For seven years, the magnets suspended in this building were constantly watched every two hours—every even hour—day and night, except on Sundays, the object being that some light might be thrown upon the laws regulating the movements of the mariner's compass; hence, that while men became wiser, navigation might be rendered safer. The chief observer—the genius loci—is Mr. Glaisher, whose name figures in the reports of the Register-General. He, with two assistants, from year to year, went on making these tedious examinations of the variations of the magnets, by means of small telescopes, fixed with great precision upon pedestals of masonry or wood fixed on the earth, and unconnected with the floor of the building, occupying a position exactly between the three magnets. This mode of proceeding had continued for some years with almost unerring regularity, and certain large quarto volumes full of figures were the results, when an ingenious medical man, Mr. Brooke, hit upon a photographic plan for removing the necessity for this perpetual watchfulness. Now, in the magnet-house, we see light and chemistry doing the tasks before performed by human labor; and doing them more faithfully than even the most vigilant of human eyes and hands. Around the magnets are cases of zinc, so perfect that they exclude all light from without. Inside those cases, in one place, is a lamp giving a single ray of prepared light, which, falling upon a mirror soldered to the magnet, moves with its motions. This wandering ray, directed toward a sheet of sensitive photographic paper, records the magnet's slightest motion! The paper moves on by clock-work, and once in four-and-twenty hours an assistant, having closed the shutters of the building, lights a lantern of yellow glass, opens the magnet-boxes, removes the paper on which the magnets have been enabled to record their own motions, and then, having put in a fresh sheet of sensitive paper, he shuts it securely in, winds up the clock-work, puts out his yellow light, and lets in the sunshine. His lantern glass is yellow, because the yellow rays are the only ones which can be safely allowed to fall upon the photographic paper during its removal from the instrument, to the dish in which its magnetic picture is to be fixed by a further chemical process. It is the blue ray of the light that gives the daguerrotypic likeness—as most persons who have had their heads off, under the hands of M. Claudet, or Mr. Beard, or any of their numerous competitors in the art of preparing sun-pictures, well know.
Since the apparatus of Mr. Brooke for the self-registration of the magnetic changes has been in operation at Greenwich, the time of Mr. Glaisher and his assistants has been more at liberty for other branches of their duties. These are numerous enough. Thermometers and barometers have to be watched as well as magnets. To these instruments the same ingenious photographic contrivance is applied.
The wooden building next to the magnet-house on the southwest contains a modification of Mr. Brooke's ingenious plan, by which the rise and fall of the temperature of the air is self-registered. Outside the building are the bulbs of thermometers freely exposed to the weather. Their shafts run through a zinc case, and as the mercury rises or falls, it moves a float having a projecting arm. Across this arm is thrown the ray of prepared light which falls then upon the sensitive paper. Thus we see the variations of the needle and the variations in heat and cold both recording their own story, within these humble-looking wooden sheds, as completely as the wind and the rain are made to do the same thing, on the top of the towers of the Observatory. The reward given to the inventor of this ingenious mode of self-registration has been recently revealed in a parliamentary paper, thus: "To Mr. Charles Brooke for his invention and establishment at the Royal Observatory, of the apparatus for the self-registration of magnetical and meteorological phenomena, £500." Every year the invention will save fully £500 worth of human toil; and the reward seems small when we see every year millions voted for warlike, sinecure, and other worse than useless purposes.
Photography, however, can not do all the work. Its records have to be cheeked by independent observations every day, and then both have to be brought to their practical value by comparison with certain tables which test their accuracy, and make them available for disclosing certain scientific results. The preparation of such tables is one of the practical triumphs of Greenwich. Many a quiet country gentleman amuses his leisure by noting day by day the variations of his thermometer and barometer. Heretofore such observations were isolated and of no general value, but now, by the tables completed by Mr. Glaisher, and published by the Royal Society, they may all be converted into scientific values, and be made available for the increase of our weather-wisdom. For nearly seventy years the Royal Society had observations made at Somerset House, but they were a dead letter—mere long columns of figures—till these tables gave them significance. And the same tables now knit into one scientific whole, the observations taken by forty scientific volunteers, who, from day to day, record for the Registrar-General of births and deaths, the temperature, moisture, &c., of their different localities, which vary from Glasgow to Guernsey, and from Cornwall to Norwich.
What the Rosetta stone is to the history of the Pharaohs, these Greenwich tables have been to the weather-hieroglyphics. They have afforded something like a key to the language in which the secrets are written; and it remains for industrious observation and scientific zeal to complete the modern victory over ancient ignorance. Already the results of the Greenwich studies of the weather have given us a number of curious morsels of knowledge. The wholesale destruction of human life induced by a fall in the temperature of London has just been noticed. Besides the manifestation of that fact, we are shown, that instead of a warm summer being followed by a cold winter, the tendency of the law of the weather is to group warm seasons together, and cold seasons together. Mr. Glaisher has made out, that the character of the weather seems to follow certain curves, so to speak, each extending over periods of fifteen years. During the first half of each of these periods, the seasons become warmer and warmer, till they reach their warmest point, and then they sink again, becoming colder and colder, till they reach the lowest point, whence they rise again. His tables range over the last seventy-nine years—from 1771 to 1849. Periods shown to be the coldest, were years memorable for high-priced food, increased mortality, popular discontent, and political changes. In his diagrams, the warm years are tinted brown, and the cold years gray, and as the sheets are turned over and the dates scanned, the fact suggest itself that a gray period saw Lord George Gordon's riots; a gray period was marked by the Reform Bill excitement; and a gray period saw the Corn Laws repealed.
A few more morsels culled from the experience of these weather-seers, and we have done.
Those seasons have been best which have enjoyed an average temperature—not too hot nor too cold.
The indications are that the climate of England is becoming warmer, and, consequently, healthier; a fact to be partly accounted for by the improved drainage and the removal of an excess of timber from the land.
The intensity of cholera was found greatest in those places where the air was stagnant; and, therefore, any means for causing its motion, as lighting fires and improving ventilation, are thus proved to be of the utmost consequence.
Some day near the 20th of January—the lucky guess, in 1838, of Murphy's Weather Almanac—will, upon the average of years, be found to be the coldest of the whole year.