"On impact, an air canister triggers the block to drop so the mercury drops into a bottom chamber" - thereby cutting off the battery connection.

In these months, skies are often gloomy and the mercury drops below freezing.

Autumn is almost as inviting as spring, as the crowds dissipate and the mercury drops to a more comfortable level.

But the last time that the mercury dropped below 25 in Miami was, well, never.

But advocates for the homeless have argued that someone evicted during a thaw could be in grave danger when the mercury dropped again.

From a daytime 70-75F the mercury drops to below freezing.

Monticello's mercury, which usually hovers between 10 and 20 degrees, dropped to near zero several times.

At one point, in midday, the mercury dropped to 68 degrees in Central Park.

Then the tube is filled, sealed and the mercury dropped and shaken.

The mercury had dropped even farther, but the air had dried.