38 ROYAL SOCIETY OF CANADA 



Icndar designed and liad constructed in the Physics Building- of McGill 

 University, a thermometer capable of mehsuring difterences in temper- 

 ature to thousandths of a degree. 



On the 15th of Februarj- this thermometer with the set of measuring 

 instruments, consisting of a compensated wire resistance box, galva- 

 nometer, reversing ke}' and battery, was takxin down to the watchman's 

 shanty on the Guard Pier. Having set up the instruments, one of the 

 ends of the thermometer was placed through a hale cut in the ice of the 

 river, and from that date on. for over four weeks, daily readings were 

 taken, with the exception of two short stops. The first stop was caused 

 by a soldered join giving out on one of the ends of the thermometer, 

 making it necessary to completely renew^ this end. This only caused the 

 loss of two days. The other stop was the breaking of the galvanometer 

 suspension, which cau.sed the loss of one day. 



During the time of the experiments it may be said that the most 

 severe part of the winter occurred. The temi)erature of the outside air 

 varied from —28° F. to + 40° F., including several cold dips, from 20° 

 to 30° above zero î'ahrenheit to several degrees below zero. 



It is to be regretted, that the observations could not have been made 

 at the foot of the Lachine rapids where it is likely that greater variations 

 exist than could be expected under the ice in the river. But from the 

 well known fact that sui'face ice grows in thickness on the underside where 

 frazil cr3-stals are found attached, a series of experiments, such as this, 

 would be indispensable in giving any complete explanation of the causes 

 underlying their formation. Tt is to be hoped, how.wer, that another 

 winter similar experiments may be tried in open water. 



Details ok the Instruments. 



Diffcnnifidl Thcniioincter. 



The thernu:)metcr con.sisted of two separate stems 25 cm. long, each 

 made of a coil of wire inclosed in à covering of lead composition tubing- 

 one cm. in diameter pressed flat over the wire. One of the coils of wire 

 was connected to compensated leads, ten feet long, contained in a strong 

 and thick compo-tube, while the other coil was connected to leads one 

 hundred feet long similarly protected. Ditferetices in the resistance of 

 the two coils were mea.sured on the wire-bridge contained in the shanty. 



The compensating leads contained w^ith the connecting leads and 

 balanced against them by the well-known Wheatstone's bridge method, 

 served to prevent the variation in the temperature of the air from affect- 

 ing the readings on the bridge. 



(For information in regard to electrical thermometer, Pi-of. Callcn- 

 dar's |)aper on the Practical Measurement of Temperature. Phil. Trans, 

 vol. 178 (1887). should be consulted.) 



