AERMOTOR PUMPS.
Aeris the first element in many compound words of Greek origin meaning air, the air, atmosphere; in this connection it is combined withmotor, defined as a machine which transforms the energy of water, steam, or electricity into mechanical energy—in this instance, is meant the changing ofthe power of moving air or windinto mechanical energy.
Wind is air put in motion.There are two ways in which the motion of the air may arise. It may be considered as an absolute motion of the air, rarefied by heat and condensed by cold; or it may be only an apparent motion, caused by the superior velocity of the earth in its daily revolution.
When any portion of the atmosphere is heated it becomes rarefied, its specific gravity is diminished, and it consequently rises. The adjacent portions immediately rush into its place to restore the equilibrium. This motion produces a current which rushes into the rarefied spot from all directions. This is what we call wind.
Meteorologyis the science which treats of the atmosphere and its phenomena, particularly of its variations of heat and cold,of its winds, etc.
This is the great division of science to which one has to turn when searching for the first principles relating to the operation ofaermotor pumps. The vast volumes of air which flow “hither and yon” are controlled by physical laws which act as accurately and unceasingly as those which control and hold in check the seemingly solid substance of the earth itself.
Note.—The portions north of the rarefied spot produce a north wind, those to the south produce a south wind, while those to the east and west in like manner, form currents moving in opposite directions. At the rare spot, agitated as it is by winds from all directions, turbulent and boisterous weather, whirlwinds, hurricanes, rain, thunder and lightning, prevail. This kind of weather occurs most frequently in the torrid zone, where the heat is greatest. The air, being more rarefied there than in any other part of the globe, is lighter, and, consequently, ascends; that about the polar regions is continually flowing from the poles towards the equator, to restore the equilibrium; while the air rising from the equator flows in an upper current towards the poles, so that the polar regions may not be exhausted.
Note.—The portions north of the rarefied spot produce a north wind, those to the south produce a south wind, while those to the east and west in like manner, form currents moving in opposite directions. At the rare spot, agitated as it is by winds from all directions, turbulent and boisterous weather, whirlwinds, hurricanes, rain, thunder and lightning, prevail. This kind of weather occurs most frequently in the torrid zone, where the heat is greatest. The air, being more rarefied there than in any other part of the globe, is lighter, and, consequently, ascends; that about the polar regions is continually flowing from the poles towards the equator, to restore the equilibrium; while the air rising from the equator flows in an upper current towards the poles, so that the polar regions may not be exhausted.
To sum up all observations, it can be said with truth that thesole forceimmediately concerned in causing the movements of the atmosphere,is gravitation.
So far as the prevailing winds are concerned it has been shown that where pressure is high, that is to say,where there is a surplus of air, out of such a region winds blow in all directions; and, on the other hand where pressure is low, or where there is a deficiency of air,towards such a region, winds blow from all directions in an in-moving special course.
This outflow of air currents from a region of air pressure upon a region of low pressure is reducible to a single principle, as already stated, viz., the principle of gravitation.
A regular east wind prevails about the equator, caused in part by the rarefaction of the air produced by the sun in his daily course from east to west. This wind, combining with that from the poles, causes a constant north-east wind for about thirty degrees north of the equator, and a south-east wind at the same distance south of the equator.
From what has now been said, it appears that there is a circulation in the atmosphere; the air in the lower strata flowing from the poles towards the equator, and in the upper strata flowing back from the equator towards the poles. It may be remarked, that the periodical winds are more regular at sea than on the land; and the reason of this is, that the land reflects into the atmosphere a much greater quantity of the sun’s rays than the water, therefore that part of the atmosphere which is over the land is more heated and rarefied than that which is over the sea. This occasions the wind to set in upon the land, as we find it regularly does on the coast of Guinea and other countries in the torrid zone. There are certain winds, called trade-winds, the theory of which may be easily explained on the principle of rarefaction, affected, as it is, by the relative position of the different parts of the earth with the sun at different seasons of the year, and at various parts of the day.
A knowledge of the laws by which these winds are controlled is of importance to the mariner. When the place of thesun with respect to the different positions of the earth at the different seasons of the year is understood, it will be seen that they all depend upon the same principle. The reason that the wind generally subsides at the going down of the sun is, that the rarefaction of the air, in the particular spot which produces the wind, diminishes as the sun declines, and, consequently, the force of the wind abates.
From its importance in practical meteorologyBuys Ballot’s lawmay be stated in these two convenient forms. (1) Stand with your back to the wind, and the center of the depression or the place where the barometer is lowest will be to your left in the northern hemisphere, and to your right in the southern hemisphere. This is the rule for sailors by which they are guided to steer with reference to storms. (2) Stand with the high barometer to your right and the low barometer to your left, and the wind will blow on your back, these positions in the southern hemisphere being reversed. It is in this form that the prevailing wind of any part of the globe may be worked out from the charts.