Admiral FitzRoy, in a communication to the _Mercantile Marine Magazine_, December, 1860, says:--"Aneroids are now made more portable, so that a pilot or chief boatman may carry one in his pocket, as a railway guard carries his timekeeper; and, thus provided, pilots cruising for expected ships would be able to caution strangers arriving, if bad weather were impending, or give warning to coasters or fishing boats. Harbours of Refuge, however excellent and important, are not always accessible, even when most wanted, as in snow, rain, or darkness, when neither land, nor buoy, nor even a lighthouse-light can be seen."
=51. Measurement of Heights by the Aneroid.=--For measuring heights not exceeding many hundred feet above the sea-level by means of the aneroid, the following simple method will suffice:--
Divide the difference between the aneroid readings at the lower and upper stations by ·0011; the quotient will give the approximate height in feet.
Thus, supposing the aneroid to read at the
Lower Station 30·385 inches. Upper Station 30·025 ------ Difference ·360 ======
As an illustration of the mode in which the aneroid should be used in measuring heights, the following example is given:--
A gentleman who ascended Helvellyn, August 12th, 1862, recorded the following observations with a pocket aneroid by Negretti and Zambra:--
Near 10 a.m., at the first milestone from Ambleside, found by survey to be 188 feet above the sea, the aneroid read 29·89 inches; about 1 p.m., at the summit of Helvellyn, 26·81; and at 5 p.m., at the milestone again, 29·76. The temperature of the lower air was 57°, of the upper, 54°. Hence the height of the mountain is deduced as follows:--
Reading at 10 a.m. 29·89 " 5 p.m. 29·76 ------ Mean 29·825 Table I.[5] 1010 Upper Reading 26·81 " 3796 ----- Difference 2786 Mean Temperature 55°·5, gives in Table II. 1·048 ----- 2920 Lat. 55° N., gives in Table III. ·9991 ----- 2917 Table IV. 5 ----- Difference of height 2922 Height of lower station 188 ----- " Helvellyn 3110
In Sir J. Herschell's _Physical Geography_ it is given as 3115 ft.
So near an agreement is attributable to the excellence of the aneroid, and the careful accuracy of the observer.
This instrument, the invention of M. Bourdon, has a great resemblance to the aneroid, but is much simpler in arrangement. The inventor has applied the same principle to the construction of metallic steam-pressure gauges. We are here, however, only concerned with it as constructed to indicate atmospheric pressure. It consists of a long slender flattened metallic tube, partially exhausted of air, and hermetically closed at each end, then fixed upon its centre, and bent round so as to make the ends face each other. The transverse section of this tube is an elongated ellipse. The principle of action is this: interior pressure tends to straighten the tube, external pressure causes it to coil more. Hence as the atmospheric pressure decreases, the ends of the tube become more apart.