"If your dog is fat," the old saying goes, "you aren't getting enough exercise." But walking the dog need not be just about a little exercise. Here are 13 cool things you can see in greater Baltimore while you hike with your dog.AIRPLANES. The BWI Airport is the only airport in America that features a recreational trail. The Thomas A.

Dixon Jr. Aircraft Observation Area on this 12.5-mile paved trail, opened in 1994, provides an ideal spot to watch the planes land directly in front of you. You won't be able to see the rubber hit the ground here but can see it from other spots along the trail. To get the feel of a big jet soaring directly over your head walk down a half-mile to the east (you'll see stop signs) and stand here. It won't be only jets using the airport either - you can spot an occasional propeller plane as well.

AMUSEMENT PARK RUINS. Although only 20 acres in size, the Bay Shore Park was considered one of the finest amusement parks ever built along the Chesapeake Bay. Built in 1906, the park featured an Edwardian-style dance hall, bowling alley and restaurant set among gardens and curving pathways. There were rides such as a water toboggan and Sea Swing.

Visitors would come out from Baltimore on a trolley line. Most of the park was torn down after its closure in 1947 but you and the dog can explore the remains of the turn-of-the-century amusement park, including the wood-framed trolley station and the restored ornamental fountain, in North Shore State Park. Complete your tour with a hike down the old Bayshore Pier which juts almost a quarter-mile into the wind-swept Bay - a diving board once operated here where benches are today.BALD EAGLES.

With nearly 13,000 acres of undeveloped space, the Patuxent Research Refuge is said to be the largest patch of green space remaining on the East Coast between Boston and Raleigh. Research done here was used by Rachel Carson to argue that the pesticide DDT was weakening the shells of bird eggs, especially bald eagles, causing them not to hatch. Her book, Silent Spring, led to the banning of DDT and launched the modern environmental movement.

Today more than 250 species - 8 of every 10 birds that can be seen in the Baltimore area - have been sighted at Patuxent, including a pair of nesting bald eagles in the North Tract in Anne Arundel County. These representatives of America's national symbol quite possibly could be the bald eagles living closest to the White House. Don't let your dog dig around at the North Tract - this land was once a testing ground for Fort Meade and may still harbor live ammunition.If you aren't lucky enough to spot the eagles in flight at the refuge, try hiking the Hashawha Trails at the Bear Branch Nature Center in Carroll County.

Here is the chance for your curious dog to look a bald eagle in the eye. The Nature Center maintains a M.A.S.H.

unit for raptors who have been injured too badly to be returned to the wild. The cages for eagles, kestrels, hawks, owls, turkey vultures and other recovering birds of prey are on the Vista Trail.CANAL LOCK. Near North Park in Havre de Grace, the 444-mile Susquehanna River is busy emptying 19 million gallons of fresh water every minute into the Chesapeake Bay that it has drained from 13 million acres of land.

The rocky river upstream from here, however, is not navigable and the 45-mile Susquehanna and Tidewater Canal opened for barges, pulled at 4 miles per hour by mules, to haul goods between Havre de Grace and Wrightsville, Pennsylvania. The first of 29 locks operated here and it has been restored to its original appearance including a pivoting footbridge that swung open to allow barge traffic to pass. The handsome brick Lock House, now a museum open on weekends, dates to the canal's opening in 1840. The large grassy lawn can be used for a first-rate game of fetch.CHOATE MINE. The first chromium mines in America were opened in rural Baltimore County in 1808 and from 1828 to 1850 just about every scrap of chrome in the world came from here.

Along the Choate Mine Trail in Soldiers Delight Natural Environmental Area you can stand in front of the entrance to the Choate Mine and look into the slanting hole kept open by half-timbered posts. So close the cool air will rustle your dog's fur. The mine once ran 200 feet deep and 160 feet across.

DAMS. After a long hike at Robert E. Lee Park around Lake Roland you can sit on top of the Greek Revival valve house completed in 1862 and look over the stone dam. Lake Roland, created after plugging up Jones Falls, was Baltimore's first reservoir. This smallish dam is just an appetizer for the dams yet to come that were built to quench Baltimore's thirst.

Others to see include hiking to the base of Liberty Dam at the end of Feezer's Lane in Patapsco State Park or using the Gunpowder South Trail in the Hereford section of Gunpowder Falls State Park to see the Prettyboy Dam, built in 1933. No tour of Baltimore's dams would be complete without a visit to Conowingo Dam, America's longest concrete slab dam across the Susquehanna River. You can take the dog to gaze out at the Conowingo Dam in Susquehanna State Park.

FORTS. At Fort Howard Park your dog can climb into an actual gun battery and scan the Patapsco River just like gunnery officers who once aimed guns capable of accurately firing 1,000 pound projectiles eight miles. Ruins abound at the former "Bulldog at Baltimore's Gate," including remainders left over from the 1960s when a mock Vietnamese village was created for training at Fort Howard. Batteries and magazines that once formed the coastal defense of Baltimore in 1899 can also be seen at Fort Armistead Park and Fort Smallwood Park.

