without these aids, that the paddle shaft will not stand awry across the vessel, nor be carried forward beyond its place by the framing shouldering up more than was expected.  As a plumb line cannot be used, recourse must be had to a square; and it will signify nothing at what angle with the deck the keelsons run, so long as the line of the shaft across the keelsons is square down from the shaft centre.  The sole plates being fixed, there is no difficulty in setting the other parts of the engine in their proper places upon them.  The paddle wheels must be hung from the top of the paddle box to enable the shaft to be rove through them, and the cross stays between the engines should be fixed in when the vessel is afloat.  To try whether the shafts are in a line, turn the paddle wheels, and try if the distance between the cranks is the same at the upper and under, and the two horizontal centres; if not, move the end of the paddle shaft up or down, backward or forward, until the distance between the cranks at all the four centres is the same.

490. Q.—­In what manner are the engines of a steam vessel secured to the hull?

A.—­The engines of a steamer are secured to the hull by means of bolts called holding down bolts, and in wooden vessels a good deal of trouble is caused by these bolts, which are generally made of iron.  Sometimes they go through the bottom of the ship, and at other times they merely go through the keelson,—­a recess being made in the floor or timbers to admit of the introduction of a nut.  The iron, however, wears rapidly away in both cases, even though the bolts are tinned; and it has been found the preferable method to make such of the bolts as pass through the bottom, or enter the bilge, of Muntz’s metal, or of copper.  In a side lever engine, four Muntz’s metal bolts may be put through the bottom at the crank end of the framing of each engine, four more at the main centre, and four more at the cylinder, making twelve through bolts to each engine; and it is more convenient to make these bolts with a nut at each end, as in that case the bolts may be dropped down from the inside, and the necessity is obviated of putting the vessel on very high blocks in the dock, in order to give room to put the bolts up from the bottom.  The remainder of the holding down bolts may be of iron, and may, by means of a square neck, be screwed into the timber of the keelsons as wood screws—­the upper part being furnished with a nut which may be screwed down upon the sole plate, so soon as the wood screw portion is in its place.  If the cylinder be a fixed one it should be bolted down to the sole plate by as many bolts as are employed to attach the cylinder cover, and they should be of copper or brass, in any situation that is not easily accessible.

491. Q.—­If the engines become loose, how do you refix them?