December 30, 1897] 



NATURE 



213 



a lake's outlet is re<julated by the discharge, and is not affected 

 by slow changes in the attitude of the basin ; but at other points 

 of the shore the water advances or retreats as the basin is tipped. 

 Consider, for example, Lake Superior. On the map (Fig. i) a 

 line has been drawn through the outlet at the head of St. Marys 

 River in a direction at right angles to the direction of tilting. 

 All points on this line, called the isobase of the outlet, are raised 

 or lowered equally by the tilting, and are unchanged with refer- 

 ence to one anotkier. All points south-west of it are lowered, 

 the amount varying with their distances from the line, and all 

 points to the north-east are raised. The water, always holding 

 its surface level, and always regulated in volume by the dis- 

 charge at the outlet, retreats from the rising north-east coasts, 

 and encroaches on the sinking south-west coasts. Assuming 

 the rate of tilting to be 0*42 foot per 100 miles per century, the 

 mean lake level is rising at Duluth 6 inches per century and 

 falling at Heron Bay 5 inches. Where the isobase intersects 

 the north-western shore, which happens to be at the inter- 

 national boundary, there is no change. 



Lake Ontario lies altogether south-west of the isobase of its 

 outlet, and the water is encroaching on all its shores. The 

 estimated vertical rise at Hamilton is 6 inches per century. 

 The whole coast of Lake Erie also is being submerged, the 

 estimated rate at Toledo and Sandusky being 8 or 9 inches per 

 century. 



The isobase of the double Lake Huron- Michigan passes south- 

 west of Lake Huron and crosses Lake Michigan. All coasts of 

 Lake Huron are therefore rising as compared to the outlet, and 

 the consequent apparent lowering of the mean water surface is 

 estimated at 6 inches per century for Mackinac, and at 10 inches 

 for the mouth of the P>ench river on Georgian Bay. In Lake 

 Michigan the line of no change passes near Manistee, Michigan. 

 At Escanaba the estimated fall of the water is 4 inches per 

 century ; at Milwaukee the estimated rise is 5 or 6 inches, and 

 at Chicago between 9 and 10 inches. 



These slow changes of mean water level are concealed from 

 ordinary observation l)y the more rapid and impressive 

 changes due to variations of volume, but they are worthy of 

 consideration in the planning of engineering works of a per- 

 manent character, and there is at least one place where their 

 influence is of moment to a large community. The city of 

 Chicago is built on a smooth plain little above the high-water 

 level of Lake Michigan. Every decade the mean level of the 

 water is an inch higher, and the margin of safety is so narrow 

 that inches are valuable. Already the older part of the city has 

 lifted itself several feet to secure better drainage, and the time 

 will surely come when other measures of protection are impera- 

 tively demanded. 



Looking to the more distant future, we may estimate the date 

 at which the geographical revolution, prophesied by Spencer, 

 will occur. Near Chicago, as already mentioned, is an old 

 channel made by the outlet of a glacial lake. The bed of the 

 channel at the summit of the pass is about 8 feet above the 

 mean level of Lake Michigan and 5 feet above the highest level. 

 In 500 or 600 years (assuming the estimated rate of tilting) high 

 >tages of the lake will reach the pass, and the artificial discharge 

 l>y canal will be supplemented by an intermittent natural dis- 

 charge. In 1000 years the discharge will occur at ordinary lake 

 stages, and after 1500 years it will be continuous. In about 

 2000 years the discharge from Lake Michigan-Huron-Erie, 

 which will then have substantially the same level, will be equally 

 divided between the western outlet at Chicago and the eastern 

 at Buffalo. In 2500 years the Niagara river will have become 

 an intermittent stream, and in 3000 years all its water will have 

 been diverted to the Chicago outlet, the Illinois river, the 

 Mississippi river, and the Gulf of Mexico, 



\ 



FORESTS AND RAINFALL} 



r" AN it be possible that the cutting away of forests affects the 

 amount of precipitation in any locality? To many, no 

 doubt, this question will seem easy of answer ; but we find the 

 results of study by no means reassuring, and recent investigations 

 'lave led to almost diametrically opposite conclusions, depending, 

 ■mewhat at least, upon the feeling of the writer. When we 

 1 cflect that our rain storms are of very wide extent, oftentimes 

 )ver 1000 miles in diameter, and may take their origin and 



1 A paper by Prof. H. A. Hazen, presented at the annual meeting of the 

 Atnencan Forestry Association at Nashville, leon., September 22. 

