Progress of Invention. 313 



is more or less danger of a dispute between, the driver of a hired 

 vehicle and the person who uses it ; this is true, even in Paris — 

 though less, perhaps, than in most other places. A driver also 

 may cheat his master, since there is generally but an imperfect check 

 upon him. These inconveniences are likely soon to come to an end, 

 as a very ingenious contrivance has been recently invented at 

 Paris, and is now being tried there, which is expected to put it out 

 of the power of the driver to impose upon either the traveller or his 

 master. It records, in a way distinctly visible, the exact distance 

 traversed, the time consumed in traversing it, the number of stop- 

 pages, and the length of each. Also, for the benefit of the master, 

 the number of journeys made in the day, with the time and 



distance corresponding to them respectively. Production of 



Magnesium. — This metal will, very probably, soon become an indis- 

 pensable requisite, not merely for photography, but for many 

 ordinary purposes. All that is required for this is that its price 

 should become moderate, which iu is probable will soon be the 

 case — it has already fallen more than fifty per cent. The cost 

 of this metal, as an illuminating agent, is very easily calcu- 

 lated. A wire of it, the one-thousandth of an inch in diameter, 

 affords a light equal to that of seventy-four Stearine candles 

 of five to the pound. Three feet of this will be burned in a minute 

 ■ — that is, a quarter of an ounce in an hour, which, at the present 

 reduced rate, would cost about two shillings and sixpence. A 

 further reduction, consequent on increased demand and improved 

 methods of production, may be anticipated ; but it never can be 

 very cheap as long as sodium is indispensable to obtaining it. There 

 are many purposes to which it will be difficult ever to apply this 

 light, on account of the large quantity of caustic magnesia which 

 is given off as a very fine powder, and which soon renders the 

 atmosphere intolerable. Proper methods of ventilation may, how- 

 ever, in a great degree, remedy this inconvenience. It does not 

 exist in photography, the light is required for such a short time. 

 The magnesium light is not to be considered a perfect substitute for 

 that of the sun, being five hundred and twenty-five times less 

 intense. Its actinic power is the one thirty-sixth of that of the 



sun. The Capabilities of ilia Steam- Engine. — Three Cornish 



engines have drained the Lake of Haarlem, which contained eight 

 hundred million tons of water, a quantity which would supply 

 London for seven yeai-s, and which covered 45,230 acres to an 

 average depth of fourteen feet. These engines, when all the pumps 

 are working, are capable of raising one hundred and nine tons of 



water ten feet at each stroke. Curious Fact in Acoustics. — If the 



immense bell, which is in a large chamber at the base of one of the 

 towers of JSTotre Dame in Paris, is struck with the closed hand, a 

 large volume of sound will be produced, and will be audible to a 

 considerable distance all round ; but it has been discovered that 

 it will be perfectly inaudible if the person advances within the 

 bell to the centre, the sound diminishing as he proceeds from the 



circumference. Substitute for Tea, etc. — It is curious that various 



portions of the human race have, without being conscious of it, 



