This fact may be demonstrated by converting the triangle into a parallelogram, of which one of the sides of the triangle will become its diagonal: the other two sides will, of course, represent two forces equivalent to such diagonal, which, acting in opposition to it, must produce a balance.
Note 41, p. [241].--Kite messengers.
The curious experiments of Mr. Faraday upon the optical effects produced by the revolutions of different wheels, might be exhibited by arrangements adjusted as messengers.
Note 42, p. [246].--Winds and storms.
The sea and land breezes which occur in the islands of the torrid zone, very strikingly illustrate the position laid down in the text, and afford a good explanation of the manner in which winds may be occasioned by a change of temperature in the air. In these, during the hottest part of the day, the wind sets in from all quarters, and appears to be blowing towards the centre of the island, while in the night it changes its direction, and blows from the centre of the land towards the sea; for since the sun’s rays produce much more heat by their reflection from land than they do from water, that portion of air which is over the land will soon become heated, and will ascend; a rarefaction and diminution of the quantity of air over the central part of the land will be thus occasioned, which must be supplied from the sides; but, as the land cools again during the night, that portion of air which had been previously heaped up will begin to descend, and by spreading and equalizing itself will produce a breeze blowing from the centre.
The trade-winds, so called from the advantage which their certainty affords to trading vessels, are another example of the same kind; they are generally stated to blow from east to west over the equator, and are occasioned by the rarefaction of the air by the sun’s heat, and the motion of the earth from west to east. While writing the present note, we have seen an essay upon the subject by Captain Basil Hall, published in an appendix to Mr. Daniel’s admirable work on Meteorology: the perusal of this paper has induced us to cancel what we had written, and to refer the reader to the essay itself; for it is quite impossible to do justice to the views it entertains, in the limited space necessarily prescribed to us in this note.
On the coast of Guinea, the wind always sets in upon the land, blowing westerly instead of easterly; this exception arises from the deserts of Africa, which lie near the equator, and being a very sandy soil, reflect a great degree of heat into the air above them, which being thus rendered lighter than that which is over the sea, the wind continually rushes in upon the land to restore the equilibrium.
Among the irregular winds, or those which are not constant, but accidental, may be noticed the whirlwind, the harmattan, and the sirocco. The first of these is occasioned by the meeting of two or more currents of wind from opposite directions, and which can only be occasioned by some temporary but violent disturbance of equilibrium. The harmattan is met with on the western coast of Africa, and is generally attended by great heat and fog; it appears to be occasioned by a conflict between the heated sands of Africa, and the regular direction of the trade-winds over that continent, and, by disturbing their progress, it is frequently the forerunner of a hurricane in the West Indies. The sirocco occurs in Egypt, the Mediterranean, and in Greece, and is chiefly characterised by its unhealthy qualities. The air, by passing over the heated sands of Egypt, becomes so dried and rarefied as to be scarcely fit for respiration, and, being thus prepared, it absorbs so much humidity on passing the Mediterranean as to form a suffocating and oppressive kind of fog.
Mr. Daniel observes, that the currents of a heated room, in some measure, exemplify the great currents of the atmosphere. If the door be opened, the flame of a candle held to the upper part will show, by its inclination, a current flowing outwards; but, if held near the floor, it will be directed inwards. If the door be closed suddenly from without, it moves with the in-coming current, and against the out-going, and a condensation of air takes place in the room; which is proved by the rattling of the windows, and the bursting open of any door in the room, if slightly closed. If the door close from within, it moves against the in-coming current, and with the out-going, and a rarefaction of the air in the room takes place; which is evidenced by the rattling of the windows, and the bursting open of another door in the contrary direction.
Meteorology has been long considered the least perfect branch of natural knowledge; so apparently capricious and irregular are its phenomena, that philosophers had almost abandoned the idea of bringing them under the operation of any general laws. Brighter lights are, however, now dawning upon us. Mr. Whewell, in his Bridgewater Treatise, has explained the manner in which the various currents of the atmosphere maintain a necessary balance in the distribution of heat and moisture around the globe, and has thus reduced to order and design phenomena which have hitherto been regarded as unconnected and fortuitous. Lieut.-Col. Reid, by his late happy investigation of the law of storms, will, no doubt, lead us into a novel path of the most important discoveries. He has satisfactorily proved, by a mass of evidence derived from numerous logbooks, that storms obey fixed laws. His attention was ardently directed to the subject by having been at Barbadoes immediately after the great hurricane of 1831, which in the short space of seven hours killed upwards of 1400 persons on that island alone. The discoveries of Col. Reid may be thus briefly stated.--That hurricanes are whirlwinds of great diameter, always revolving according to an invariable law, viz. from right to left (supposing yourself standing in the centre), or in the opposite way to the hands of a watch, in the northern hemisphere, and in a contrary direction in southern latitudes; at the same time they have a progressive motion in a curved line, and as they advance their diameters appear to enlarge and their violence to diminish; it has been also found that in the centre of the vortex there is a lull, or calm. Col. Reid observes that the simplest mode of illustrating the subject is to cut out concentric circles, so as to represent progressive whirlwinds, by moving which over any tract, the veering of the wind will be easily understood. The reader may form a more familiar idea by causing the water to circulate in a basin, which will represent the violent circular motion of the storm-wind, with a calm in the centre of the vortex. Suppose this to be also moving onward at a rate of about seven miles an hour, and he will have a correct notion of the subject. Since the storms expand in size and diminish in force as they proceed towards the poles, and the meridians at the same time approach each other, gales become huddled together; and hence, apparently, the true cause of the very complicated nature of the winds in our latitude. Observations would also appear to render it probable that there exists an accordance of the force of storms with the law of magnetic intensity; for example, it is at its minimum at St. Helena, where storms never occur; on the contrary, the lines of greatest intensity seem to correspond with the latitudes of typhoons and hurricanes. To what important discoveries may not the pursuit of this enquiry lead us?