8o POPULAR SCIENCE MONTHLY. 



or other rock existing as a ledge. Owing to the loss of the ele- 

 ment of elasticity, the road would soon become weakened in its 

 coherency, and the rate of wear would be much increased. It is 

 therefore customar}^ to excavate, when a cut in rock is necessary, 

 some four inches below subgrade, and to fill in to subgrade with 

 gravel on which the broken stone is placed as before. 



Another modification is practiced when clayey, wet ground is 

 encountered. Under these circumstances it is generally best to 

 excavate some sixteen inches below the finished grade and spread 

 on a layer of gravel four inches deep. Upon this Telford founda- 

 tion is laid by hand to a depth of eight or ten inches and care- 

 fully rolled (Plate III). A layer of broken stone is then put on, 

 and a finishing coating of screenings is added as before. 



As to the character of the roads already constructed in Massa- 

 chusetts, Prof. N. S. Shaler, of the Highway -Commission, in- 

 formed the writer that, in his opinion, they are in no way inferior, 

 in so far as quality and durability are concerned, to the celebrated 

 Swiss roads. 



So well pleased are the people of Massachusetts with the State 

 roads already constructed, and so active are they in the cause of 

 good roads, that the Legislature appropriated the additional sum 

 of four hundred thousand dollars for highway construction dur- 

 ing the year 1895 and five hundred thousand dollars in 1896. 



At first glance it would seem that the engineering skill neces- 

 sary to construct a Macadam road would not be of a particularly 

 high order ; and yet the problems involved in building roads in 

 the latitude of Massachusetts, where great variations exist in the 

 character of the soil, owing to the glacial conditions that once 

 existed here, call for engineering ability of a peculiar kind, as 

 well as an extended experience in the treatment of special cases 

 and the economical application of the materials at hand. 



As an adjunct of the Highway Commission, a laboratory has 

 been established in Cambridge, where the systematic study of 

 road materials is carried on. It has come to be generally recog- 

 nized that materials which possess the necessary qualities for a 

 good road stone are both limited in kind and in quantity. It is 

 the object of these laboratory investigations to classify the road 

 stones of the State in the order of their fitness for this purpose, 

 and to prepare a map showing the area and location of the most 

 desirable varieties. Here are investigated the questions of the 

 rate of wear of stones under impact, and the cementing and re- 

 cementing value of the powdered rock on which the life of the 

 road depends in a large measure. The hardness and toughness 

 also come within the scope of the experimental work. Experi- 

 ment has shown that a stone must possess certain all-around 

 properties in order to come up to the desired standard. For 



