168 



SCIENCE. 



[N. S. Vol. III. No. 57. 



July twenty-eighth, eighteen hundred and sixty- 

 six, shall be the tables of equivalents which may 

 be lawfully used for computing, determining, 

 and expressing in customary weights and meas- 

 ures the weights and measures of the metric 

 system." 



IMPROVED BLACKBOARD. 



Editor of Science : Several persons have 

 enquired about the blackboard mentioned in 

 your columns recently. May I describe it 

 briefly : A sheet of ground glass a meter square 

 is framed and the frame is hinged into a very 

 shallow cupboard fastened to the wall. A false 

 bottom covered with padded serge fits this cup- 

 board loosely, and when the door is closed and 

 fastened presses firmly against the glass on the 

 inside. It then forms a fine blackboard as the 

 ground glass surface is perfect for use with 

 crayons. 



If the door be opened and a sheet' of white 

 paper fastened to the false bottom by thumb 

 tacks, it becomes an equally useful drawing 

 slate for colored crayons. If in the place of 

 the white paper a sheet of drawings as of crys- 

 tal forms or geometrical figures, or outline maps 

 be put behind the glass they show through so 

 that all modifications of the primai-y form be- 

 neath can be drawn on the glass and in proper 

 relation to this primary. It is only needful that 

 the false bottom shall press firmly against the 

 glass, and this is easiljr effected by having it 

 held in place by four screws placed near the 

 corners whose heads are countersunk in the 

 false bottom. The latter moves freely on these 

 screws and four spiral springs which are slid on 

 the screws behind it press the serge firmly against 

 the glass. Ben. K. Emerson. 



Amherst, Mass., January 14, 1896. 



SCIENTIFIC LITERATURE. 

 Elementary Physical Geography. By Ralph S. 



Tarr. 12 mo., pp. 1-xxxl., 1-488, 29 plates 



and charts, 267 diagrams and photographs. 



Macmillan & Co. 1895. Price $1.40. 



Physical geography is no longer a mere de- 

 scription of the earth's surface, but includes 

 also an enquiry as to how its features came to 

 be what they are. The recent ideas that have 

 vivified this study and placed it on a scientific 



basis may be seen by contrasting the writings of 

 Hitter, Humboldt, Guyot and others of what 

 may in all courtesy be termed the old school, 

 with the book before us. In the older books, 

 which are by many persons still considered 

 fountains of geographical knowledge, the lead- 

 ing theme is the description of the earth ; in 

 Tarr's physical geography the dominant idea is 

 how the features of the earth came to have 

 their present characteristics. 



In descriptive physical geography the conti- 

 nents are sometimes treated as fragments of 

 broken china, which, by the exercise of much 

 ingenuity and an active imagination, are made 

 to fit together with more or less accuracy, 

 thus leading the student to fancy that at one 

 time they were united. In rational physical 

 geography each continent is shown to have a 

 life history, and to have been modified by ele- 

 vation and subsidence, and varied in relief by 

 erosion and sedimentation. In the modern view 

 of nature even the largest of land masses are 

 found to be unstable forms ; the processes to 

 which they owe their elevation above the sea, 

 as well as their outlines and relief, are still 

 active, and additional changes are to come. 

 Mountains are no longer to be studied as fin- 

 ished forms, but as representing all stages of 

 growth, adolescence, maturity and old age. 

 River valleys are not merely drainage canals, 

 the lengths and breadths of which are to be 

 memorized, but each one has a history written 

 in its terraces and flood plains, in which evi- 

 dences of elevation and depression of the land, 

 climatic changes, the influence of rock structure, 

 etc., can be read. 



The modern ideas referred to, which, so to 

 speak, have blown away the mist from the 

 landscape and revealed its varied beauties, are 

 truthfully reflected in the book before us. One 

 who is familiar with the progress of geological 

 study in America sees, as he turns its pages, an 

 epitome of the results brought by many consci- 

 entious workers from the mountains and val- 

 leys, with much labor and thought. Most of 

 all, it is flavored with the studies of Prof. Davis, 

 of Harvard, in whose class room and from ■ 

 whose writings Prof Tarr has gained much of 

 his inspiration. The great sources both of facts 

 and ideas, as must of necessity be the case in 



