand around the dial the necessary number of times, allowing the clock to strike the full amount each time the hand passes the XII point before approaching that point again. This method is especially convenient when the clock strikes one or two strokes less than it should. When it strikes more strokes than it should, the same method can be used, or the minute hand can be turned ahead rapidly so that it will again pass through the XII point while the clock is still striking for the previous hour. By so doing the striking mechanism is not released to strike the following hour, and thus an hour is gained in the face indication of the clock compared with the striking. This can be repeated as many times as the number of strokes by which the striking mechanism was in error. The clock can then be set to correct time in the usual way, allowing it to strike the full amount on each passage of the XII point, or the clock can be stopped for as many hours as it is fast, until it again indicates the correct hour, when it can be started and set correct without the necessity of striking all the 9, 10, or 11 hours that may have intervened.

FIG. 58. Rear view of a common pendulum clock movement Showing the hook H at which the pendulum bob may be unhooked for transportation, and the loop L through which the pendulum rod passes and in which the rod should be adjusted when the clock is set up if the loop is not closed. The nut N when turned to the right or left raises or lowers the bob and makes the clock go faster or slower. The adjusting device A is lowered or raised along the thin piece of steel below by means of the screw S operated from the front of the clock by a key and effects the fine adjustment of the rate of the clock. The striking mechanism can be released by raising the pin P or by lifting the rods at the points marked R, by doing which the clock can be made to strike correctly.