against a small lever, and gradually raises the keeper toward the magnet, during the inactive minute as shewn in the figure, it has just elevated the keeper to its maximum; and now leaving it to be sustained by the magnet, it passes on in order that the keeper may have free space to fall when the observation is made.

From this description, we should almost be led to expect that a separate striking movement and type-wheels would be required for each instrument; but a very simple contrivance has enabled Mr. WHEATSTONE to register the indications of all the instruments by the same apparatus. E, fig. 2, Plate XI. is a rheotome, or cutter-off of currents, being a circle of ten sectors of brass insulated from each other by ivory: each sector has a small brass peg, to which the connecting wires are attached; the wires of the barometer, thermometer, and psychrometer, occupy three of these sectors; two sectors remain open for the attachment of other instruments; the remaining five are in common connection with the copper plate of the battery: the metal index completes the circuit, by connecting the right-hand sectors with one of those on the left. This index makes a complete revolution in one hour; and passes over each division in six minutes; during five minutes, which correspond to the ascent of the wire, it passes along the metal sector, and during the remaining one minute, corresponding to the fall of the wire, it passes over the ivory division to the next sector. Now, as each instrument is connected with a different sector, and each sector is insulated, only one instrument at a time is included in the circuit; so that, when the barometric observation is made, the index passes on to the next division, and introduces the psychrometer, for instance, into the circuit; and then passes on farther to the sector connected with the thermometer, and so on. In Plate x. fig. 1, are shewn the wires h i, and pulleys, by which the two last-mentioned instruments are included in the circuit; the range of both is from -5° to + 95°: the scale, therefore, has a greater length than that of the barometer, and, consequently, the pulleys are larger, as shewn in the figure; in all other respects, the arrangements are similar. It will be seen, from the above description, that three observations are recorded in each eighteen minutes, one of the barometer, another of the thermometer, and the third of the psychrometer. The instrument requires no attention for a week, during which time it registers 1008 observations. The immersion of the wire in the mercury of the tubes, for the time being, slightly elevates the mercury; but as the observation is not made until the wire is leaving the mercury, no error is committed. As there is no necessity that the circuit should be completed with mercury, there are very few meteorological instruments to which this register cannot be applied. It is scarcely necessary to remark that the part containing the instruments must be properly exposed. We have mentioned that this register is for the Kew Observatory, where, we hope, very soon, to see it placed.

OF

AUTHORS AND OBSERVERS

CITED IN THIS WORK.

Daniell, 77, 78, 82, 88, 252, 269, 270, Gadbury, 356

Deluc, 90, 95, 105, 112, 304, 305, 306, Gersdorf, 555

Erman (Adol.) 206, 208, 276, 278, 518 Henley, 553, 572