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for the use of seamen, the Dip-micrometer and Dip-sector have been contrived for the purpose of measuring the actual dip at the time of any observation.
In the Dip-micrometer, the eye-end of the telescope is turned at right angles to its length, in order that the observations may be made with greater convenience in opposite positions of the instrument.
The instrument being held in a vertical position, two opposite points of the horizon are seen by reflection through two lateral openings at the larger end. When the two images have been made to coincide by means of the tangent screw at bottom, the divided head of the micrometer must then be set to zero, and the observation must be repeated with the length of the instrument inverted. Since in one position the arc measured is 1800 through the zenith, INCREASED by the dip of each horizon, and since in the opposite position of the instrument it measures the opposite arc through the nadir, or 180° DECREASED by the same double dip; hence, the difference of the two observations is really four times the dip; but in order to avoid the arithmetical operation of dividing by four, the divisions on the micrometer head are made larger in that proportion, so as to shew at once the actual dip without division.
In the Dip-sector the eye-end of the telescope is set at right angles to the plane of the instrument, in order that the head of the observer may not intercept the view of that horizon, which is seen by reflection.
If the plane of the instrument be held truly in a vertical position, the opposite points of the horizon that are seen, will appear parallel to each other; but, on the contrary, they will appear to cross each other, if the plane of the instrument be inclined either to or from the observer: hence, this mark becomes a sure guide in giving a correct position to the instrument, which requires a little practice to effect.
The index is moved by a tangent screw, as in a common sextant; and when, by means of it, the two horizons have been brought to coincide, the place of the index is to be noted down
to minutes and seconds. But the observation is not complete without being repeated with the instrument inverted. If the index glass was held uppermost in the former position, the instrument then measured the arc through the zenith of 180°, increased by the dip of each horizon; but when the instrument is inverted, so that the index-glass is lowermost, then it mea. sures the opposite arc through the nadir, or 180° decreased by the double dip. Hence, the difference of the two arcs is four times the dip; consequently, when the two horizons have again been made to coincide in the new position of the instrument, the place of the index is to be again noted down, and the difference of the two readings divided by four, shews the actual dip observed, without any regard to the index-error, which is purposely made considerable, in order to avoid any negative reading.
There is a screw for equalizing the light of the two horizons in the first observation; but in the second, there will be no occasion to move the screw, if the observer turns his face round to the opposite direction at the same time that he reverses the instrument, for then the same horizon, as before, is seen as reflected image.
Since the principal known cause of variation of the dip is a difference between the temperature of the sea and air, it would be desirable, as often as may be, to observe the state of the thermometer in the air, and also to ascertain the temperature of the sea, and to record them regularly whenever the dip observed is found to differ from that shewn in the common tables, with a view to perfecting future tables for the same purposc.
The macrometer is intended to measure directly the distance of inaccessible objects, by means of two reflectors, mounted as in a common sextant, but at a greater distance from each other.
The first reflector admits of adjustment by a capstan-headed screw, for the purpose of correcting lateral error of the images. The index-glass does not admit of adjustment, but the indexerror must be occasionally ascertained, as usual, by means of the sun or other celestial objects. And as the instrument is intended to be used solely in the vertical position, the index-error
should be examined as nearly as may be in the same position
When any terrestrial object at a moderate distance is ob-
The measure depends on the angle which the interval between the mirrors subtends at the object observed; and if the interval of the mirrors (one yard and a half) be considered as radius, the distance is as cotangent of the angle measured.
Use of the Electrical Apparatus.-H.D.
The analogy of the Aurora Borealis and Australis to electrical light, strongly impresses on the mind the probability of these phenomena being electrical; and it becomes an interesting question, whether the earth may not possess electrical as well as magnetic poles. An electrical apparatus is furnished for each Expedition, in order to determine if there is any thing peculiar in the electricity of the atmosphere in the polar regions. The use of this apparatus must be obvious, from its simple construction: it consists of a chain of copper, attached by glass to a rope, by which it is to be elevated so as to be out of the reach of conductors, and as far as possible above the surface of the vessel and the water; the electrometers to which the chain is to be attached, are likewise insulated. The sphere of the balance is brought in contact with the brass ball when the instrument is to be used: the degree of repulsion will indicate the degree of electricity of the atmosphere.
When the ball is observed to be repelled, a stick of sealing wax is to be rubbed with woollen cloth, and presented to the brass ball; if the repulsion of the sphere of the balance ceases, or is diminished, the electricity is positive; if it be increased, the electricity is negative. The observation should be registered in a journal, and the experiments made at different times in the twenty-four hours; and the state of the weather, temperature of the barometer, clouds, &c. at the time, noted.
When the balance does not indicate electricity, the gold leaf electrometer may be tried. If the repulsion of the leaves is increased by rubbed sealing wax, the electricity is negative; if the contrary, the electricity is positive.
Use of the Apparatus for taking up Sea Water from given Depths. H.D.
The possibility of reaching the Pole by the Expedition, must depend upon this circumstance, whether there is at the Pole an ocean so deep, that the heat stored up in it during the six months of summer, is sufficient to prevent the formation of ice
upon it in winter. If there be such an ocean, the waters of it must greatly expand during summer, and the laws of the motion of the earth would tend to pour them through the narrow seas or channels, separating Asia from America, and Spitsbergen from Greenland. If a current is found, it will consequently be of great importance to ascertain if the great body of it be comparatively fresh, or salt; and for this purpose the deeper parts of it should be examined, for snow or ice water may float at the surface, and salt water be beneath.
The Apparatus sent with the Expedition, consists of a copper vessel furnished with a stop-cock, which is opened by a piston moving in consequence of the compression of air when the instrument is sunk in the sea. The piston may be set so as to collect the water from five to eighty fathoms. As the volume f elastic fluids are inversely as their compressing weights, and as the compression of about thirty-two feet of water diminishes the volume of air to one-half, the gradation will point out the use of the instrument. It is needless to say, that the temperature of the water as well as all other circumstances should be registered.
If the current be ice cold, and comparatively fresh, there can be little hope of reaching a deep sea in that direction.
Of the state of the Atmosphere in high northern Regions.—H. D.
As there can be little or no change from vegetable or animal life or decomposition in the polar atmosphere, it will be interesting to ascertain the relative proportions of oxygene, azote, and carbonic acid in the air, as well as the nature of the air expelled from sea water. An apparatus for the analysis of air is sent with the Expedition. The use of it is so simple, that a description of it is unnecessary.