2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



tance at which a Secchi Disc would just disappear if it were immersed 

 in water and if that water were at the surface. 



As an example, if an Equivalent Secchi Disc Reading were given 

 as 10 feet for water at a depth of 100 feet, this would mean that if 

 the water mass at a depth of 100 feet were brought to the surface a 

 Secchi Disc would disappear from view at a distance of 10 feet in 

 this transposed volume of water. 



The Secchi Disc is admittedly a crude indicator of water trans- 

 parency, since it was originally used by marine biologists to measure 

 the so-called extinction coefficient. This is a measure of the amount 

 of light reaching a horizontal surface at some depth. Unfortunately, 

 the extinction coefficient is not only a measure of the water transpar- 

 ency but also a function of such things as sea state, cloud cover, 

 altitude of sun, and other factors. Even so, however, the Secchi Disc 

 reading is probably a reasonably good indicator of water clarity if 

 it is taken with the sun fairly high in the sky and if it is viewed 

 through a glass-bottom viewer or hydroscope [30] . 



In addition, the Secchi Disc reading is an easily understood unit, 

 generating an intuitive feeling for the existing conditions, so that 

 it has become fairly universal in its use as an indicator of water 

 transparency. 



Of course, the actual Secchi Disc reading gives an average value 

 of the transparency of the surface layers, so that if a layer of mark- 

 edly different water exists somewhere from top to bottom, it will not 

 be seen. For this reason, other instruments which measure trans- 

 parency of relatively small volumes of water were used in conjunction 

 with the disc. These will be discussed in a later section. 



The writers wish to express their appreciation to Dr. Ruth Patrick, 

 Curator, and Miss Margaret Le Mesurier, Librarian, of the Depart- 

 ment of Limnology, Academy of Natural Sciences of Philadelphia, for 

 their indispensable aid in the preparation of this manuscript. Ap- 

 preciation is also expressed to the Smithsonian Institution for material 

 aid and advice in this project and publication of the paper, and to the 

 Academy of Natural Sciences of Philadelphia for its contribution of 

 personnel and materials in the carrying out of this program. We 

 regret that space does not permit the listing of over 50 other persons 

 and institutions to whom we are indebted for advice and assistance 

 rendered. 



INSTRUMENTS 



The instruments utilized in the accumulation of the data presented 

 herein can roughly be divided into two classes : ( 1 ) those that meas- 



