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verits use mer it is a tised in till be appoint G,
pendicular to it there are five others; the centre one, AB, bisecting DF in C, while the other four are placed at equal distances, two on each side of AB.
As the time in which a heavenly body passes the field of view of a telescope varies according both to the diameter of the instrument and the polar distance of the body, it would be nearly impossible to ascertain when its centre coincided with the axis of the telescope, if it were not furnished with some such apparatus as this. The state of modern science, however, requires this precision; and a brief explanation of its use will enable our readers to comprehend in what manner it is attainable.
When the instrument is fixed in the plane of the meridian, the motion of the body will be apparently horizontal; and both the centre C and the point G, which is the middle of ES, will be in the vertical line bisecting DF, and consequently in the plane of the meridian. If the subject of observation be a star, and the apparent diameter of the wire be equal to that of the star, then the moment it is entirely hidden by the central wire will be that of its passing the meridian of the place of observation. In most cases, however, the metallic thread is sufficiently fine to bisect the star; and then, at the moment of its passage, an equal portion of the disk will be seen on each side of the wire; and as the body is so small, the eye, accustomed to such observations, will readily seize this moment, within a small fraction of a second. The same may obviously be done for each of the other four vertical wires, which, being at equal distances from one another, the times of the star passing them will form an arithmetical series; there, fore, by taking the mean of the five observations, we shall have the time of the star passing the centre wire more correctly than could be obtained in any single instance. If the observation be made with care, this method may be depended upon for giving the result within a tenth of a second.
If the instrument be not placed in the plane of the meridian, the motion of the star will be oblique, and then it is necessary to give the same inclination to the wire DE, so that the motion in ES may be parallel to it, as shown in Fig. 2: this is done by a proper apparatus applied to the telescope for that purpose. The greatest obstacle which is experienced in observations of this kind is when the night is very dark, and there is not sufficient light to see the wires, except at the moments when the star is bisected by one of them : this, however, is so instantaneous, that the observer is not always prepared for noting the time of the bisection. To avoid this, the following method of enlightening the tube is frequently adopted :-A small hole is made in the side of the telescope, and generally in the axis on which it turns; and opposite this aperture a mirror is placed, having an inclination of 45° to the axis of the telescope. The light of a small lamp is then made to fall on the mirror, and having the angle of incidence of its rays equal to 45°, it is reflected in the same angle, and the rays are consequently parallel to the axis of the instrument, and render the wires sufficiently visible. If the star upon which the observation is made be small, care must be taken that the artificial light is not too strong, otherwise the star will not be perceptible.
Observations of this kind are not, however, confied merely to the stars; both the Sun and the Moon are frequently the subjects upon which they are made. But the comparative magnitudes of these render it necessary to follow a different process to ascertain the moment when their centres pass the meridian, or rather the axis of the instrument. For this purpose, the several instants when the eastern or western limb of the body comes successively into contact with each of the five wires must be carefully noted, and the sum of these times, divided by five, will be the moment when that limb passed the central
wire. When this is accomplished, the other limb will be about leaving the first wire, and the five instants of its transit must be noted in precisely the same manner as for the first limb; and when their sum is divided by 5, the quotient will, in like manner, be the time of its passing the central wire. Consequently the mean of the times for the opposite limbs will give the transit of the centre.
These operations are so simple, and the reasons upon which they are founded so obvious, that it will not be necessary to enter into any further explanation of the subject, particularly as a little experience will be more instructive than any lengthened description. When this telescope is used for astronomical purposes, it is chiefly attached to other instruments; and the precision above described relative to the passage of the body over the axis of the telescope would be of little use, unless equal accuracy could be obtained in determining the direction of that axis, both with respect to its horizontal and vertical position. We shall, therefore, pursue the subject next month, in describing the instruments to which it is usually attached, as well as in explaining the nature of their adjustments, so as to obtain the necessary precision.
both. We shallibing tail as in
The Naturalist's Diary
For MARCH 1823.
And deluging tempests. O'er the moistened fields
Beside the garden-path, the crocus now
Turn to the OCEAN-how the scene is changed !
Listens, and answers with responsive note. Tue superabundant moisture of the earth being dried up, the process of vegetation is gradually brought on: those trees which, in the last month,
Blackwood's Magazine for March 1822, p. 304.
were budding, now begin to put forth their leaves; and the various appearances of Nature announce the approach of SPRING.
The melody of birds now gradually swells upon the ear. The throstle (turdus musicus), second only to the nightingale in song, charms us with the sweetness and variety of its lays. The linnet and the goldfinch join the general concert in this month, and the golden-crowned wren (motacilla regulus) begins its song. The lark, also, must not be forgotten :
In the golden lightning
Of the sunken sun,
Thou dost float and run,
The pale purple even
Melts around thy flight;
* In the broad daylight Thou art unseen, but yet I hear thy shrill delight. In this month, black ants (formica nigra) are observed; the blackbird and the turkey (meleagris gallopavo) lay; and house pigeons sit. The greenfinch (loxia chloris) sings; the bat (vespertilio) is seen flitting about; and the viper uncoils itself from its winter sleep. The wheatear (sylvia onanthe), or English ortolan, again pays its annual visit, leaving England in September. ;
Those birds which have passed the winter in England now take their departure for more northerly regions; as the fieldfare (turdus pilaris), the red-wing (turdus iliacus), and the woodcock (scolopax rusticola). Some other birds, as the crane and stork, formerly natives of this island, have quitted it entirely, since our cultivation and population have so rapidly increased.
On the 20th, the vernal equinox takes place, and all nature feels her renovating sway, and seems to rejoice at the retreat of winter.
The general or great flow of sap in most trees