hardness of another; some we try to stretch, others to tear. To try what things are is the object of this kind of experiment.

20. Often, in a somewhat similar way, experiments are undertaken to ascertain the relative fitness of different materials for a special purpose, and the extent to which their properties may be relied upon. Thus, previous to the construction of the celebrated Tubular Bridge over the Menai Straits, a long course of experiments was entered upon to find out the relative values of wrought and cast iron employed in different ways. By these it was found generally that cast iron is best for bearing a crushing force, and wrought for enduring a strain; and, in accordance with this, the two were most advantageously employed in different parts of the structure. Recently an experiment has been made to manufacture wrought-iron guns, which has failed, from the tendency of the metal to resume its natural, or crystallized, condition when subjected to vibrations. This fact, that iron returns from the fibrous to the crystalline texture, was previously known by the observation of axles of railway carriages, which had become fractured after long use; so that it affords an example of what generally occurs, that observation and experiment, when carefully made, confirm each other.

21. Frequently the purpose of experiment is to determine the extent of a truth, so as to ascertain how far it may be considered a principle applicable to a class, or whether it is only a solitary fact. Thus, it was long observed that an object seen through water appeared displaced; substituting other transparent media for water this was found to be generally true-i. e., an object seen through any transparent substance appears displaced. But on examination it was found not to exhibit equal degrees of displacement, being distorted more or less according to the angle under which it was viewed, even when the same transparent substance was experimented with, and varying for the same angle when different substances were used. A careful comparison of the changes resulting under these different circumstances enabled Snellius to give to the world his

well-known law of refraction, and other observers to determine the refractive powers of different substances.

22. It is a different matter when we wish to find out any hitherto unknown properties in substances which we know, or to discover new substances. Then truthseeking experiments may be said to commence. The substance must be placed under as many different conditions as the experimenter's ingenuity can devise, and the resulting changes ascertained and compared. It was by patient labour in such a mode of cross-examining natural objects that many of the great truths of natural science have been established. Experience, of course, and acquaintance with all that has been previously discovered about the examined substance will often suggest the best line of inquiry in which to proceed, and save much waste of time and fruitless labour. No one should endeavour to seek for a new truth in a science until he has mastered those established by others, or he will run the risk of much useless toil; and, even supposing him successful, may only re-discover something already known and published by persons better informed than himself. It is a well-known fact that much ingenuity which might have been turned to better account has been thus wasted, from ignorance of the actual progress of science and its applications; and many undoubted discoverers, so far as is meant by finding a truth for themselves, have been bitterly disappointed after long labours, and the momentary triumph of success, at finding that their results had been long before anticipated by a former labourer in the same field of investigation.-E. PURCELL.

ENTERPRISE.

WHAT though this ancient earth be trod
No more by step of demigod,

Mounting from glorious deed to deed
As thou from clime to clime didst lead;
Yet still, the bosom beating high,
And the hushed farewell of an eye
Where no procrastinating gaze
A last infirmity betrays,

Prove that thy heaven-descended sway
Shall ne'er submit to cold decay.
By thy divinity impelled

The stripling seeks the tented field ;
Inflamed by thee, the blooming boy
Makes of the whistling shrouds a toy,
And of the ocean's dismal breast
A play-ground,- —or a couch of rest;
'Mid the blank world of snow and ice,
Thou to his dangers dost enchain
The chamois-chaser awed in vain
By chasm or dizzy precipice;
And hast thou not with triumph seen
How soaring mortals glide between

Or through the clouds, and brave the light
With bolder than Icarian flight?

How they, in bells of crystal, dive—
Where winds and waters cease to strive-
For no unholy visitings,

Among the monsters of the deep;
And all the sad and precious things
Which there in ghastly silence sleep?
Or, adverse tides and currents headed,
And breathless calms no longer dreaded,
In never-slackening voyage go
Straight as an arrow from the bow;
And, slighting sails and scorning oars,
Keep faith with Time on distant shores?
Within our fearless reach are placed
The secrets of the burning waste;
Egyptian tombs unlock their dead,
Nile trembles at his fountain-head.
Thou speak'st-and lo! the polar seas
Unbosom their last mysteries.

But thou O goddess! in thy favourite Isle
(Freedom's impregnable redoubt,

The wide earth's store-house fenced about

With breakers roaring to the gales

That stretch a thousand thousand sails),

Quicken the slothful, and exalt the vile!-
Thy impulse is the life of fame;
Glad hope would almost cease to be
If torn from thy society;

And Love, when worthiest of his name,
Is proud to walk the earth with thee!

WORDSWORTH.

Geology.

BY J. BEETE JUKES.

LESSON I.

LITHOLOGY.

"Finds tongues in trees, books in the running brooks,
Sermons in stones, and good in everything.'

1. THERE is a saying in old books of Natural History, which is adopted by Linnæus, that "Stones grow, vegetables grow and live, animals grow, live, and feel." This saying is untrue so far as "stones" are concerned, if the word "grow" is used in the same sense throughout. The growth of animals and plants is a process altogether different from that by which "stones" or any mineral substances are formed; and the term "growth,' as describing an internal and spontaneous increase of substance, could never be properly applied to the formation or increase of the latter.

2. In the chapter of Vol. I., describing "Minerals and Rocks, and their Uses," this subject was touched upon; but we propose now to go into it a little more fully, and describe what was the method by which stones and rocks were formed. To do this fully we shall be obliged also to describe the method by which the external crust of the earth was produced; for we cannot explain the formation of any individual lumps or masses of rock, stone, or earth, without also explaining how all the earthy matter which is anywhere open to our examination over the whole face of the globe came into its present form and condition.

And here a difficulty meets us at the outset, which it is quite right we should overcome before we go any farther. Until the science of geology was properly understood, people naturally imagined that the earth was originally created very nearly as we now find it. Just as, till the science of astronomy was properly understood, they as naturally imagined that the sun went round the earth, instead of the earth round the sun. Neither of these sciences, nor any other of the natural sciences, was made the subject of revelation,

and, therefore, there is nothing in the Bible to enlighten us on these points. Incidental expressions are used therein which seem rather to favour the common notions; but that is to be expected, because we have no reason to suppose that the sacred writers were supernaturally enlightened on such matters, and they therefore necessarily used the expressions which would naturally occur to them. There is nothing in the science of astronomy, or in that of geology, at all opposed to any of the great doctrines of revealed religion, and some of the most pious and most eminent divines of all forms of the Christian religion, without exception, have been, and still are, eminent geologists. When, therefore, we speak of the earth as having passed through many different states, and as having existed as a habitable globe through untold ages anterior to the creation of man, we speak of things which the Bible does not contradict, but simply says nothing about them.

3. Geology does not pretend to explain the origin of the earth. Some people have supposed it once to have existed as a gaseous body, and after that as a mass of molten matter. We will not discuss the

question, but we will take the earth as we find it—a globe, the surface of which is composed of land and water, surrounded by an atmosphere of air producing winds, rains, and snows,-and from the interior of which are here and there vomited forth molten masses of stone called lava, accompanied by cinders and ashes, that produce mountains which we call volcanoes. On the earth in this state there are two principal agencies at work producing new rocks, these agencies are FIRE and WATER.

4. IGNEOUS ACTION.-Suppose we take that of Fire first, as the most striking and obvious.

We

What happens at the eruption of a volcano? all know that vast quantities of powder, dust, sand, and ashes are blown out of the interior of the mountain, and fall all over the country round its foot. In some of the tremendous eruptions in the larger volcanoes of the globe these materials have been carried even