What is a mystery clock? How does it work? Magicians never tell the secrets of their trade. But the secret of the mystery clock, first made in the 1860s, has been exposed.

The most famous mystery clock had a figure, often a goddess, who held a long pendulum that swung for no obvious reason. She stood on a circular base with a platform. The platform moved slightly, making the pendulum swing and the clock hands move to tell the time.

A different type of mystery clock was made in the 20th century. It was very popular from the 1950s to the 1980s. A normal-looking circular face that was secretly made of four glass discs was set in a modern looking case, usually rectangular. The hands were glued to the discs. The numbers for the hours were on a band on the case that went around the glass, so the hands seem to float on the glass while keeping the time. Many of these clocks were made with gold and jewels and sold for high prices.

Ceres, the goddess of agriculture, is featured in a mystery clock that sold late in November 2014 for $5,843. She is holding a ball-shaped black clock suspended on chains from her left hand. The round black clock has raised Roman numerals, and it furnishes the necessary rotation.

Buddy L truck

Q: I have an old 20-inch toy Army truck by Buddy L. It was given to my brother in 1940. The truck has a green metal body, a canvas cover and rubber wheels. The side door is embossed with "Buddy L." The truck is in good condition. What is it worth?

A: Your Army transport truck with the Buddy "L" bar and circle logo embossed on the door was made from 1939 to 1940. The truck was sold alone or with a trailer. It's worth $90 to $175, depending on its condition. The Buddy "L" company goes back to the Moline Pressed Steel Co. that was started by Fred A. Lundahl in 1913 in Moline, Ill. His son, Arthur Brown Lundahl, was born in 1915 and nicknamed Buddy. Another boy named Buddy lived in their neighborhood, and to avoid confusion, Buddy Lundahl was called Buddy "L." His father's company began making toys under the name, Buddy "L" in 1921. Buddy "L" toys still are being made.

Dresden cup

Q: I have a delicate blue and gold cup and saucer with flowers that is marked "Dresden, Saxony" along with an animal that looks like a calf. I'd love it if you could give me some information about it.

A: The mark on your cup and saucer was used by Ambrosius Lamm, a decorator who opened a studio in Dresden, Germany, in 1887. He decorated porcelain made by the famous manufacturers in the nearby town of Meissen. His mark shows a lamb over the word "Dresden." The word "Saxony" was added in 1915 and used until about 1934, when the mark was changed to say "Made in Saxony." When Lamm retired in 1934, his daughter took over the business and continued to operate it until 1949. The fancy blue and gold decoration on your cup and saucer suggests the set is worth $100 to $150.

Airplane china

Q: I have six luncheon plates and six cups and saucers marked on the bottom "Designed for Pan Am" above an oval mark for "Bauscher Weiden, Bavaria, Germany." They are solid white with a gold border and are surprisingly sturdy. Do they have any value, or should I just give them to my granddaughter for her kitchen play?

A: Pan American Airlines was in business from 1927 until 1991. Early Pan Am planes had "dining rooms," where passengers dined at tables set with china plates, silverware, and real glassware, not plastic. Dinnerware was made especially for Pan Am by several different manufacturers from the 1930s to the 1980s. Airline china, like railroad china, and other restaurant ware, is heavier, making it less likely to break with constant use. Your dishes are the "Gold Rim" pattern originally made by Noritake in Japan in the early 1970s. Later, the same pattern was made by Bauscher Weiden. Another white pattern, with a wavy border design, was made for Pan Am by Bauscher Weiden beginning in 1986. There are collectors who search for vintage airline and railroad china. Six-place sets like yours might auction for $150 to $250 depending on where and when it is sold. It will sell best at an auction with other airplane collectibles.

Snooker table

Q: I have a 10- by 5-foot snooker table. I believe the small size indicates this table is an American style rather than English, which would most likely be 12 feet by 6 feet. The brass badge on the table said "Ehrlich, St. Joseph, Mo., Dan Patrick cushions." Any information you might have would be greatly appreciated.

A: The brass badge probably reads "Dan Patch cushions," not "Dan Patrick." H. Ehrlich & Sons Mfg. Co. was listed as a maker of billiard and pool table cushions in 1912. The standard size for tournament snooker tables of 11 feet 8 ½ inches by 5 feet 10 inches, plus or minus half an inch, is usually called a 12- by 6-foot table. Smaller tables like yours were made for use in homes, pubs and other places. The Dan Patch cushions were named after the horse that set a world's record for the fastest mile in harness racing in 1906.

Terry and Kim Kovel will answer as many letters from readers as possible through the column only. For return of a photograph, include a self-addressed, stamped (55 cents) envelope. Write to: The Kovels, c/o King Features Syndicate, 300 W. 57th St., New York, NY 10019. The website is www.kovels.com.

current prices

Prices are from shows nationwide.

Campbell Kids mugs, kids eating sandwiches, M'm M'm Good, Westwood, 15 ounces, 3 1/2 inches, $5.

Cigar lighter, silver, Art Deco, linear design, flower etched, square base, J. Knewite, about 1900, 6 inches, $360.

Bisque figurine, women, seated, holding arrows, wheat, scythe, cold painted, turquoise base, 19 inches, pair, $530.

Drop earrings, Gripoix glass, cobalt blue, red, goldtone setting, clip on, Chanel, 1993, 2 inches, $540.

Brass sculpture, puzzle, Portrait of Michelle, Miguel Berrocal, Spain, 1969, 4 by 3 inches, $565.

San Ildefonso Indian, vase, pueblo, serpent design, black on black, incised Marie, 9 inches, $805.

Advertising sign, Dr. Swett's Root Beer, woman, holding mug, tin, curled edges, eight-sided, about 1905, 13 inches, $900.

Minaudière, Panda, multicolor crystals, goldtone chain shoulder strap, Judith Leiber, 5 by 3 1/2 inches, $980.

Lounge chair, walnut, wood, high, triangular back, Adrian Pearsall, Kraft, 1960s, 40 by 34 inches, $1,215.