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THE HONGKONG GOVERNMENT GAZETTE, 3RD SEPTEMBER, 1881.

of physics. The magnet of the Declination magnetometer is placed in the magnetic meridian; that for measuring variations of the Horizontal force is compelled by torsion to take a position transverse to the meridian; the third, for measuring variations of Vertical force, is balanced on knife-edges.

The Unifilar Magnetometer is used for ascertaining the absolute value of the horizontal component It is also of the total force in the direction of the dip, by observations of deflection and vibration. used for determining the absolute declination.

The Dip-Circle affords measures of the magnetic inclination, or "dip," and can be used under certain circumstances for measuring variations of the total force.

The chief magnetic building, for the differential instruments, should be a basement or vault, almost wholly underground, and contain an instrument-chamber and a photographic laboratory. I recommend about 22 feet by 20 feet and 12 feet by 10 feet as suitable dimensions for those rooms, and the build- ing should be laid out with the long side of the instrument-room inclined 15 degrees to the west of the magnetic north, the laboratory being at the northern end.

The annexed small drawing will give an idea of the character of the building which I propose, It shows that the actual chamber for the magnetograph has and of the positions of the instruments. an arched roof, and is wholly inclosed within a similar building, the walls and roofs of the two being The floor of the chamber is 15 feet below the surface of the ground, separated by a space of two feet. and the highest point of the external arch is 21 feet above the floor. Covering this arch, there will be (1) 2 inch asphalte, (2) six inches of fine concrete, (3) two feet of earth; the surface being finally turfed. Small relieving arches will need to be built at intervals between the side walls of the outer and inner chambers, in order to meet the thrust upon the inner walls. The main chamber will be quite dark, but a small amount of yellow light will be admitted to the laboratory.

Although the vault form of construction is the best for differential magnetic observations, since it secures the great desideratum of an equable temperature, there are obvious complications and diffi- culties connected with it. Thorough drainage must be carefully provided, in order to keep it dry; and there must be arrangements for carrying off the heat and smoke of lamps. Ventilation also presents a grave difficulty, especially in a climate so hot and damp as that of Hongkong is during several months of the year. It is evident that, if warm air laden with moisture be admitted to an underground chamber which is appreciably cooler, it will produce a constant state of saturation, and be continually depositing its moisture on the delicate instruments and apparatus. A long underground air-flue, connected with vertical shafts, must therefore be provided, in order that the incoming air may be cooled to the temperature of the vault, as nearly as possible, before reaching it. These devices are of course expensive, but it is better to put up with the cost than to face the alternative of a building above ground, in which the great variations of temperature would be fatal to satisfactory observation.

For the above and other essential details, Mr. BOWDLER's past experience in designing and erecting the excellent Magnetic Observatory at Mauritius will prove of the utmost service in the present case. Mr. BOWDLER has, indeed, been good enough to prepare a detailed drawing after the pattern of the Mauritius building, and although I propose, for the sake of economy, to depart in some important respects from the plan there carried out, his drawing furnishes much practical and useful information, applicable to all works of this kind. It has been of great service to me in preparing the accompanying design.

It remains to consider the material which shall be used for the magnetic building, and which, as needs hardly be stated, must be practically free from iron. Unfortunately, the granite of this region is out of the question, on account of the large percentage of protoxide of iron which it contains. For the same reason we must condemn the concrete that is in common use in Hongkong, since its main ingre- dient is broken granite. Even the bricks made of Canton River mud contain iron, though to a less extent than the granite. I suggest that an analysis be made of the limestone of the White Mountain Range, near Canton, which, if found sufficiently free from iron, might be imported for our purpose, and used both for the masonry and the concrete, if not too costly. A fine concrete, of suitable quality, might be made with washed gravel of this island, i.e., the common decomposed granite (taking care to select a coarse, gritty sample), dug from a nullah side, and washed until only the quartz particles remain. This of course would be expensive. I have little doubt, however, that, on careful inquiry, it will be found neither very difficult nor very costly to procure, near at hand or somewhere on the coast, a material that will fulfil all requirements.

The same rule as regards the exclusion of iron must be followed in every detail of construction. All nails, locks, keys, hinges, bolts, hooks, gratings, pipes, &c., should be of copper, brass, or zinc ; and it is important to bear in mind that the copper articles ought to be of European manufacture, as those made in China contain a large percentage of iron. The Reverend M. DECHREVENS, S.J., director of the Zi Ka Wei Observatory, with whom I have entered into correspondence, and to whom I am indebted for some valuable suggestions relating to the Magnetic building, as well as for plans and photographs of his own establishment, is emphatic in his caution on this point, and recommends me "n'employer (C pour

les portes, les fenêtres, les jonctions, etc., que du cuivre, et autant que possible que des objets,