lated situation between decks, where many persons habitually congregate, or which, from any other cause, is usually or periodically damp. The whole instrument should be protected with a cap of wirework to defend it from injury: this, if it interfere with the readings, should be removed a quarter of an hour before the observation. In reading the thermometers begin with the dry one, and use all the precautions recommended with respect to the "external" thermometer. Enter the simple readings, but at the head of each column place the zero correction (with its proper sign) required for its thermometer (a general rule for all thermometric entries), and leave a blank column for the "hygrometric depression," in calculating which subsequently the zeros must be applied. The reduction of the observations to derive the elastic force of vapour at the dew-point is effected by the formula of Dr. Apjohn:

(a) to be used when the reading of the wet thermometer is above 32°, and (b) when below. In these, d is the hygrometric depression, h the height of the barometer, ƒ the elastic force of vapour for the temperature shown by the wet thermometer, to be taken from Table III., Appendix, and F the elastic force of vapour at the dew-point, which (all the other quantities being known) these formulæ enable us to calculate. With F so calculated enter the same table under the column of Force of Vapour, and the corresponding temperature is the dew-point, which, however, is not wanted to be known except as a matter of curiosity.*

* The discussion respecting the formulæ and coefficients of reduction of observations of this nature can hardly be regarded as satisfactorily terminated; and it cannot be denied that great difficulty still subsists in determining, by any mere reading of instruments, the exact hygrometric state of the air. In the absence of direct observation of the Dew Point, the actual absorption and weighing of the water contained in a given volume of air seems to be the only method free from

The Rain-Gauge.-This may be of very simple construction. A cubical box of tin or zinc, exactly ten inches by the side, open above, receives at an inch below its edge a square funnel, sloping to a small central hole. On one of the lateral edges of the box, close to the top of the cavity, is soldered a short pipe, in which a cork is loosely fitted; the whole should be well painted. The water which enters the reservoir through the funnel-hole is poured through the short tube into a cylindrical glass vessel graduated to cubic inches and fifths of cubic inches. Hence, one inch in depth of rain in the gauge will be measured by 100 inches of the graduated vessel, and a thousandth of an inch may easily be read off. It is very difficult to place the raingauge properly on shipboard, and its entries therefore require constant explanatory notes, pointing out causes tending to disturb its influence. In fact, excepting the mast-head (and there upon a gimbal), it seems hardly possible to devise a tolerably permanent situation for it. On land, a perfectly open exposure on the ground, or very little elevated above it, should be chosen. The quantity of water should be daily measured and registered at 9 A.M., unless the fall of rain be so heavy as to endanger filling the instrument within the 24 hours, when this operation should be performed as often as needed. Snow collected or water frozen in the reservoir should be melted.

The Anemometer.-Lind's would appear to be the only anemometer which can conveniently be used on shipboard. It is adjusted by filling it with water till the liquid in both legs of the syphon corresponds with zero of the scale. It is to be held perpendicularly with the mouth of the kneed tube turned towards the wind, and the amount of depression in the one leg and of elevation in the other is to be noted. The sum of the two is the height of the column of water which the pressure of the wind is able to support: and the

theoretical objection, and it might not be very difficult to contrive a portable apparatus for this purpose.

[For all calculations relating to the humidity of the atmosphere, Glaisher's Hygrometrical Tables will be found very valuable.—R. M.]

force of the wind on a square foot is obtained from this height by Table IV. of the Appendix. In great degrees of cold a saturated brine may be used which does not freeze, and whose specific gravity being 1.244, the force given by the table must be multiplied by this factor. In addition to the regular hours of observation this instrument should be observed in storms, white squalls, or other circumstances of interest; the direction of the wind, as well as its force, should be registered at each observation; and for this it is well to have a small compass with a vane of card or thin and very moveable sheet brass, which may be fastened on the top of the anemometer, and which will indicate the direction in which its opening should be turned. In concluding the direction and the force of the wind from the vane and anemometer readings, a correction depending on the direction and velocity of the ship's motion is in strictness required. But such corrections are not usually applied, and it may be doubted whether the observations can be made accurately enough to render it worth while to apply them.*

The Actinometer.-This consists of a large hollow cylinder of glass, soldered at one end to a thermometer-tube, terminated at the upper end by a ball drawn out to a point, and broken off, so as to leave the end open. The other end of the cylinder is closed by a silver or silver-plated cap, cemented on it, and furnished with a screw, also of silver, passing through a collar of waxed leather, which is pressed into forcible contact with its thread, by a tightening screw of large diameter enclosing it, and working into the silver cap, and driven home by the aid of a strong steel key or wrench, which accompanies the instrument.

