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Rh not an easy thing to secure this result, for as the machines are used the cutting edges wear down and require regrinding and resetting. Hence a tool is not allowed to make more than a given quantity of parts without being examined and readjusted, and from time to time the pieces being put out are tested with callipers. The parts thus made are put in groups and sorted into boxes, which are then given over to the watch-adjusters, who put the parts together and make the watch go. The work of adjustment for common watches is a simple matter. But expert adjusters select their pieces, measure them and correct errors with their tools. The finest watches are thus largely machine-made, but hand-finished. The prejudice against machine-made watches has been very strong in England, but is dying out—not, unfortunately, before much of the trade has been lost. A flourishing watch industry exists in Switzerland in the neighbourhood of Neuchatel. A watch in a stamped steel case can now be made for about five shillings. There is no reason why in such a neighbourhood as Birmingham the English watch industry should not revive.

The use of jewelled bearings for watch pivots was introduced by Nicholas Facio about the beginning of the 18th century. Diamonds and sapphires are usually employed and pierced either by diamond drills or by drills covered with diamond dust. Rubies are not a very favourite stone for jewels, but as they and sapphires can now be made artificially for about two shillings a carat the difficulty of obtaining material for watch jewelling has nearly disappeared.

Watches have also been fitted with machinery whereby electric contacts are made by them at intervals, so that if wires are led to and away from them, they can be made to give electric signals and thus mark dots at regular intervals on a moving strip of paper.

As in the case of clocks, the accuracy of going of a watch is estimated by observation of the variations of its mean daily rate. This is officially done at Kew Observatory, near Richmond, and also for admiralty purposes at Greenwich. At Richmond watches are divided into two classes, A and B. For an A certificate the trials last for forty-five days, and include tests in temperatures varying from 40° to 90° F., going in every position with dial vertical, face up and face down. The average daily departure from the mean daily rate, that is the average error due to irregular departures from the average going rate, must not exceed 2 seconds a day except where due to position, when it may amount to 5 seconds. The errors should not increase more than 0·3 seconds a day for each 1° F. The trial for the B certificate is somewhat similar but less severe. Chronometers are put through trials lasting 55 days, and their average error from mean rate is expected not to exceed 0·5 seconds per diem. The fees for these tests are various sums from two guineas down-Wards. In estimating the time-keeping qualities of a watch or clock, the error of rate is of no consequence. It is simply due to the timekeeper going too fast or too slow, and this can easily be corrected. What is wanted for a good watch is that the rate, whatever it is, shall be constant. The daily error is of no account provided it is a uniform daily error and not an irregular one. Hence the object of the trials is to determine not merely the daily rate but the variations of the daily rate, and on the smallness of these the value of the watch as a time-keeper depends. 

WATER. Strictly speaking, water is the oxide of hydrogen which is usually stated to have the formula H2O (see below), but in popular use the term is applied to a great variety of different substances, all of which agree, however, in being the water of the chemist modified differently in the several varieties by the nature or proportion of impurities. In all ordinary waters, such as are used for primary purposes, the impurities amount to very little by weight—as a rule to less than th of 1%.

Of all natural stores of water the ocean is by far the most abundant, and from it all other water may be said to be derived. From the surface of the ocean a continuous stream of vapour is rising up into the atmosphere to be recondensed in colder regions and precipitated as rain, snow or sleet, &c. Some ths of these precipitates of course return directly to the ocean; the rest, falling on land, collects into pools, lakes, rivers, &c., or else penetrates into the earth, perhaps to reappear as springs or wells. As all the saline components of the ocean are nonvolatile, rain water, in its natural state, can be contaminated only with the ordinary atmospheric gases—oxygen, nitrogen and carbon dioxide. Rain water also contains perceptible traces of ammonia, combined as a rule, at least partly, with the nitric acid, which is produced wherever an electric discharge pervades the atmosphere.

Lake waters, as a class, are relatively pure, especially if the mountain slopes over which the rain collects into a lake are relatively free of soluble components. For example, the water of Loch Katrine (Scotland) is almost chemically pure, apart from small, but perceptible, traces of richly carboniferous matter

taken up from the peat of the surrounding hills, and which impart to it a faint brownish hue, while really pure water is blue when viewed through a considerable thickness.

