88 
THE NATURE AND 
[sect. II. 
elastic force, equivalent to the pressures : and it would be as the velocity acquired 
by falling through the difference between the heights of the columns equivalent to 
the pressure at any other instant; the height to be ascertained for the instant at 
which the velocity is required. After a certain time, the pressures or elastic forces 
would become equal, and the velocity of course would be nothing. 
131. The consideration of chimneys is another case of the motion of elastic 
fluids, where, by increase of temperature, a part of an atmospheric column is ren 
dered of a different density. Some mistakes have been committed in treating this 
case ; but we must proceed to treat of the motions which take place in engines, and 
first of the allowances to be made for contractions. 
132. In the motion of elastic fluids, it appears from experiments, that oblique 
action produces nearly the same effect as in the motion of water, in the passage of 
apertures; and that eddies take place under the same circumstances, tending to 
retard the motions in a considerable degree. 
133. The velocity of motion that would result from the direct unretarded 
action of the column of the fluid which produces it being unity - - - 1 ’000 or 8 
The velocity through an aperture in a thin plate by the same pressure is - *625 1 or 5 
Through a tube from two to three diameters in length projecting outward - *813 or 6‘5 
Through a tube of the same length projecting inwards ----- *681 or 5’45 
Through a conical tube, or mouth piece, of the form of the contracted vein - '983 or 7'9 
134. Every enlargement of a pipe which is succeeded by a contraction 
reduces the velocity of the motion, and in proportion to the nature of the con 
traction, and every bend and angle in a pipe, is attended with a diminution of 
velocity. Hence, as far as convenience will admit, these causes of loss should be 
avoided; and where they must be introduced, such forms should be given as will, 
lessen the defect as much as possible. 
Of the Motion of Steam in an Engine. 
135. We have stated (art. 129.) that the most convenient mode of determining 
the motion of steam, is, by finding the height of a column of the same fluid which 
would produce an equal pressure upon a base of equal area : the manner of deter 
mining this column is therefore the first point to be considered. The force of 
steam is sometimes expressed by the pounds on a square inch ; sometimes by the 
inches in height of a column of mercury, and not unfrequently by the number of 
atmospheres : it will therefore be an advantage to find the height of a column of 
water equivalent to each of these measures; and then, that being multiplied by the 
1 According to experiments on air made by Mr. Banks, 0'634. See Power of Machines, p. 13.