Fig. 396.—Face.
Fig. 397.—Back.
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839.—Watkin's New Patent Mountain Aneroid Barometer.—This instrument, of which both a front and back view is shown above at Figs. 396 and 397, is the invention of Colonel H. S. Watkin. The special feature is that it can be put in or out of action as required, and when out of action is impervious to the influence of variations in atmospheric pressure. This relieves the strain on the mechanism of the aneroid, as it is only put into action when a reading is required. The lower portion of the vacuum-box instead of being a fixture (as is the case with ordinary instruments) is allowed to rise, which is effected by attaching to the lower portion of the vacuum box a screw arrangement actuated by a fly nut on the outside of the case. Under ordinary conditions this screw is released, and the vacuum-box put out of strain. When a reading is required, the fly nut is screwed up as far as it will go, thus bringing the instrument into the normal condition in which it was graduated.
It has an aluminium case for lightness, is made in two sizes (3 inch and 4½ inch), and has a sling leather case.
These plans are again on their trial. It is the author's opinion on the subject, knowing the delicacy and skill shown in Vidie's work, that little improvement is likely to be obtained by magnification of the small motion of the vacuum chamber by mechanical means, which must necessarily be by a process both delicate and highly frictional. Attempts, he thinks, may otherwise be successfully made in the magnification of the small motion of the hand in a frictionless manner by optical means to obtain clearer definition.
840.—An improvement was made in the aneroid in one direction by the late Thomas Cooke[64] by replacing the chain by a thin gold band upon, and leading from, the drum. This obviated the small difference of rate of displacement due to separate jointed links as they leave the tangent of the drum. It is said, however, to cause a little springiness at this point, where it should be very dead, which somewhat minimises the improvement; so that it has not been very generally adopted.
841.—Bourdon's Aneroid, invented by C. Bourdon in 1849.[65] The motor of this instrument consists of a flat, oval tube bent into a circular form. This tube opens to greater and lesser curvature by difference of external pressure upon it. The small motion given at one free end of the tube is multiplied up by gearwork. This instrument is found to act most delicately as a steam gauge; but experience has shown that it is not so sensitive or durable for indicating atmospheric pressure as the vacuum-chamber aneroid last described.
842.—Hypsometer, or Boiling-point Thermometer.—That water or any other liquid boils at a certain temperature, according to the amount of atmospheric pressure surrounding it, is easily observed by placing a cup of boiling hot water under the receiver of an air-pump. At first the surface will remain still, but as the pressure of the air is pumped off it may be made to boil time after time until it arrives at a low temperature. The temperature at which the water boils as the air is rarified may be easily followed by observation of a thermometer immersed in the cup of water; and at the same time, if a barometer be placed in connection with the receiver it will indicate the pressure, from which the scale of differences may be practically made. For the civil engineer this instrument, accompanied by the aneroid, is in every way superior to the mountain barometer, which must necessarily have a three-feet tube, as the hypsometer is much lighter, more portable, and less liable to injury, and perhaps, from the uncertainty of keeping a pure vacuum in the barometer, safer as a means of observation.