538 



SCIENCE. 



[N. S. Vol. XI. No. 275. 



neous rocks (wLich are generally classed as 

 acid and basic volcanics) are limited to the 

 extreme eastern portion of tnis appalachian 

 region, and have not yet been employed to 

 any considerable extent for highway pur- 

 poses. 



In part VI. of this report, Dr. H. F. Eeid 

 describes the qualities of good road metals 

 and the methods of testing them. He clas- 

 sifies the forces tending to destroy roads 

 into two groups : 



First, the wear and tear of travel : (1) "by blows 

 of the horses' feet; (2) By the blows of the wheels; 

 (3) By the action of the horses' feet in pulling or hold- 

 ing back, tending to pull the stones out of place ; (4) 

 By friction of the wheels, especially when trucks are 

 used ; (5) By the pressure on the road surface due to 

 the weight of the vehicle and horses. 



Second, the forces of nature, under which he in- 

 cludes : (1) Heavy rains "which tend to wash the 

 road ; (2) The winds which tend to sweep away 

 all the fine material formed on the road surface by 

 travel instead of allowing it to become consolidated 

 again with the mass of the road ; (3) The solution 

 and general chemical decomposition of the surface 

 material through the action of water; (4) The 

 changes of temperature ; (5) The heaving action of 

 frost and subsequent breaking up of the surface at 

 times of thawing. 



Under the ' Methods of testing Eoad 

 Material,' Mr. Eeid describes briefly : (1) 

 the microscopic examination as showing 

 the structure of the rock'; (2) abrasion 

 test, which has been so long practiced in 

 France and other countries ; (3) the crush- 

 ing test, which shows the resistance of the 

 rock to crushing action or blows ; (4) the 

 cementation test. Of these abrasion and 

 cementation tests are undoubtedly the most 

 important. For many years the French 

 highway engineers depended entirely upon 

 the abrasion test, but one of the important 

 results arrived at through the investigations 

 of the highway laboratory of the Lawrence 

 Scientific School, under Professor Shaler's 

 direction, is the demonstration of the fact 

 that the cementing power of the road metal, 

 as the surface is ground under the wheels 



and hoofs, is as important, and in some cases 

 more important than the power of resisting 

 wear ; for however hard and tough the road 

 material may be, under the action of the 

 wheels it will be gradually ground into 

 powder, and if it does not possess this ce- 

 menting power in a high degree, it will be 

 continuously removed from the road surface. 

 With this cementing power developed in a 

 high degree, however, it has been found 

 that fairly soft stone may be used for road 

 surfacing in many cases with decided 

 success. 



Part IV., on the present condition of 

 highwaj's in Maryland, by Mr. A. N. John- 

 son, is of much interest to the student of 

 this great problem of highway improvement 

 in America. Some 2500 miles of these 

 roads were travelled over by Mr. Johnson 

 in carrjung on his investigation. The total 

 number of miles of public roads given in 

 Maryland (with a total land area of 9860 

 square miles) is 14,483 ; or 1.47 miles of 

 road for each square mile of area. Of this 

 there are 890 miles of stone road, 225 miles 

 of gravel road, 250 miles of shell road, and 

 13,118 miles of typical American earth 

 road. Of the 890 square miles of stone 

 road about 630 miles were built by turnpike 

 companies and 260 miles have been built 

 by the counties. Unfortunately most of 

 these improved roads were constructed 

 without competent engineering supervision 

 and without proper care as to the grades ; 

 and the folly of such practice can not 

 be stated too emphatically nor too often. 

 Perhaps the first urgent need of the typical 

 American highway to-day is that it be 

 properly relocated by a competent engineer 

 before there is anj^ expensive improvement 

 of the surface. 



Mr. Johnson also brings out the fact that 

 the average length of the haul for farm 

 products in Maryland, including the dis- 

 tance from farms to the markets and rail- 

 way stations, is for the State 6.7 miles. 



