comfort.  Time is his great object.  No age of the world has conquered such difficulties as our engineers have had to deal with, and the result is now before the eye of every thinking traveller.  Our engineers were at first self-taught, and many a self-taught man has had reason to rejoice in the time he spent in his education.  Of these men we have examples in Brindley, who was at first a labourer and afterwards a millwright; Telford was a stone-mason; Rennie a farmer’s son apprenticed to a millwright; and George Stephenson was a brakesman at a colliery.  Perseverance with genius, and a determination to overcome, made them the great men they were.  That you may so persevere and strive is the earnest wish of him who has this evening had the great pleasure of giving you this lecture, and who feels so greatly obliged to you for the very patient hearing you have given him.

ON ATTRACTION.[B]

Gravitation.—­Attraction, which may be illustrated by the effect a magnet has on a piece of iron, may be viewed generally as an influence which two bodies, say, exert on each other, under which, though at a distance, they tend to move towards each other till they come into contact.  The force by which a body has weight, and, when free, falls to the ground, is of this nature; and it is called, from gravis, “heavy,” the gravitating force of the earth, because it causes weight, and because, though emanating in a small degree from the falling body, it is mainly exerted by the earth itself.  It is under the action of gravity that a pendulum oscillates:  it is by that unseen influence it begins to sway alternately downward and upward as soon as it is moved to a side; and it is only because it is withheld by the rod that the ball or bob keeps traversing the arc of a circle and does not fall straight to the earth.

All material substances, however small, and however light, buoyant, and ethereal they may seem, are subject to this force:  the tiniest speck in a sunbeam and the most volatile vapour, equally with the heaviest metal and the hugest block, the particles of bodies as well as the bodies themselves.  The rising of a balloon in the air may seem an exception to this law; but it is not so; for the balloon rises, not because the particles of the gas with which it is inflated are not acted upon by the earth’s attraction, but because the air outside being bulk for bulk heavier than the air inside, its particles press in below the balloon and buoy it up, until it reaches a stratum of the atmosphere where, the pressure being less, the air outside is no heavier than the air within—­a fact which rather proves than disproves the universal action of gravitation; because the greater weight of the air in the lower strata of the atmosphere is due to the pressure of the air in those above, and the balloon ceases to ascend because it has reached a point where the air outside is the same weight as the air within, and the weight in both cases is caused by the attraction of the earth.