The axis of this screw is pierced to allow the stem of a spirit thermometer to pass out through it, the bulb (a very long one) being within the cylinder, to take the temperature

*For a description of an improved form of Lind's anemometer by Sir W. Snow Harris, with a detailed explanation of its construction and mode of use, the reader is referred to The Nautical Magazine for March, 1858.-R. M.

of the enclosed liquid. The graduation is in the stem of the screw, which is prolonged to receive and defend it.

The cylinder is filled with a deep blue liquid (ammoniosulphate of copper); and the ball at the top being purposely left full of air, and the point closed with melted wax, it becomes, in any given position of the screw, a thermometer of great delicacy, capable of being read off on a divided. scale attached. The cylinder is enclosed in a chamber blackened on three sides, and on the fourth, or face, defended from currents of air by a thick glass, removable at pleasure.

The action of the screw is to diminish or increase at pleasure the capacity of the hollow of the cylinder, and thus to drive, if necessary, a portion of the liquid up into the ball, which acts as a reservoir, or, if necessary, to draw back from the reservoir such a quantity as shall just fill it, leaving no bubble of air in the cylinder. The interior thermometer indicates approximately the temperature of the blue liquid for the subsequent reduction of the observation.

To use the instrument, examine first whether there be any air in the cylinder, which is easily seen by holding it level, and tilting it, when the air, if any, will be seen to run along it. If there be any, hold it upright in the left hand, and the air will ascend to the root of the thermometer-tube. Then, by alternate screwing and unscrewing the screw with the right hand, as the case may require, it will always be practicable to drive the air out of the cylinder into the ball, and suck down the liquid, if any, from the ball, to supply its place, till the air is entirely evacuated from the cylinder, and the latter, as well as the whole stem of the thermometer-tube, is full of the liquid in an unbroken column. Then, holding it horizontally, face upwards, slowly and cautiously unscrew the screw till the liquid retreats to the zero of the scale.

The upper bulb is drawn out into a fine tube, which is stopped with wax. When it is needed to empty, cleanse, and refill the instrument, liquid must be first forced up into the ball, so as to compress the air in it. On warming the

end, the wax will be forced out, and the screw being then totally unscrewed, and the liquid poured out, the interior of the instrument may be washed with water slightly acidulated, and the tube, ball, &c., cleansed, in the same way, after which the wax must be replaced, and the instrument refilled.

To make an observation with the actinometer, the observer must station himself in the sunshine, or in some sharply terminated shadow, so that, without inconvenience, or materially altering his situation, or the exposure of the instrument in other respects, he can hold it at pleasure either in full sun or total shadow. If placed in the sun, he must provide himself with a screen of pasteboard or tin plate, large enough to shade the whole of the lower part or chamber of the instrument, which should be placed not less than two feet from the instrument, and should be removeable in an instant of time. The best station is a room with closed doors, before an open window, or under an opening in the roof into which the sun shines freely. Draughts of air should be prevented as much as possible. If the observations be made out of doors, shelter from gusts of wind, and freedom from all penumbral shadows, as of ropes, rigging, branches, &c., should be sought. Generally the more the observer is at his ease, with his watch and writing table beside him, the better. He should have a watch or chronometer beating at least twice in a second, and provided with a seconds hand; also a pencil and paper, ruled according to the form subjoined, for registering the observations. Let him then grasp the instrument in his left hand, or, if he have a proper stand (which is preferable on shore or in a building *), otherwise firmly support it, so as to expose its face perpendicularly to the direct rays of the sun, as exactly as may be.

The liquid, as soon as exposed, will mount rapidly in the

*This may consist of two deal boards, eighteen inches long, connected by a hinge, and kept at any required angle by an iron, pointed at each end. The upper should have a little rabate or moulding fitting loosely round the actinometer, to prevent its slipping off.