A B737 CREW EXPERIENCED DECREASING HYD QUANTITY DURING FLT; LANDED AT ATL.

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ON DEP FROM BDL. HYD FLUID QUANTITY GAUGE READ JUST ABOVE THE RF LINE. AFTER TKOF; THE USUAL DECREASE IN QUANTITY WAS NOTED FOR GEAR UP AND FLAPS RETRACTED. DURING CRUISE; THE QUANTITY CONTINUED TO SLOWLY DECREASE AND FLT CTL AND MAINT CTL WERE CONTACTED VIA RADIO. AS WE CONTINUED TO MONITOR THE HYD QUANTITY; WE REVIEWED CHKLISTS WE MIGHT HAVE TO USE. WE WERE OVERHEAD FL310 WHEN THE HYD FLUID DECREASED BELOW THE BAL LINE LEVEL OF 1.8 GALLONS. WE COORDINATED WITH FLT CTL AND MAINT TO DIVERT TO ATLANTA. WE INITIATED OUR DSCNT INTO ATLANTA AS A PRECAUTIONARY LNDG; BUT LATER DECLARED AN EMER WITH ATL APCH AND REQUESTED RWY 27R FOR RWY LENGTH. WHILE CONFIGURING WITH FLAPS 5 DEGS (SPD BRAKES RETRACTED) AND SPD LESS THAN 200 KTS; THE ACFT EXPERIENCED A SIGNIFICANT SHUDDERING THAT LASTED UNTIL THE GEAR WAS EXTENDED. NO ASYMMETRY WAS NOTED AND FLAPS 15 DEGS WAS SELECTED AS A FINAL FLAP SETTING. THE LNDG WAS 500 LBS OVERWT (103.5) WITH A 50 FPM DSCNT. WE STOPPED ON THE RWY TO DETERMINE HYD QUANTITY AND BRAKE PRESSURE AND THEN TAXIED CLR. WE SAW THE HYD QUANTITY WAS NOW APPROX 2.0 WITH GOOD BRAKE PRESSURE AND CANCELED THE EMER. WITH EXTREME VIGILANCE; WE TAXIED TO THE GATE AND WERE MET BY MAINT AND PAX SVC. LOGBOOK WRITE-UPS WERE MADE FOR THE HYD MALFUNCTION AND THE OVERWT LNDG.