20 Things not generally Known. 



1 dropped my slipper until I heard the sound as it fell in the street, 

 which, according to the calculations made of the time taken by bodies 

 in their accelerated fall, and of that employed by the sonorous undula- 

 tions of the air to arrive from the street to my ear, gave the height of 

 my apartment as 94 feet 3 inches 1 tenth (French measure), supposing 

 that my heart, agitated as it was, beat 120 times in a minute." 



A person travelling may ascertain his rate of walking by the aid of a 

 slight string with a piece of lead at one end, and the use of a seconds 

 watch ; the string being knotted at distances of 44 feet, the 120th part 

 of an English mile, and bearing the same proportion to a mile that half 

 a minute bears to an hour. If the traveller, when going at his usual 

 rate, drops the lead, and suffers the string to slip through his hand, the 

 number of knots which pass in half a minute indicate the number of 

 miles he walks in an hour. This contrivance is similar to a log-line for 

 ascertaining a ship's rate at sea : the lead is enclosed in wood (whence 

 the name log), that it may float, and the divisions, which are called 

 knots, are measured for nautical miles. Thus, if ten knots are passed in 

 half a minute, they show that the vessel is sailing at the rate often knots, 

 or miles, an hour : a seconds watch would here be of great service, but 

 the half-minute sand-glass is in general use. 



The rapidity of a river may be ascertained by thro wing- in a light 

 floating substance, which, if not agitated by the wind, will move with 

 the same celerity as the water : the distance it floats in a certain number 

 of seconds will give the rapidity of the stream ; and this indicates the 

 height of its source, the nature of its bottom, &c. See Sir Howard 

 Douglas on Bridges. Thomson's Time and Time-keepers. 



SAND IN THE HOUE-GLASS. 



It is a noteworthy fact, that the flow of Sand in the Hour- 

 glass is perfectly equable, whatever may be the quantity in the 

 glass ; that is, the sand runs no faster when the upper half of 

 the glass is quite full than when it is nearly empty. It would, 

 however, be natural enough to conclude, that when full of sand 

 it would be more swiftly urged through the aperture than when 

 the glass was only a quarter full, and near the close of the hour. 



The fact of the even flow of sand may be proved by a very 

 simple experiment. Provide some silver sand, dry it over or 

 before the fire, and pass it through a tolerably fine sieve. Then 

 take a tube, of any length or diameter, closed at one end, in 

 which make a small hole, say the eighth of an inch ; stop this 

 with a peg, and fill up the tube with the sifted sand. Hold the 

 tube steadily, or fix it to a wall or frame at any height from a 

 table ; remove the peg, and permit the sand to flow in any mea- 

 sure for any given time, and note the quantity. Then let the 

 tube be emptied, and only half or a quarter filled with sand ; 

 measure again for a like time, and the same quantity of sand 

 will flow : even if you press the sand in the tube with a ruler 

 or stick, the flow of the sand through the hole will not be in- 

 creased. 



The above is explained by the fact, that when the sand is 

 poured into the tube, it fills it with a succession of conical 



