CLEPSYDEA SAND-GLASS. 



sand-glass, and gives the signal when the sand has run out. 

 The number of knots run off from the reel is then the number of 

 miles per hour in the rate of the vessel. The intervals between 

 the knots, the quantity of sand in the glass, and the aperture 

 through which it falls, are so adapted to each other as to give this 

 result. 



9. Notwithstanding the great perfection to which the art of 

 constructing chronometers has attained, an apparatus was not long 

 since proposed by the late Captain Kater for the measurement of 

 very small intervals of time, fractions of a second, for example, 

 which is a modification of the clepsydra. A quantity of pure and 

 clean mercury is poured into a funnel with a small aperture at its 

 apex, so that a stream of the quicksilver shall fall through it. 

 The flow is rendered uniform, by keeping the mercury in the 

 funnel at a constant level. The apparatus is intended in scientific 

 researches to note the exact duration of phenomena, and it is 

 so managed, that the stream issuing from the funnel, is turned 

 over a small receiver at the instant the phenomenon to be 

 observed commences, and is turned away from it the instant the 

 phenomenon ceases. The mercury discharged into the receiver is 

 then accurately weighed, and the number of grains, and parts 

 of a grain it contains, being divided by the number of grains 

 which would be discharged in a second, the number of seconds, 

 and the parts of a second, which elapsed during the continuance 

 of the phenomenon is found. 



10. For the purposes of civil life, as well as for the more 

 precise objects of scientific research, all these contrivances have 

 been superseded by clocks and watches, which are now so 

 universal as to constitute a necessary article of furniture in the 

 most humble dwellings, and a necessary appendage of the person 

 in all civilised countries. 



All varieties of this most useful mechanical contrivance include 

 five essential parts. 



1. A moving power. 



2. An indicator, by whose uniform motion time is measured. 



3. An accurately divided scale, upon which the indicator moves 

 and by which its motion is measured. 



4. Mechanism, by which the motion proceeding from the 

 moving power is imparted to the indicator. 



5. A regulator, which renders the motion imparted to the 

 indicator uniform, and which fixes its celerity at the required rate. 



Thus, for example, in a common clock, the moving power is the 

 weight suspended by cords over a pulley fixed upon the axle of a 

 wheel, to which the weight in descending imparts a motion of 

 rotation. The indicator is the hand. The scale is the dial plate 



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