PHYSICS OF THE GLOBE. 203 



in 1847. The total area of the water-surface has increased by 

 about 25 per cent., by which expansion the surface for evap- 

 oration was increased. This extension of the lake is shown 

 to be clearly an anomaly in its history, and to explain it Mr. 

 Gilbert states that he has reason to believe that the indus- 

 tries of the settlers have so modified the surface of the land 

 that a larger share of the snow and rain finds its way into 

 the watercourses and the lake. He believes that the tax 

 imposed upon the streams by the work of irrigation is more 

 than repaid by the effects of the draining of marshes and 

 the destruction of herbage and timber. 



The influence of wind and climate upon the migrations 

 and spread of the grasshoppers has been very fully consid- 

 ered in the reports of Riley and Whitman, State entomolo- 

 gists for Missouri and Minnesota respectively (see also the 

 report for 18*76 of the Commissioner of Statistics for Minne- 

 sota). The report for 1S77 of the United States Entomolog- 

 ical Commission is remarkably full on this point. 



Hellmann, in Petermann's Mittheilungen, calls attention to 

 the possibility of predicting the invasions of grasshoppers or 

 locusts, which, leaving the Sahara in the spring with south- 

 west winds, are carried over Algeria and Egypt, and do more 

 damage than the severest storms. A similar duty has been 

 frequently urged by Dr. Packard and others upon our Signal 

 Service ; and in this connection it may be well to call atten- 

 tion to a theoretical explanation of the grasshopper migra- 

 tions which has lately been proposed by Abbe, and which is 

 said to account for most of the phenomena that have been 

 observed. According to this explanation, the grasshopper is 

 an insect at home and comfortable only in a rather dry at- 

 mosphere, and possibly a diminished atmospheric pressure ; 

 air that is either too dry or too moist is equally liable to 

 make the insect uncomfortable, and in either case he seeks 

 relief in flight, not knowing whither he shall go. Now the 

 very dry winds are the westerly winds, that bear him rapid- 

 ly eastward to the Missouri and Mississippi valleys. The 

 very moist winds are the south and southeast winds of the 

 Mississippi valley, that bear him or his progeny in the next 

 vear back to his original breedinor-oTounds. It will be curi- 

 ous to show whether this hypothesis holds good for the Af- 

 rican as well as it does for the American insect. 



