EXPERIMENTS IN THE KEEPING OF PLANKTON ANIMALS. 557 



days of 5-8°, the average temperature being 10-8°. During the summer 

 months, the changes must have been very much greater. 



(B) Here the vessels rested on a table close to a large double 

 window, with a western aspect, partly lighted also from the northern 

 side, and the light was consequently good. The position in the building 

 being more central than Position A, the changes in temperature were 

 probably, on the whole, less in extent and less sudden than in the latter. 

 At times, however, they were greater, as, for example, in the month of 

 July, when on four successive days the following temperatures of water 

 in the bell-jars were recorded : July 17th, 25° ; July 18th, 20° ; July 19th, 

 16-3° ; July 20th, 17°. 



(C) In this part of the building, the General Laboratory, well lighted 

 with large windows, both on the southern and on the northern sides, 

 the light obtained was always good, and often so strong as to need 

 screening. In the colder weather the air temperature is kept by hot- 

 water pipes between 13° and 18°, and in general it is subject to 

 very much less change than in either of the other two positions. As, 

 however, in all experiments here carried out, the vessels were kept sub- 

 merged, either in the sea-water of the general circulation or in water 

 artificially adjusted to a nearly constant temperature, the changes in 

 air temperature were of minor importance and, as affecting the experi- 

 ments, almost negligible. The water of the experiments, which was 

 necessarily regulated by the temperature of the tank water, showed 

 over a number of observations made between November 26th and 

 February 7th a varying range of no more than 1-6°, a maximum daily 

 variation of 1°, and an average temperature of 12-3°. 



The vessels were covered with squares of glass or with watch glasses, 

 as a provision against dust. Except in one special experiment (p. 562), 

 there was no circulation of water through the vessels, the water being 

 left standing, subject only to stirring by aeration or otherwise, and 

 being only changed in certain occasional instances. 



