Ultrasonic surveyor's distance measuring instrument

Claim 1

A DISTANCE MEASURING SYSTEM FOR OPERATING BETWEEN A STATION OF ORIGIN AND A DISTANCE STATION COMPRISING A FIRST TRANSMITTING TRANSDUCER AT SAID STATION OF ORIGIN FOR SENDING SOUND WAVES OF A FIRST FREQUENCY TO SAID DISTANCE STATION, A FIRST RECEIVING MEANS FOR DETECTING SAID SOUND WAVES OF SAID FIRST FREQUENCY AT SAID DISTANCE STATION, A SECOND TRANSMITTING TRANSDUCER AT SAID DISTANCE STATION ACTIVATED RESPONSIVE TO SAID FIRST RECEIVING MEANS DETECTING SAID SOUND WAVES AT SAID FIRST FREQUENCY TO SEND SOUND WAVES AT A SECOND FREQUENCY BACK TO SAID STATION OF ORIGIN, A SECOND RECEIVING MEANS FOR DETECTING SAID SOUND WAVES AT SAID SECOND FREQUENCY AT SAID STATION OF ORIGIN, AN INTERVAL COUNTER MEANS OPERATIVELY ASSOCIATED WITH SAID FIRST TRANSMITTING TRANSDUCER AND SAID SECOND RECEIVING MEANS AND HAVING A COUNT INTERVAL CONTROLLED BY THE TOTAL ELAPSE OF TRAVEL TIME OF SAID SOUND WAVES OF SAID FIRST FREQUENCY FROM SAID FIRST TRANSMITTING TRANSDUCER TO SAID FIRST RECEIVER AND THE TRAVEL TIME OF SAID SOUND WAVES OF SAID SECOND FREQUENCY FROM SAID SECOND TRANSMITTING TRANSDUCER TO SAID SECOND RECEIVER, AN OSCILLATOR OPERATIVELY ASSOCIATED WITH SAID INTERVAL COUNTER MEANS TO FURNISH A COUNTABLE TRAIN OF PULSES TO SAID INTERVAL COUNTER MEANS, A TEMPERATURE COMPENSATING DEVICE COUPLED TO SAID OSCILLATOR TO CONTROL THE FREQUENCY OF SAID TRAIN OF PULSES, SAID INTERVAL COUNTER MEANS HAVING READ-OUT MEANS TO INDICATE THE DISTANCE BETWEEN SAID STATION OF ORIGIN AND SAID DISTANCE STATION ACCORDING TO SAID COUNT INTERVAL.