Table showing the Number of Threads to an Inch in V-thread Screws.

The depth of the threads should be half their pitch. The diameter of a screw, to work in the teeth of a wheel, should be such, that the angle of the threads does not exceed 10°.

CAST IRON expands 162000 of its length for one degree of heat; greatest change in the shade, in this climate, T of its length; exposed to the sun's rays, Tooo; shrinks in cooling from tog of its length; is crushed by a force of 93,000 lbs. upon a square inch; will bear, without permanent alteration, 15,300 lbs. upon a square inch, and an extension of T20 of its length. Weight of modulus of elasticity for a base of an inch square, 18,400,000 lbs.; height of modulus of elasticity, 5,750,000 feet.

WROUGHT IRON expands T43000 of its length for one degree of heat; will bear, on a square inch, without permanent alteration, 17,800 lbs., and an extension in length of Too; cohesive force is diminished 3000 by an increase of one degree of heat. Weight of modulus of elasticity for a base of an inch square, 24,920,000 lbs.; height of modulus of elasticity 7,550,000 feet.

PART SECOND.

INTRODUCTION.

CHOICE OF A PROFESSION: RESPECTABILITY OF MECHANICAL TRADES.

THE choice of a pursuit in life, one of the most important practical questions upon which a young person is ever called to decide, is often determined by the most trifling circumstances, and without the slightest aid from judgment or reflection. One youth becomes a soldier because his great grandfather was at the taking of Cape Breton, or his great uncle signalized himself in Braddock's fight; another studies medicine, and hopes to be almost an infallible doctor, because he is the seventh son of a seventh son; while a third chooses the profession of the law for no better reason than that his sponsors at the baptismal font, chose to call him William Wirt, or Daniel Webster, or John Sergeant. Surely this is not that practical wisdom which adapts the fittest means to the noblest ends The choice of a profession

in life is at least worthy of such a consideration as common sense would dictate in any other case, where success in an enterprise depends upon fitness for undertaking it. Men do not expect to gather grapes from thorns, nor figs from thistles; yet they expect their sons and daughters to succeed in pursuits for which they are wholly incapacitated by talents, disposition or education; and what is still more unreasonable, they expect them to be happy in situations which are totally uncongenial to their

nature.

One reason why parents and guardians fall so frequently into errors on this point, errors, too, which they lead those under their charge to embrace, is the vain imagination that there is a great and essential difference in the respecta bility of those pursuits which are generally admitted to be honest. The respectability of a profession, I suppose it will be admitted, must depend in a great measure on the respectable character of its members, taken collectively, or regarded with reference to the most brilliant examples. If we adopt this standard, it will be found no easy matter to establish a claim to superior respectability in favour of any one

trade or profession, or of any class of trades or professions.

If it should be asserted that the learned professions of law, physic and divinity are more respectable than the pursuits of commerce, mechanics or agriculture, it might be easily shown that taken collectively, the members of these latter professions or trades possess more wealth, ease and independence than those of the learned ones; and moreover, that among them as brilliant examples of mental pre-eminence, patriotism and public spirit may be pointed out as among those of the more learned professions

In fact, in a country like ours, such a claim of superior respectability on behalf of any profession is preposterous; and yet it is constantly assigned by purse-proud fathers and silly mothers as a reason for determining their children's pursuits in life. There is a very general impression that a merchant, a clergyman, doctor or lawyer stands higher, and should stand higher, in the social scale than a mechanic or farmer. But such is not the fact, as a general principle; or, which results in the same thing, if in a particular instance, a particular merchant, for example, stands higher in social estimation than a particular mechanic, it is not on account