River water varies very much in composition even in the same bed, as a river in the course of its journey towards the ocean passes from one kind of earth to others; while, compared with spring waters, relatively poor in dissolved salts, rivers are liable to be contaminated with more or less of suspended matter.

Spring waters, having been filtered through more or less considerable strata of earth, are, as a class, clear of suspended, but rich in dissolved, mineral and organic matter, and may also contain gases in solution. Of ordinarily occurring minerals only a few are perceptibly soluble in water, and of these calcium carbonate and sulphate and common salt are most widely diffused. Common salt, however, in its natural occurrence, is very much localized; and so it comes that spring and well waters are contaminated chiefly with calcium carbonate and sulphate. Of these two salts, however, the former is held in solution only by the carbonic acid of the water, as calcium bicarbonate. But a carbonate-of-lime water, if exposed to the atmosphere, even at ordinary temperatures, loses its carbonic acid, and the calcium carbonate is precipitated. The (q.v.) which adorn the roofs and sides of certain caverns are produced in this manner. Many waters are valuable medicinal agents owing to their contained gases and salts (see ).

In addition to its natural components, water is liable to be contaminated through accidental influxes of foreign matter. Thus, for instance, all the Scottish Highland lochs are brown through the presence in them of dissolved peaty matter. Rivers flowing through, or wells sunk in, populous districts may be contaminated with excrementitious matter, discharges from industrial establishments, &c. The presence of especially nitrogenous organic matter is a serious source of danger, inasmuch as such matter forms the natural food or soil for the development of micro-organisms, including those kinds of bacteria which are now supposed to propagate infectious diseases. Happily nature has provided a remedy. The nitrogenous organic matter dissolved in (say) a river speedily suffers disintegration by the action of certain kinds of bacteria, with formation of ammonia and other (harmless) products; and the ammonia, again, is no sooner formed than, by the conjoint action of other bacteria and atmospheric oxygen, it passes first into (salts of) nitrous and then nitric acid A water which contains combined nitrogen in the form of nitrates only is, as a rule, safe organically; if nitrites are present it becomes liable to suspicion; the presence of ammonia is a worse symptom, and if actual nitrogenous organic matter is found in more than microscopic traces the water is possibly (not necessarily) a dangerous water to drink.

All waters, unless very impure, become safe by boiling, which process kills any bacteria or germs that may be present.

Of the ordinary saline components of waters, soluble magnesium and calcium salts are the only ones which are objectionable sanitarily if present in relatively large proportion. Calcium carbonate is harmless; but, on the other hand, the notion that the presence of this component adds to the value of a water as a drinking water is a mistake. The farinaceous part of food alone is sufficient to supply all the lime the body needs, besides, it is questionable whether lime introduced in any other form than that of phosphate is available for the formation of, for instance, bone tissue.

The fitness of a water for washing is determined by its degree of softness. A water which contains lime or magnesia salts decomposes soap with formation of insoluble lime or magnesia salts of the fatty acids of the soap used. So much of the soap is simply wasted; only the surplus can effect any detergent action. Several methods for determining the hardness of a water have been devised. The most exact method is to determine the lime and magnesia gravimetrically or by alkalimetry; or by Clark's soap test, but this process frequently gives inaccurate results. In this method, which, however, is largely used, a measured volume of the water is placed in a stoppered bottle, and a standard solution of soap is then dropped in from a graduated vessel, until the mixture, by addition of the last drop of soap, has acquired the property of throwing up a peculiar kind of creamy froth when violently shaken, which shows that all the soap-destroying components have been precipitated. The volume of soap required measures the hardness of the water. The soap-solution is referred to a standard by means of a water of a known degree of hardness prepared from a known weight of carbonate of lime by converting it into neutral chloride of calcium, dissolving this in water and diluting to a certain volume. The hardness is variously expressed. On Clark's scale it is the grains of calcium carbonate per gallon of 70,000 grains; in Germany the parts of lime per 100,000 of water, and in France the parts of calcium carbonate per 100,000. 