Part 21 (1/2)
The vacuu manufacture and sealed It would naturally assuainst the atmospheric pressure, pulls it out As the pressure varies, so does the spring rise or sink; and the slightestarood aneroid is so delicate that it will register the difference in pressure caused by raising it from the floor to the table, where it has a couple of feet less of air-colu upon it An aneroid is therefore a valuable help totheir altitude above sea-level
BAROMETERS AND WEATHER
Wethe weather byto keep in ht_ recorder Hoeather connected with atenerally brings eather, the north and east winds fine weather; the reason for this being that the first reaches us after passing over the Atlantic and picking up a quantity of moisture, while the second and third have co us
A sinking of the barometer heralds the approach of heated air--that is,colder air sheds its moisture So when the lass” rises, we know that colder air is co, and as colder air comes from a dry quarter we anticipate fine weather It does not follow that the saions which have the ocean to the east or the north, the winds blowing thence would be the rainy winds, while south-westerly windshot and dry weather
THE DIVING-BELL
Water is nearly 773 tie a barometer a very little way below the surface of a water tank, we shall at once observe a rise of the mercury colued object is 15 lbs to the square inch, in addition to the atmospheric pressure of 15 lbs per square inch--that is, there would be a 30-lb _absolute_ pressure As a rule, when speaking of hydraulic pressures, we start with the normal atmospheric pressure as zero, and ill here observe the practice
[Illustration: FIG 158--A diving bell]
The diving-bell is used to enable people to work under water without having recourse to the diving-dress A sketch of an ordinary diving-bell is given in Fig 158 It may be described as a square iron box without a botto chain, and s, protected by grids; also a nozzle for the air-tube
[Illustration: FIG 159]
A silass tuh the bottolass into a vessel of water The water rises a certain way up the interior, until the air within has been compressed to a pressure equal to that of the water at the level of the surface inside The further the tuher does the water rise inside it
Evidently -bell which is invaded thus by water It is imperative to keep the water at bay This we can do by attaching a tube to the tap (Fig 160) and blowing into the tumbler till the air-pressure exceeds that of the water, which is shown by bubbles rising to the surface The diving-bell therefore has attached to it a hose through which air is forced by pumps from the atmosphere above, at a pressure sufficient to keep the water out of the bell This puen for the workers
[Illustration: FIG 160]
Inside the bell is tackle for grappling any object that has to be -bell is usedeither from a crane on the masonry already built above sea-level, or froe, comes into action The block is lowered by its own crane on to the bottom The bell descends upon it, and the crew seize it with tackle suspended inside the bell Instructions are sent up as to the direction in which the bell should be moved with its burden, and as soon as the exact spot has been reached the signal for lowering is given, and the stone settles on to the cement laid ready for it”[34]
For many purposes it is necessary that the worker should have more freedom of action than is possible when he is cooped up inside an iron box Hence the invention of the
DIVING-DRESS,
which consists of two161) is made of copper A breastplate, B, shaped to fit the shoulders, has at the neck a segmental screw bayonet-joint The headpiece is fitted with a corresponding screhich can be attached or ree of the dress, which is s, arms, body and all, is attached to the breastplate by -nuts N N, the bolts of which pass through the breastplate Air enters the helh tubes along the inside to the front This valve closes automatically if any accident cuts off the air supply, and encloses sufficient air in the dress to allow the diver to regain the surface
The outlet valve O V can be adjusted by the diver to maintain any pressure At the sides of the headpiece are two hooks, H, over which pass the cords connecting the heavy lead weights of 40 lbs each hanging on the diver's breast and back These weights are also attached to the knobs K K A pair of boots, having 17 lbs of lead each in the soles, colazed s are placed in the headpiece, that in the front, R W, being reain free access to the air when he is above water without being obliged to take off the helmet
[Illustration: FIG 161--A diver's helmet]
By means of telephone wires built into the life-line (which passes under the diver's ar) easy communication is established between the diver and his attendants above
The transmitter of the telephone is placed inside the helmet between the front and a side , the receiver and the button of an electric bell in the crown This last he can press by raising his head The life-line so 162) used by the diver at depths to which daylight cannot penetrate
The pressure on a diver's body increases in the ratio of 4-1/3 lbs per square inch for every 10 feet that he descends The ordinary working lireater pressures The record is held by one Jao of the _Cape Horn_ sunk off the South American coast, made seven descents of 201 feet, one of which lasted for forty-two minutes
[Illustration: FIG 162--Diver's electric la beloater with pneu to his buoyancy a diver has little depressing or pushi+ng power, and he cannot bore a hole in a post with an auger unless he is able to rest his back against so toolssufficient resistance to the very rapid blohich they make
[Illustration: FIG 163--Divers at work beloater with pneumatic tools]