# Page:Popular Science Monthly Volume 91.djvu/763

How Old Is the Earth?

the Salt of the Sea

THE salt of the ocean has been made to give an answer to the question of the age of the earth. The general idea seems simple enough to be worked out by anyone with a plentiful supply of good long pencils and sufficient paper.

Salt is carried down year after year into the sea, where it accumulates. If, then, we can divide the entire amount of salt at present in the ocean by the amount carried down in a year, the quotient will be the probable age of the earth. Simple? Oh yes. Pro- fessor Joly has worked it out to ninety- nine million years, but to be on the safe side he reduces that number to ninety million. A little matter of nine million years is negligible to those who can think in such numbers.

��Popular Science Monthly As Old as

��747

���The steam shovel moved along by its own power on its self-laid track at the rate of about four blocks a day through the side streets of the city

��Low temperature contact points

High temperature

contact points

��The Temperature Alarm. It Tells You Whether You Are Too Warm or Too Cold

THE burglar alarm now has a rival in the temperature alarm. A bell rings to inform you that you are too warm, or too cold, as the case may be.

Let us assume that the damper on your furnace is open, which, of course, causes the fire to burn fiercely. A chain is attached to the damper at one end, and at the other to a metallic ring. The ring is part of an electric cir- cuit in which the bell is located. A thermostat (a kind of thermometer) is inserted in the circuit near the contact point, and swells when the house becomes too hot. This causes the contact points of the thermostat to touch, and the circuit is thus closed, so that the bell rings.

Upon hearing the bell ring, you realize that you are too warm. You move the metallic ring down to another position which breaks the circuit, causes the bell to stop ringing, closes the damper and cools the house.

���A thermostat (a kind of thermometer) inserted in a bell circuit causes the bell to ring when the temperature is too high or too low

��How a Railroad Steam-Shovel Traveled Through New York City

IN excavating the site for a new hospital in New York city a contractor made use of a gigantic steam-shovel mounted on a stan- dard railroad flat-car. Upon the comple- tion of the work the shovel had to be trans- ported to the next scene of operations, in this case several miles away, the route being entirely through city streets.

No alternative pre- sented itself to the mind of the contractor to a plan to lay tracks for this self-propelled steam- shovel and to pick them up again after the shovel had passed on. So, mak- ing use of three short sec- tions of track consisting of standard rails held the required distance apart by iron rods, the steam-shovel started on its queer journey. As it passed, under its own power, from the last sec- tion to the second sec- tion, a team of horses hauled the last section around to the front of the shovel to form a new length of track.

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