 (Abridged from the Monthly Weather Rcz'iciv ) 



NO. 1470, VOL. 57J 



bring their moisture from distances of 1000 miles or more, the 

 thought that man, by his puny eflbrts, may change their action, 

 or modify it in any manner, seems ridiculous in the extreme. 



It has been well established that forests have a most important 

 bearing upon the conservation of rainfall ; that the forest floor 

 permits a seepage of water to the source of springs, antl thus 

 maintains their steady flow ; that they hold back the precipita- 

 tion that falls, especially in the form of snow, thus preventing or 

 ameliorating the effects of dangerous freshets. There is not the 

 slightest doubt of their great importance to the welfare of man, 

 but all these facts do not affect the question of their influence 

 upon precipitation. The following paper is prepared from the 

 standpoint of a meteorologist, and is an attempt to present facts. 



The Historical Argument. 

 Formerly the historical argument was a favourite one. I 

 quote one of these : " It is a familiar fact that there are many 

 regions in Asia and southern Europe, once exceedingly fertile 

 and densely populated, that are now utterly sterile and desolate. 

 The country bordering on the Euphrates and portions of Turkey, 

 Greece, Egypt, Italy and Spain are now incapable of cultiva- 

 tion from lack of rain due to deforestation." The most fertile 

 of all provinces in Bucharia was that of Sogd. Malte Brun said, 

 in 1826, " For eight days we may travel and not be out of one 

 delicious garden." In 1876 another writer says of this same 

 region : " Within thirty years this was one of the most fertile 

 spots of central Asia, a country which, when well wooded and. 

 watered, was a terrestrial paradise. But within the last twenty- 

 five years a mania of clearing has seized upon the people, and\ 

 all the great forests have been cut away, and the little that* 

 remained was ravaged by fire during a civil war. The con- 

 sequences followed quickly, and this country has been trans- 

 formed into a kind of arid desert. The water-couiies are dried 

 up and the irrigating canals are empty." It has also been said- 

 that in the older settled portions of New England and the 

 Middle States there are arid hills and worn-out fields, due to the 

 falling off of precipitation from the cutting away of the forest" 

 growth. Such quotations and statements might be made to fill, 

 a large volume. Without more precise data as to rainfall it 

 would be hazardous to conclude that we have here a case of 

 cause and effect. It is certain that the fertility of these regions 

 in ancient times was due to stupendous irrigating devices and 

 canals, and when these were neglected, through wars and other 

 untoward circumstances, the fertility necessarily ceased. It is 

 certain that there are ruins of enormous irrigating ditches and 

 canals in Babylonia, where history indicates that there was once 

 a teeming population and great fertility, but where now only a 

 sandy desert greets the eye. 



Constancy of Rainfall. 

 It has been said that where our densest forests are found there 

 we have the greatest precipitation. There is no way whereby 

 we can see that such forests would have started unless favoured 

 by rainfall, so that the presence of the forest rather indicated 

 the earlier occurrence of practically the same rainfall as at 

 present. Meteorologists are agreed that there has been, 

 practically no change in the climate of the world since the 

 earliest mention of such climates. Plants found in mummy, 

 cases in Egypt that were plucked thousands of years ago show 

 the same size as those now found in that land. The " early and- 

 the latter rains" are experienced in Palestine to-day just as they 

 were four thousand years ago. Jordan " overflows all its banks"' 

 to-day, in February, precisely as it did in Joshua's day. When* 

 we come down to recent times and to the records of rainfall; 

 measured in New England for more than one hundred years, or» 

 at least, before and since the forests were cut, we find a con- 

 stancy in the rainfall which shows its entire independence of 

 man's efforts. Here it should be noted that totally barren lands 

 of any extent, in New England for example, are to be found 

 only in imagination. Even where the forest has been cut away 

 mercilessly there springs up a growth of sprouts which covers 

 the ground, and answers almost the same purpose in causing 

 rainfall (if there is any effect of that kind) as the forest. Even 

 where land is entirely cleared of a forest we have at times the 

 green pasture, and at others still heavier crops which leave the 

 ground anything but a sandy waste. 



Rainfall Measurements in Forests and Open Fields. 

 But the strongest argument adduced in the past to show the 

 influence of forest on rainfall has existed in a comparison 

 between rain-gauge measures in the forest and the open field 



