94 THE ERESH-WATER LOCHS OF SCOTLAND 
the survey was carried on under the direction of Sir John Murray 
and Mr Laurence Pullar, and temperature observations were made in 
practically all the lochs surveyed. In 
Loch Ness observations were made in the 
years 1903,1904, and 1905, which in their 
completeness are, I believe, unique in 
the history of limnology, though in the 
light of after experience the observations 
leave much to be desired. I have sub- 
sequentlv endeavoured to examine the 
temperature changes occurring in fresh- 
water lakes experimentally, and during 
the first half of the year 1908 I was able 
to make numerous observations in Loch 
Garry, Liverness-shire, which are of great 
value as confirming deductions based on 
the observations in Loch Ness. An at- 
tempt has also been made (the first of 
its kind) to observe directly currents in 
lakes, with a view to a better understand- 
ing of the temperature changes which 
have been observed. 
Reference may be made in passing to 
the different kinds of thermometers which 
have been used from time to time in mak- 
ing temperature observations in lakes. 
The instrument which is chiefly used 
now is the Negretti & Zambra reversing 
thermometer. It consists of a mercury 
thermometer with a constriction in the 
bore of the stem about an inch above 
the bulb. As long as the thermometer 
is upright the mercury is continuous from 
bulb to stem, but if the thermometer be 
turned upside down, the mercury breaks 
at the constriction, and the portion in the 
stem falls down. The stem is graduated 
from the point to the bulb, so that the 
temperature at the moment of inversion 
is read off on the stem. This thermo- 
FiG. 35. 
meter is enclosed in a strong glass case to 
protect it against the pressure to which 
it is subjected when immersed in water, and the whole is fitted 
in a metal frame which can be attached to the sounding-line. 
