Page images

that part of the cassein which is insoluble in water. The curds may be made to disappear by putting some soda into the curd and whey, to make up for that which has been abstracted from the cassein to form the lactate of soda.

Rennet curdles milk by acting on the milk of sugar and changing it into the new form lactic acid, which abstracts the soda of the cheesy matter as previously mentioned, and leaves apparent the insoluble curd. Most acids, and other animal membranes, besides rennet, act in the same way.

See Johnston's Lectures on Agricultural Chemistry.


Give one reason, and that the simplest, for be-

lieving1. That the earth is isolated in space. 2. That the form of the earth is nearly that of a sphere. 3. That the dimensions of the earth are those assigned

to it in books on astronomy.

SECTION II. 1. Explain the phases of the moon, and illustrate your

explanation by a diagram. 2. Under what circumstances is an eclipse of the sun

total, or partial, or annular? 3. How often would similar eclipses return if there were no regression of the moon's nodes?

SECTION III. 1. Describe the apparent path of the sun in a summer's

day in the arctic circle. 2. Show that more of the sun's rays fall on a given

portion of the earth's surface when they are inci

dent vertically, than when obliquely. 3. The extreme summer heat of Moscow is equal to

that of Nantes, that of Tobolsk to that of Cherbourg, and that of Astrachan to that of Bordeaux, Account for these and similar facts.

Section IV. 1. Account for the apparently retrograde motions of

the planets. 2. On what causes do the variations of brightness in

the planet Venus depend ? 3. What is meant by parallax? To what uses is the

consideration of parallax applied, and what conclusions have been drawn from it?


“Give one reason, and that the simplest, for believing—

1. « That the earth is isolated in space.

2. “That the form of the earth is nearly that of a sphere.

3. “That the dimensions of the earth are those assigned to it in books on astronomy."

1. Whatever part of the world we make the scene of our observations, similar appearances in the relative positions of the heavenly bodies, at different hours of the day, present themselves. The sun, moon, and stars, rise in one direction, and set in the opposite ; and no part of the world affords an example of the absence of these phenomena. Now if these risings and settings are to be seen in all points of the horizon, and from every place, no other conclusion is admissible than that the earth is isolated in space.

2. At any point above the earth's surface and especially above that of the sea—our extent of vision is bounded, except in cases of mists or other intervening obstacles, by a clear, well-defined line, extending all round us.

That this line is a circle we conclude from the similarity of all its parts, and from its apparent distance from us, being the same in whatever direction we observe it. Now every part of the earth affords the same appearances, and no other figure than that of a sphere will do so : hence the shape of the earth is globular, or spherical.

3. Our confidence in the modes of admeasurement that have been adopted, and in the persons by whom the admeasurements have been made, are corroborated by the rough teachings of our individual experience in changing our latitude. We know, practically, that in removing due north or south from any given latitude to any other latitude, the distance, in miles, between our two localities is just what is due to their stated difference of latitude. Thus, if a person go from the town of Buckingham, due south, through Reading, to Petersfield, he will have passed from lat. 52° to lat. 51°; and the direct distance between the two places, about 70 English miles, is just the same part of the circumference of the earth, as given in works on astronomy, that 1° is of 360°, the whole circumference.


1. “ Explain the phases of the moon, and illustrate your explanation by a diagram."

In the following diagram, E is the earth—1, 2, 3, 4, 5, 6, 7, 8, different positions of the moon in its orbit; and S the direction of the sun. The enlightened halves are those towards the sun; the dark parts from the sun. Its position 1, the illuminated hemisphere of the moon is directed wholly from, and its dark half towards, the earth: this is new moon. In positions 3 and 7 half the light and half the dark sides of the moon are towards

the earth : these are the first and third quarters of the moon. At 5 the whole bright surface of the moon is towards the earth ; and, accordingly, it is full moon.

[ocr errors]

In the intermediate positions, 2, 4, 6, 8, the earth has less, more, more, less than half the bright side of the moon towards it. These different aspects of the moon, viewed from the earth, are termed the phases of the


2. “Under what circumstances is an eclipse of the sun total, partial, or annular?

Solar eclipses occur at the time of new moon, i.e., when the sun and moon are in conjunction ; or, when the sun, moon, and earth are in one line, and the moon between the sun and the earth. If the orbit of the moon coincided with that of the earth an eclipse of the sun would take place at each new moon. But the lunar orbit is inclined 5° 8' 48" to the ecliptic, so that the conjunction may happen when the moon is in that part of her orbit too far removed from the ecliptic to allow the disc of the moon to overlap that of the sun, as seen from the earth. When the disc of the moon completely

« PreviousContinue »