The standard measure for the British empire is a brass rod, into which two pins of gold are inserted; the upper surfaces of these are sunk to the half thickness of the bar; and a small dot is made in the middle of each. The distance between the centers of these 'dots, taken when the temperature is at 62°Fahr., is declared to be the true yard.
In the same way as the standard of measure so must the standard of weight be estab lished. A piece of heavy metal is made of the desired weight, and is duly authenti cated. The preservation of the standard of weight is a matter of very considerable difficulty. Every occasion on which it is used, each removal of dust from its surface, the actions of the oxygen of the air and of the products of combustion which are always floating about, produce a sure though slow waste; and all that can be done is to retard this waste as much as possible. Perhaps a lump of platinum would make the best standard; but its softness is a decided objection.
In the use of a standard of weight another matter has to be taken into consideration. The apparent weight of any substance is less than its true weight by the weight of as much air as is displaced by it. N ow, the density of the air is not constant—air, when warmed, expands very much more than any solid body; and therefore a piece of metal appears to weigh more in warm than in cold weather. Not only so, air is rendered more dense by an increase of pressure, and so, when the barometer is high, all heavy bodies become apparently lighter.; when the barometer sinks they appear to become heavier. Thus the apparent weight of the standard pound is continually changing. If we accu rately adjust two weights of brass when the barometer is low and the air warm, and afterward compare them when the barometer is high and the weather cold, we can per ceive no change, for, though each has lost weight, they have lost alike. But if we had adjusted a weight of iron to a weight of platinum in light air, and again compared them in dense air, the change would have been at once seen. For, since a pound of iron is more bulky than a pound of platinum it displaces more air, and its apparent weight undergoes a greater change than does that of the platinum. Fortunately, these changes are too small to have any perceptible influence on mercantile transactions, yet they are sufficient to create the necessity for it being enacted that the standard weight must be held as true when the air is in a specified state as to warmth and pressure. The stand
and brass pound, which serves for the British empire, is to be used when Pahrenhcit's thermometer is at 62°, and the barometer is at 30 inches. (See note at the end.) The thought naturally arises, what if, in the course of time, the original standards be lost or destroyed? Tithe was when a seed of wheat gathered from a well-ripened ear served sufficiently well to define a grain weight; and even now the eastern jewelers weigh their gems against the sand or carob-bean, the hardness and uniformity of which seem to justify the selection of it. But for the extended purposes of modern commerce, and particu larly for the more delicate requirements of scientific research, it is indispensable that we lInd some unchanging object of comparison; and none can be preferred to the earth itself as the most universally acceptable and as the best defined. For the purposes of geogra phers and navigators, the circumference of the earth is divided into degrees and minutes, the length of one minute being the geographical or nautical mile; and it certainly would have been convenient if the common or statute mile had agreed with this. The dimen sions of the earth are now known with a precision far greater titan is needed for ordinary purposes; the entire length of the circumference of a meridian circle being 131,236,000 of our standard feet, so that the length of a nautical mile is 6,075 ft. and about 9 in.; and it is highly probable that subsequent and more accurate measurements will not alter this determination more than an inch or two either way. It is usual to divide the minute into 60 seconds, so that a second of the earth's circumference is 101.25, and thus if our standard foot had happened to be one-eightieth part longer than it is, there would have been exactly 100 ft. in a second, and 0,000 ft. in a nautical mile. When we reflect on the disparity of the foot used by different nations, and recollect that 100 Vienna ft. make 103.6 English, as many Amsterdam feet 92.7, as many Berlin feet 99.2, we can hardly help regretting that our forefathers had not happened to hit upon the exact 100.