64 



Lattin: Introduction 



some other means must be used. The reversing the, 

 mometer may be used in recording temperature series 

 in a lake. The thermometer is lowered to the desired 

 depth and allowed to remain there one or two minutes 

 to permit the instrument to adjust to the temperature 

 of that level. Then a metal messenger is sent down 

 the line; it trips the mechanism and the thermometer 

 swings upside down, breaking the column of mercury 

 at the recorded temperature. The instrument is pulled 

 to the surface, read, reset, and returned to the next 

 level. A maximum-minimum thermometer may also be 

 used at various levels in lakes. The instrument is 

 set, lowered to the desired depth, and allowed to 

 remain in position five minutes; then it is pulled 

 back to the surface and read. Still another method 

 is to read temperatures from the thermometer that is 

 built into the Kemmerer water sampler. Unfortunately, 

 these methods require considerable time in order to 

 take a complete series of temperatures. The electric- 

 resistance thermometer is more efficient and saves 

 a great deal of time. It is based on the principle that 

 different metals have different resistance at different 

 temperatures. It consists of a box containing batteries, 

 galvanometer, Wheatstone bridge dial, and a switch. 

 A cable is calibrated in feet and has a terminal bulb 

 containing temperature sensitive ends. The device is 

 lowered into the water, the switch closed, and the 

 temperature read on the Wheatstone bridge dial in 

 the box. The advantages of this type of recorder are 

 that the adjustment interval for each level is very 

 short, and it is not necessary to bring the instrument 

 back to the surface after each temperature has been 

 read. Although the initial expense is higher than with 

 the other two types mentioned, the increased effi- 

 ciency and greater accuracy offset the cost. 



Relative humidity. — The sling psychrometer is used 

 to obtain relative humidity readings. The wet bulb 

 is moistened with water and the device swung in 

 circles for three to five minutes. The two temperatures 

 are read, one from the dry bulb and the other trom 

 the wet bulb. A calibrated chart will then give the 

 relative humidity. 



Current velocity. — The speed of current is deter- 

 mined most efficiently by means of a current meter. 

 This consists of a propeller mounted on a finlike or 

 rudderlike metal base that serves for orientation in 

 the stream. An electric sounding device records a 

 click for each revolution of the propeller, and the 

 number of clicks per minute as detected with ear- 

 phones is recorded. Measurements should be made at 

 various depths and in a transect across a stream 

 because the current speed is greatest toward the 

 center and at intermediate depths. 



Stream velocity may also be determined in a rough 

 way by means of a surface float. The float is placed 

 in the water and timed as it is carried over a known 

 distance. By this means the velocity is determined 

 in feet per second. Since the surface speed is gener- 

 ally faster than the average channel speed, it is 

 necessary to divide the above mentioned figure by 

 1.05 for channels less than 2 feet deep and by 1.33 

 for channels 10 or more feet in depth. A subsurface 

 float can be made that will give a reasonably accurate 



recording of the channel speed. The float is made of 

 a small plastic bottle with a screw top that tapers to 

 a small hole. By squeezing the bottle, air is forced 

 out and water can be drawn in. With a little practice, 

 the proper proportions of air and water will be attained 

 so that the bottle will float beneath the surface. The 

 smaller the bottle, the less resistance is offered. 

 Plastic bottles will not break and the narrow-holed 

 top permits small amounts of water or air to be added. 

 The volume of flow is given in feet per second or, in 

 small streams, in gallons per minute. The flow is 

 obtained by multiplying the average velocity in feet 

 per second by the cross section area of the stream 

 in square feet. The cross section is obtained by 

 multiplying the width of the stream by the average 

 depth. The average depth is obtained by measuring 

 the depth of water at uniform intervals across the 

 stream and dividing the sum of these depths by the 

 number of measurements plus one. 



Light. — On land, the intensity of light is measured 

 by a photoelectric cell or light meter. For measuring 

 the light beneath the water, the same instrument can 

 be used except that it must be connected to a galvanom- 

 eter by means of a long wire. A simple device, known 

 as a Secchi disc, is often used to measure the relative 

 penetration of light. The Secchi disc is a white, round 

 plate eight inches in diameter divided into four alter- 

 nating white and black quarters. The plate is lowered 

 until it disappears, dropped a few feet further, and 

 raised until it appears again. The average is taken 

 of the two depths. Although giving only relative 

 results, the Secchi disc is a convenient instrument 

 and has been used by limnologists for years. It serves 

 in a general way to indicate the limits of effective 

 light penetration for photosynthesis. 



Chemical Factors 



Water sampling. — Water for chemical analysis is 

 usually collected from a stream in 500 cc. glass- 

 stoppered bottles. Care should be taken when filling 

 the bottles that the water is not disturbed too much 

 since that might alter the gaseous content. Water to be 

 analyzed for carbon dioxide should be collected in a 

 low-form Nessler tube (200 x 32 mm.). Water samples 

 taken from ponds and lakes are collected by means 

 of a Kemmerer water sampler (intro. fig. 94). The 

 samples are taken at the surface, at or near the 

 thermocline, and at the bottom. In deep lakes, it may 

 be necessary to take a longer series of water samples 

 in order to have a complete picture of the distribution 

 of the various gases through the water levels. The 

 open sampler is lowered to the desired depth on a 

 graduated rope, then a metal messenger is sent down 

 to trip the closing mechanism, shutting the upper and 

 lower valves. Two or three minutes are allowed for 

 the thermometer to adjust to the water temperature. 

 Then the instrument is pulled to the surface, the 

 thermometer read, and the water drained into a water 

 sample bottle by means of the rubber hose on the 

 bottom valve. Water should be permitted to overflow 

 the sample bottle until the contents have been changed 

 twice. 



