322 



KANSAS CITY REVIEW OF SCIENCE. 



MEASUREMENT OF WATER. 



To measure water roughly in an open stream, take from four to twelve dif- 

 ferent points in a straight line across the stream, and measure the depth at each 

 of these points, and adding them all together, divide by the number of measure- 

 ments taken. This quotient will give you the average depth which should be 

 measured in feet. Multiply this average depth in feet by the width in feet, and 

 this will give you the square feet of cross-section of the stream. Multiply this by 

 the velocity of stream in feet per minute, and you will have the cubic feet per 

 minute of the stream. The velocity of the stream can be found by laying off loo 

 feet on the bank, and then throwing a board into the stream at the middle, note 

 the time passing over the hundred feet, and dividing the loo feet by the time and 

 multiplying by sixty gives the velocity in feet per minute at the surface. The 

 velocity at the center is only eighty-three per cent of that at the surface, and so 

 only eighty-three per cent should be calculated. For example, suppose the float 

 passes loo feet in ten seconds, then divided by ten and multiplied by sixty (sec- 

 onds in the minute) gives 600 feet per minute, as the velocity and eighty-three 

 per cent of this gives 498 feet per minute, as the velocity of the stream at the 

 center, and the area of the cross-section multiplied by this will give you the num- 

 ber of cubic feet per minute in the stream. This, of course, is only a rough way 

 of calculating, but it is often used, and is a good and simple way to obtain data 

 to select a wheel by. — Craig Ridgway df Son. 



EDITORIAL NOTES. 



Prof, W. J. McGee has contributed a val- 

 uable paper to be read at the meeting of the 

 American Association for the Advancement 

 of Science. The object of this paper is to 

 show in the language of a writer who gives 

 a digest of it, that, to whatever latitude a 

 polar ice-field may extend, precipitation can 

 take place only along its outer margin, and 

 that the temperature of its central portions 

 must sink too low to sustain appreciable 

 quantities of aqueous vapor. By an ingen- 

 ious system of calculations, he finds that the 

 accumulation of ice is in proportion to the 

 vapor tension, and that, if the thickness of 

 ice at any latitude is known, that at all other 

 latitudes can be readily computed. For in- 

 stance, if it be assumed that in latitude 40° 

 the ice be three miles thick, with the temper- 



ature 56.5 ; in latitude 50°, temperature 41. 7, 

 it would be 1,733 miles; in latitude 60°, 

 temperature 30.2, 1,103 miles; and so on, 

 until in latitude 90°, temperature 2.3, the 

 thickness of ice would be but .315 of a mile. 

 There can, therefore, be little or no ice at 

 the pole. If it were not for this law, if the 

 law were inversed, that the ice-cap should be 

 thickest at the pole, it might be sufficient to 

 displace seriously the earth's centre of gravi- 

 ty. Ice-streams have motion, as rivers of 

 water have. When the slope of the channel 

 increases, the depth decreases and the motion 

 is more rapid at the surface than at the 

 bottom. The velocity decreases in propor- 

 tion to depth ; that is, the shallower the ice- 

 stream the more rapidly it descends. Ice 

 may be bent to any shape by gradual pres- 



