30.  Another contrivance for regulating the effect of machinery consists in a vane or fly, of little weight, but presenting a large surface.  This revolves rapidly, and soon acquires a uniform rate, which it cannot greatly exceed, because any addition to its velocity produces a much greater addition to the resistance it meets with from the air.  The interval between the strokes on the bell of a clock is regulated in this way, and the fly is so contrived, that the interval may be altered by presenting the arms of it more or less obliquely to the direction in which they move.  This kind of fly, or vane, is generally used in the smaller kinds of mechanism, and, unlike the heavy fly, it is a destroyer instead of a preserver of force.  It is the regulator used in musical boxes, and in almost all mechanical toys.

31.  The action of a fly, or vane, suggests the principle of an instrument for measuring the altitude of mountains, which perhaps deserves a trial, since, if it succeed only tolerably, it will form a much more portable instrument than the barometer.  It is well known that the barometer indicates the weight of a column of the atmosphere above it, whose base is equal to the bore of the tube.  It is also known that the density of the air adjacent to the instrument will depend both on the weight of air above it, and on the heat of the air at that place.  If, therefore, we can measure the density of the air, and its temperature, the height of a column of mercury which it would support in the barometer can be found by calculation.  Now the thermometer gives information respecting the temperature of the air immediately; and its density might be ascertained by means of a watch and a small instrument, in which the number of turns made by a vane moved by a constant force, should be registered.  The less dense the air in which the vane revolves, the greater will be the number of its revolutions in a given time:  and tables could be formed from experiments in partially exhausted vessels, aided by calculation, from which, if the temperature of the air, and the number of revolutions of the vane are given, the corresponding height of the barometer might be found.(1*)

Notes: 

1.  To persons who may be inclined to experiment upon this or any other instrument, I would beg to suggest the perusal of the section ‘On the art of Observing’, Observations on the Decline of Science in England, p. 170, Fellowes, 1828.

Chapter 4

Increase and Diminution of Velocity

32.  The fatigue produced on the muscles of the human frame does not altogether depend on the actual force employed in each effort, but partly on the frequency with which it is exerted.  The exertion necessary to accomplish every operation consists of two parts:  one of these is the expenditure of force which is necessary to drive the tool or instrument; and the other is the effort required for the motion of some