As frequently noted in these columns practical matters depend so much on correct deductions, that what would be nothing but abstract science to most persons, becomes very much more to those in our profession. It is of first importance that what passes for science should be absolutely correct. This has been the ground taken by this magazine in regard to whatever there is in the question of rainfall and forestry. The ideas emphasised by Marsh in his " Man and Nature," mainly through our efforts, are now thoroughly exploded. It is now conceded that forests are a result and not a cause of climate, and their reflex influence very small. The only claim left now is that forests are distributors of moisture. Perhaps they are. Indeed facts incline that way; but inclination is not definite proof, and much that is offered is but assumption. Below is an extract from the recently issued report of the Water Department of Philadelphia, whose Chief, Col. Ludlow, is one of the most intelligent gentlemen who has ever occupied that position:

" During the past 60 years the Schuylkill has displayed a marked diminution in its minimum flow. In 1816 this was estimated at 500,000,000 gallons per day; in 1825, at 440,000,000; in 1867, 400,000,000; in 1874, 245,500,000. This remarkable decrease, not being accompanied by any great change in the rainfall, nor probably in the total annual discharge of the river, is no doubt largely due to the destruction of the forests within the drainage area, whereby the conservative action of the woodland is lost, and the rainfall is permitted to descend rapidly to the bed and pass off in a succession of freshets." Page 70, Report, 1884.

The Schuylkill is the river that supplies Philadelphia with water. By minimum flow is the least quantity that has flowed during any 24 hours in the year. Col. Ludlow admits that there has been no certain method by which the flow could be measured, and he tells us at page 75 that next year he will try and inaugurate a better system. We may perhaps doubt if the smallest quantity that came down in any one day in 1816 was five hundred million gallons a day. Still it is possible that the remarkable difference between 1816 and 1874 stated, should not wholly exist.

Presuming then that there has been a decrease, is there ground for Col. Ludlow's statement " no doubt the decrease is largely due to the destruction of the forests, etc.?"

Singularly enough there was little if any forest destruction on the headwaters of the Schuylkill between 1816 and 1825, yet the figures show a. falling off of 60,000,000 gallons. The greatest forest destruction was between 1825 and 1867, yet though there is four times the period of time the falling off has been no greater than in the single ten years when there were no trees of any consequence cut. In the last ten years, between 1867 and 1874, when there was comparatively little timber left to cut, when the amount of timber was nearly the same at the end as at the beginning of the period, the river fell off nearly one-half.

It is clear that whatever falling off there may be, or is, the fact has no connection with the forestry question. It will be an extremely interesting study what is the cause of this remarkable difference in the river's summer flow; but we shall never know what the cause is if we accept without hesitation Col. Ludlow's " no doubt " as true science.