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Updated: May 1, 2025
Modern OTHR installations have different transmitting and receiving sites, often located many miles apart. From the early 1950s pulsed oblique ionosphere sounders had shown that the normal ionosphere is much more stable than had previously been thought to be. The physical reason for this is that the incredibly tenuous ionized gas which does the reflecting has a molasses-like viscosity.
The viscosity at the highest temperature obtained, about 90° C., was then taken and repeated for every fall of 4° C. till the water reached the temperature of the air. The values so obtained gradually diminished with the increase of temperature.
Through study of the viscosity of gases that is to say, of the degree of frictional opposition they show to an object moving through them or to another current of gas an idea was gained, with the aid of mathematics, of the rate of speed at which the particles of the gas are moving, and the number of collisions which each particle must experience in a given time, and of the length of the average free path traversed by the molecule between collisions, These measurements were confirmed by study of the rate of diffusion at which different gases mix together, and also by the rate of diffusion of heat through a gas, both these phenomena being chiefly due to the helter-skelter flight of the molecules.
Clausius' and Maxwell's explanations of the diffusion of gases, and of thermal conduction in gases, their charmingly intelligible conclusion that in gases the diffusion of heat is just a little more rapid than the diffusion of molecules, because of the interchange of energy in collisions between molecules, while the chief transference of heat is by actual transport of the molecules themselves, and Maxwell's explanation of the viscosity of gases, with the absolute numerical relations which the work of those two great discoverers found among the three properties of diffusion, thermal conduction, and viscosity, have annexed to the domain of science a vast and ever growing province.
No man will ever doubt the plasticity, the "viscosity" of ice, as it used to be styled in the old glacier controversies, who has passed over the "rubber" ice that forms under certain circumstances and at certain seasons on these rivers. We would never, I am sure, have attempted that ice had not William been with us. We would have struck a blow with the axe and declared it unsafe.
If this hypothesis were correct, it would explain the apparent anomaly of Ghattis giving solutions of higher viscosity than gum arabics, although they leave insoluble matter behind. The increase in viscosity would be due to the thickening of the arabic acid by the metarabin.
It is to be observed, however, that the trachytic lavas nowhere appear cropping out along with the sheets of basalt around the escarpments overlooking the sea, or inland; showing that they did not spread very far from their vents of eruption; a fact illustrating the lower viscosity, or fluidity, of the acid lavas as compared with those of the basic type.
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