The construction of the Cambridge subway, which began a year ago last summer, has progressed very rapidly. At present there are only two portions of the work that are not completed. One of these sections is in front of Hampden Hall, which will be finished some time this week, and the other is a short section between the Post Office and the Co-operative Society's store, where work has been delayed on account of the large water main, which passes through the Square at that point.

The plan of construction used in the vicinity of Harvard Square is known as the "open-cut" method. As the name implies, the excavation is carried on from the surface. Cable-ways and hoisting machines, all electrically operated, are used as a means of removing the material, which is employed in reclaiming land along the Charles River. Through the Square the subway consists of two parallel tubes. The tube in which trains go toward Boston is of a slightly lower level than the other. This difference in level is to make it more convenient for passengers coming from Boston who wish to change cars for Newton and Arlington, and vice versa.

Except at the stations the subway consists of two tubes instead of a double-tracked tunnel. The trains passing in the tube put the air in circulation in one direction only, this making the ventilation better than it is in a double-tracked tunnel, where the trains, passing in opposite directions, neutralize each other and do not aid in the ventilation.

The work in Harvard Square has been further complicated by the size of the underground station, which is being constructed with special reference to the quick handling of large crowds. In the centre of the Square there will be erected an exit and entrance station similar to the one at Scollay Square. Further entrances are being placed in front of the Co-operative Society's branch store and at the waiting room, adjoining Amee's Bookstore. Exits are being arranged near Dane Hall and across Massachusetts avenue from Holyoke street.

The subway itself will run through Brattle street to Murray street, where the "elevated trains," as the subway cars will be designated, will ascend to the street. Surface cars from North Cambridge and Newton will pass underground at the Cambridge Common and at Murray street, respectively.

When the construction work on the subway began, the engineers estimated that it would take between three and four years to complete it, but now they expect to have trains running through it to Boston next October.

At the conclusion of all construction work connected with the subway, surface tracks will be relaid through Harvard Square and along Massachusetts avenue. Surface traffic then will eventually be resumed under the same conditions that existed before the commencement of subway construction.