the column BC is greater than column BH, the effective pressure of atmosphere in direction HB is greater than the effective pressure of atmosphere in direction CB, and therefore the fluid will be driven by the effective atmospheric pressure in a continuous stream in the direction HBC.

91. On intermitting Springs.

Intermitting Springs are springs which run for a time, then stop for a time, and then begin to run again.

This phenomenon is explained by the principle of the Siphon.

Let A be a reservoir in a hill in which water is gradually collected through fissures, as B, C, D, communicating with the external air.

Now suppose a channel MNR to run from A, first ascending to N and then descending to R, a place lower than the reservoir.

As the water collects in A it gradually rises in the channel to N, and then flows along NR, and by the principle of the Siphon it will continue to flow till A is completely drained. Then the flow ceases till the water in A has collected sufficiently to reach N.

92. Bramah's Press.

The Hydrostatic Press, generally called Bramah's Press, is a machine by which an enormous pressure is obtained by means of water, the only assignable limits to its power being the strength of the materials of which it is formed.

AC is a forcing-pump, by the action of which water is forced into a tube BD, which has a valve B opening inwards.

E is a strong cylindrical piston, with a base many times larger than the base of the piston A, working in a water-tight collar at M, N.

Between the top of the piston E and a fixed beam FG, a bale of goods, such as paper, cotton or wool, is placed.

Suppose the area of the base of E to be 200 times that of the base of A.

Then if a pressure of 100 lbs. be applied to A, a pressure of (200 × 100) lbs. or 20,000 lbs. will be conveyed to the base of E. Thus any amount of pressure may be applied to W, either by increasing the pressure applied to A, or by making the base of E larger in comparison with the base of A.

(1) What will be the effect of making a small aperture in the barrel of a Forcing Pump? If the piston work uniformly up and down the length of the barrel, and a small aperture be made one-third of the way up the barrel, how much more time than before will be consumed in filling a tank?

(2) If the upward motion of the piston of a Common Pump be stopped, when the water has risen to the height of 16 feet in the supply pipe, but has not yet reached the piston, find the tension of the piston-rod, the area of the piston being 4 square inches, and the atmospheric pressure 15 lbs. on the square inch.

(3) What would be the effect of opening a small hole at any point in the Siphon, first above, secondly below the surface of the fluid in the vessel ?

(4) What is the greatest height above the surface of a spring over which its water may be carried by means of a siphon-tube, when the barometer stands at 29 inches, the specific gravity of mercury being 13'57 ?

(5) What would take place in a siphon at work if the pressure of the atmosphere were removed?

(6) Will the siphon act better at the top or the bottom of a mountain?

(7) Could a siphon be employed to pump water out of the hold of a ship floating in a harbour?

(8) What is the greatest height over which water can be carried by means of a siphon when the mercurial barometer stands at 30 inches ?

(9) If the ends of a siphon were immersed in two fluids of the same kind and the air were removed, describe what would take place.

(10) A hollow tube is introduced into the bottom of a cylindrical vessel through an air-tight collar; and a large tube, of which the top is closed, suspended over it, so as not quite to touch the bottom: consider the effect of gradually pouring water into the cylinder, until it reaches the level of the top of the inverted tube.

(11) A siphon is placed with one end in a vessel full of water, and the other in a similar empty one, both of which are on the plate of an air-pump. As soon as the water has covered the lower end of the siphon, a receiver is put on, and the air rapidly exhausted, and then gradually readmitted: describe the effects produced.

(12) A siphon, filled with water, has its ends inserted in vessels filled with water; state what will take place when the vertical distances of the highest point of the siphon above the surface of the fluid are both less, both greater, and one greater and the other less than the height of the Water-Barometer.

(13) What is the length of the smallest siphon that can empty a vessel 2 feet deep?

CHAPTER VII.

On the Thermometer.

93. THE general consequence of imparting heat to bodies is the expansion of their volume.

The particles which compose a solid body, as for instance a block of lead, are held together by the force of cohesion. It requires a force of great magnitude to increase or to decrease the volume of a block of lead, though lead is a soft metal. The application of heat, by weakening the force of cohesion, reduces lead and other metals to a liquid state, pushes the particles more widely apart, and thus increases the volume of the bodies to which it is applied.

If heat be applied to a liquid, as water, the cohesion of the particles is weakened, and they ultimately acquire a tendency to break away from each other and assume the form of a

vapour.

If heat be applied to an elastic fluid, as air, it causes it to expand. Thus if a bladder, partly full of air, be placed before a fire, the air will expand and distend the bladder.

Again, if a piston P exactly fits a cylindrical tube AB, and is supported by the condensed air in PB, if heat be applied to the air in PB it will expand and raise the piston.

A

P

B