As for Baltimore's most famous fort, dogs are also welcome at Fort McHenry National Monument. Unlike the others, your best friend won't be able to explore the actual fort but there is plenty of fresh grass to romp on outside the bastion walls.HENRYTON TUNNEL. The Baltimore & Ohio Railroad built its first line west along the Patapsco River and the trails at Henryton Road in Patapsco State Park follow a particularly historic stretch of the Old Main Line. On a rainy night in 1830 Irish laborers, tired of waiting for back pay, rioted and managed to destroy all this track for five miles to Sykesville.

The disturbance prompted the first ever American troop transport by train when the Baltimore militia rode out to squelch the rampage. When the trail crosses this section of railroad track look to the west and see the Henryton Tunnel. Opened in 1850, it is the second-oldest tunnel in the world that remains in active railroad use.MODEL TRAINS. Thomas Winans made his fortune building the Russian transcontinental railroad for Czar Nicholas I. He learned railroading from his father Ross who invented the swivel wheel truck that enabled trains to negotiate curves.

Their railroad heritage is preserved at Leakin Park in Baltimore by the Chesapeake & Allegheny Live Steamers who maintain three miles of track for miniature steam trains that carry passengers (sorry, no dogs) free of charge the second Sunday of every month. Capable of speeds of 25 mph, the trains rumble along instead at a passenger-friendly 6 mph.MODERN ART.

The natural beauty of Quiet Waters Park in Annapolis is augmented by the outdoor sculptures that grace the grounds. Sculptures are chosen by jury from national and international artists working with a variety of material and installed on a rotating basis. When your dog tires of sniffing the statuary, you can take her to Anne Arundel County's first dog park at the back of Quiet Waters. Not only are there two large fenced-in enclosures for big and small dogs but there is a dog beach on the South River for serious dog paddling.

POT ROCKS. From the parking lot on US 1 at the Big Gunpowder Falls there is great canine hiking on both sides of the river in either direction. On the opposite bank heading downstream on the Big Gunpowder Trail, about a mile down, are the Pot Rocks. You and the dog can walk out and examine the conical depressions created in the bedrock by swirling waters armed with millions of years worth of grinding cobbles.

These unique potholes can be a foot or more deep. Keep hiking another two miles down the river and you reach the last series of rapids on the Gunpowder as the water leaves the hilly Piedmont region and slips into the flat Coastal Plain.RARE TREES.

Growing unobtrusivley beside the parking lot at Tridelphia Recreation Area is one of the rarest native ornamental trees in the world, the Franklinia Alatamaha. A relative of the camelia, this flowering tree is prized at any time of the year - in the winter for its striped bark, in the summer for its palm-sized snow white flowers, and in the fall for its deep red leaves. The Franklinia was discovered by Philadelphia botanist John Bartram in 1765 in a remote corner of Georgia along the Alatamaha River and named for his friend Benjamin Franklin. It has not been found growing in the wild since 1790.

For a true arboreal education however, treat the dog to Cylburn Arboretum in Baltimore, one of the few such tree museums that permit dogs on the grounds. The collection at Cylburn features several Maryland Big Tree Champions including an Italian maple and a paperback maple. Two easy champions to see are on the lawn in the right front of the mansion: a castor aralia with large glossy leaves and an Amur maackia. Both trees are native to Asia and are resilient to pests. The maackia is a member of the pea family discovered by 19th century explorer Karlovich Maack along the Amur River between Siberia and China.

UNUSUAL BRIDGES. Hiking in Gunpowder Falls State Park in Harford County, downstream from Jerusalem Mill about 1/2 mile, is Jericho Covered Bridge, one of only six remaining covered bridges in Maryland and the only one of its kind in Baltimore and Harford counties. Old folk wisdom held that these bridges were built to resemble a barn so as to entice a wary horse across water but the bridges are covered simply to protect the expensive wooden decks.

The ford at this point across the Little Gunpowder Falls dates to Colonial times; the bridge was constructed in 1865. Builder Thomas F. used three truss types in its construction: the simple Multiple King Post; the horizontal Queen Post extension; and the Burr Arch, patented in 1804 by Theodore Burr, for stability. Renovated in 1981, the Jericho Covered Bridge still carries traffic.In Howard County's Savage Park, on Foundry Road at the trailhead for the Historic Mill Trail, is the last remaining Bollman Truss bridge in the world.

Your dog can trot across the first successful iron bridge used by railroads, patented by Wendell A. Bolman in 1852. This example, a National Historic Civil Engineering Landmark, originally carried traffic on the Baltimore & Ohio main line but was disassembled and put into service here for Savage Mill in 1887.copyright 2006.

I am the author of over 20 books, including 8 on hiking with your dog, including the widely praised The Canine Hiker's Bible. As publisher of Cruden Bay Books, we produce the innovative A Bark In The Park series of canine hiking books found at http://www.hikewithyourdog.com During the warm months I lead canine hikes as tour leader for hikewithyourdog.com tours, leading packs of dogs and humans on day and overnight trips. My lead dog is Katie, a German Shepherd-Border Collie mix, who has hiked in all of the Lower 48 states and is on a quest to swim in all the great waters of North America - http://web.

mac.com/crudbay/iWeb/Katies%20Blog/Katies%20Quest.html I am currently building a hikewithyourdog.

com tours trailer to use on our expeditions and its progress can be viewed at http://web.mac.com/crudbay/iWeb/Teardrop%20Trailer/Building%20A%20Tour%20Trailer.html.

By: Doug Gelbert