screen necessary for precise measurement of 

 illumination, but the incident light readings did 

 provide an index for comparing the illumina- 

 tion between and within days. Aboard the 

 Rorqual an irradiance meter (Model C-la, Ma- 

 rine Advisors, Inc.) equipped with Weston pho- 

 tronic cells measured subsurface changes in 

 light penetration. This unit had a filter with 

 a peak sensitivity of 550 millimicrons and a 

 range of 390 to 760 millimicrons. The irra- 

 diance meter provided the ratio of the amount 

 of radiation at the depth of the submerged cell to 

 a reference cell on deck. Secchi-disk readings 

 also were taken at regular intervals. Plankton 

 samples were taken at half-hourly intervals 

 with Miller high-speed samplers at Bar Har- 

 bor, and at hourly intervals with the Clarke- 

 Bumpus sampler at Boothbay Harbor. Tem- 

 perature and salinity were recorded and echo 

 soundings were made continuously during the 

 sampling period. 



PHYSICAL CHANGES 



Measurements at Boothbay Harbor showed 

 that illuminances on the day before the eclipse 

 were (control day) and the day of the eclipse 

 was not closely comparable (table 1). Except 

 for the period of totality, surface illumination 

 was far greater on the day of the eclipse than on 

 the control day. This difference was also evi- 

 dent from other data. Visibility on July 19 was 

 limited to 9 km., and nine-tenths of the sky 

 was covered by cirrostratus clouds ; on July 20, 

 objects were visible at 16 km. and the stratus 



17 



Wednesday 



-I 



18 



Thursda y 



JULY 

 19 



Fridau 



cloud cover of seven-tenths was generally dis- 

 solving. These differences limited the com- 

 parisons which could be made between test and 

 control days. 



The decrease in surface illuminance before 

 and at totality and the subsequent increase are 

 documented in table 1. An hour before totality. 



Table I. — Surface illuminnnce nt Boothbiy Il/irbir, July 

 V) end .?(\ 1063 



> Totality. 

 '^ Sunset. 



75,000 luxes were recorded, 900 at totality, and 

 28,000 within the hour after totality. Darkness 

 at totality (1745) approximated that which oc- 

 curred one-half hour before sunset. Though 

 the primary purpose of Secchi-disk observations 

 was to measure water clarity, the results also 

 provided information on the submarine light 

 penetration during the eclipse. The extinction 

 depth of the Secchi-disk was 7.0 m. at 1600, 



/- 



20 



Saturday 



4- 



21 



Sundau 



Twi 



a 8 10 1^? 2 4 6 8 10 M' ? « 6 a 10 1,; 2 4 g B 10 M' ? 4 6 B 10 1.2 2 4 « 8 10 M' 2 4 6 8 10 1.2 2 4 6 8 10 M' 2 4 6 8 10 1,2 2 4 6 8 10 M' 2 4 



BAROMETRIC PRESSURE 



^3Q50 / f^^ t / -i-fe30J5(t: 



Figure 2. — Barometric pressure, mm. of mercury, adjusted to sea level, at Boothbay Harbor. The arrow indicates 

 time of totality. 



MARINE ORGANISMS DURING SOLAR ECLIPSE 



261 



