opposite the door, having a double sash. The outer one is three feet. high by two feet wide, and the inner one three feet high by two and a half wide; the casing is beveled so as to admit light to all parts of the room. Both sashes are screwed in with felt between the casing and the sashes, leaving an air space of six inches between the glasses. The room is heated by circulation of hot water (the fuel being gas controlled by electricity), and is cooled by ice in the refrigerator when a temperature is required below that of the outside atmosphere. In a small room, two and a half feet by three and a half, and six feet high, built between the temperature and main chronometer rooms, and connecting by door with the small passage-way, is a small copper boiler, made in the shape of the frustum of a cone. The boiler and heating space underneath are covered with a casing of copper, leaving a space of half an inch between the boiler and casing, which is filled with asbestos packing.

From the top of the boiler leads the circulating pipe (of iron and an inch in diameter), which goes through the wall, where it is surrounded by asbestos packing, into the temperature room, thence twice around the room, back through the wall and into the boiler at its lowest point, making a fall of eighteen inches in circulating. A water-cock is placed on the return pipe near the boiler for drawing off the water when not in use.

Above the boiler and connecting with the circulating pipe is a supply tank of galvanized iron, holding about one gallon of water. This is fitted with an opening at the top for filling, a glass water-gauge at the side, and a waste-pipe leading from the top out through the side of the building. This latter is a safety as well as an overflow pipe, for should the control cease to act and too much heat be generated, the steam would escape through this pipe and relieve the pressure in the circulating coil.

A gas pipe leads from the main pipe of the Observatory through the foundation up into the temperature room, with its main cock just above the floor. Thence the pipe is led up the wall about four feet, over a little shelf; thence down to the floor, through the wall into the heating room, and under the boiler where the two Bunsen burners are attached. In the horizontal part of this pipe, over the small shelf, is a spring valve acting perpendicularly, the spring keeping the valve open. The stem of the valve projects above the box, and is worked automatically by a lever attached to the armature of an electromagnet.

A small gas pipe leads from the main pipe, before it reaches the automatic valve, to the top of the room, with its burner under a funnel-shaped ventilator, for increasing the draught and regulating the hygrometric state of the room.

Another small gas pipe leads from the same point through the wall into the heating room and under the boiler, where two minute burners are attached, the flames from which are directed, one over each Bunsen burner. These are always kept burning when the room is in use.

A two-inch pipe leads through the floor and foundation to the outside for supplying cold air as required. This, with the ventilator, is regulated by hand.

Suspended over the Bunsen burners in the hollow space of the boiler is a fire-pot to deflect the flames against the sides, and from the top of this space a small copper pipe leads up to the ceiling and out through the wall, for the escape of the products of combustion. Directly under the burners, leading through the floor and out through the foundation, is a two-inch lead pipe to carry off condensed vapor and to supply oxygen to the burners. A small, tight-fitting door opens into the combustion chamber.

The supply of gas is controlled by electricity through a mercurial thermostat, the stem of which is made and graduated like an ordinary thermometer, but which is open at the top.

The bulb is made of a thin glass tube coiled into a flat spiral. A fine platinum wire is fused into the end and connects with the mercury, its other end being secured to a binding-post. The thermostat is secured in a vertical position to a stand which is placed on the centre of the table upon which the chronometers to be tested are placed. A small platinum wire passes down into the upper end of the thermostat, and is secured at its upper end to a binding-post on the top of the stand.

A delicate and plainly graduated maximum and minimum thermometer is also attached to the stand, and, with the bulb of the thermostat, is placed on a level with the chronometers.

The controlling circuit leads from one pole of the battery to the binding-screw at the top of the stand, from there through the platinum wire, mercury column, and platinum wire to the other bindingscrew; thence to the spools of an electro-magnet placed on the small shelf by the automatic valve, and thence back to the battery.

A condenser, or spark-arrester, is placed in the circuit between the binding-posts of the thermostat.