"I want you to think of our lesson on the thermometer," said Mr. Wilson. "We followed the various steps of making, testing, filling, and sealing the tubes. Now let us see what we have got. We have an instrument which will tell us that one body is hotter than another. That is all. We want something more than this. We want to say how much hotter it is. We want to show the steps or degrees of heat.
"To do this the bulb is first placed in a vessel of melting ice. After it has stood there for a short time, the mercury contracts and sinks to a certain point, but it will not sink lower. This point is marked in some way on the tube, and it shows the melting-point of ice, or, as we morefrequently call it, the freezing-point of water. Both meanthe same thing.
"The instrument is next suspended in a vessel, in which water is being boiled, so that it is surrounded on all sides by the steam of the boiling water. The mercury expands with the heat, and rises in the tube to a certain point, but after reaching that point, it will not rise higher, however long it is kept there. This point is marked on the tube asthe boiling-point of water. We have now found two very important points-one, that at which water freezes; the other, that at which water boils. Between these two points we may make any steps we please.
"All thermometers are made in one way up to this stage of the process, but there are different methods of dividing the space between the freezing and boiling points. In one thermometer the space is divided into 100equal steps; the freezing-point is marked 0, and the boiling-point 100. This is a very simple arrangement, and the instrument is known as the centigrade thermometer-fromthe Latin centum, "a hundred," and
gradus, "a step."
There are 100 steps or grades between the freezing-point and the boiling-point. This thermometer is not commonly used in our country. "The one mostly in use placesthe freezing-point at 32 degrees, and the boiling-point at 212 degrees, thus making 180 steps or degrees between the two points. We call this instrument the Fahrenheitthermometer, from the name of the man who invented it, and we write the steps 32° or 212° (32 degrees or 212 degrees).
"This freezing-point of water is not the lowest temperature that can be reached. Fahrenheit found, by making a mixture of snow and salt, a temperature 32° lower than the freezing-point, and he thought he had actually reached the extreme limit. He called this point 0° or zero, and worked upwards from it. Lower temperatures, however, have been found since his time. Ten degrees below zero on the Fahrenheit thermometer means 32° + 10° = 42° below the freezing-point.
"In the centigrade thermometer 10 grades belowzero signifies 10 grades below the actual freezing-point, because the freezing-point in that instrument is zero."Lesson 56
The Mouths of Animals
Just as the limbs of animals are specially adapted to their mode of locomotion, so are the teeth, the form of the mouth, and even the movements of the jaw adapted to the kind of food the creatures eat. It will be very advantageous at this stage to compare the various classes of vertebrate animals in these respects.
Taking first the Insectivora-bats, hedgehogs, and moles-we find the teeth bristling with sharp points, for crushing the hard horny cases of beetles and other insects, which form their natural food. They feed on insects, not by accident, but because their teeth are formed to crush such food. Some of the bats live on fruit. Their teeth have no sharp cutting points, but are broad and rounded for grinding purposes.
Passing next to the Carnivora, we notice in all of them the enormous development of the four canine teeth, and the sharp cutting edges of the others. The great canine teeth are meant for seizing the prey; the sharp jagged edges of the other teeth are for cutting through the flesh; and this work is still further assisted by the peculiar up-and-down movement of the jaw.
In the cat and dog families the tongue is long, fleshy, and very flexible. These animals drink by lapping with the tongue. In the cats the tongue is furnished with sharp horny spikes on its upper surface, and is used for rasping the flesh from the bones. The dog"s tongue is smooth. He perspires through his tongue.
The seal feeds mostly on fish. Its teeth are furnished with sharp saw-like edges, to serve the double purpose of seizing and holding such slippery prey.
In the Rodents (gnawing animals) the four incisor teeth become the chief peculiarity. These teeth are of more rapid growth than the teeth of most animals; they develop into sharp-edged chisels; their very work of gnawing tends to sharpen them for further use. The cleft in the upper lip is designed to assist in the work of gnawing. There are nocanines. Between the last of the incisors and the first of the molars, in each jaw, there is a space where the canine teeth of other animals are set. The molars are meant for grinding, not cutting, and the movements of the jaw, from side to side, as well as up and down, are designed to assist in the work.
The mouths of the Ruminants (cud-chewers) present their peculiarity, and it is in the entire absence of incisors in the upper jaw. The place of these teeth is occupied by a hard gristly pad, which the animals use in collecting up and tearing off the tufts of grass, preparatory to the act of swallowing them. The molars are all largely developed, butthey have broad crowns for grinding, and the jaw has the usual double movement designed for the same purpose.
The so-called Toothless animals differ considerably among themselves, according to the nature of their food. In none of the family do we find canine teeth ; in some the incisors are wanting; and others, again, are entirely without teeth. In most of them the lips are hard andsufficiently horny to crush the insects and soft-bodied animals on which they live.