[baenes] temperature OF THE LACHINE RAPIDS 19 



the corner where this coflter-dam joins the outer dam, the water attained 

 a considerable velocity, with a great deal of suî-face agitation in parts. 

 It was decided to locate the observation shanty at this point in preference 

 to any other, for the following reasons : The shanty would be on the 

 solid earth of the pier, and hence remain steadier for the measurements ; 

 the water was in nearly every state of agitation within easy reach of the 

 thermometer stem ; and the depth of water varied so in the immediate 

 vicinity, that while in some parts it was sufficiently shallow to permit of 

 observations being made of the growth of anchor ice, in others it attained 

 a depth of nearly 20 feet. Moreover, the current sweeping around the 

 point caused a large sheet of comparatively quiet back water to work up 

 from a considei-able distance below. This remained open, except in the 

 severest weather, when frazil, swej^t in from the currents, aided by sur- 

 face-formed ice, became compacted into a thin moving heterogeneous sur- 

 face sheet soon dispersed in milder weather. The especial advantage of 

 having this quiet water and the swift current in juxtaposition , was to enable 

 the measurements to be made at close intervals, of the temperature of the 

 Avater in such different conditions, coming together from opposite directions. 

 The river in winter above the place where the shanty was located, is 

 open for a distance of six or eight miles, and flows so swiftly that it is 

 being continually stirred to the bottom by surface currents carried down 

 and lower layers brought to the surface. All along the bottom there are 

 formed immense quantities of anchor ice, and, owing to the surface agita- 

 tion, lai'ge quantities of fine floating ice as well. The conditions are as 

 favourable, therefore, at this point for producing an extreme temperature 

 in the water as they might ever be expected to be. 



Instruments used in the Investigation. 



The instruments used in the present series are the same as those de- 

 scribed in the writer's previous paper, and consisted of the differential 

 thermometer, comiDensated wire resistance box, low resistance galvan- 

 ometer, reversing key and battery. 



The observation shanty was provided by the harbour commissioners 

 of Montreal, through the kindness of Mr. Kennedy, the chief engineer, and 

 although smaller than the one provided the previous winter, served the 

 purpose sufficiently well. It was necessary to provide a smaller cabin 

 than that of the previous wintej-, owing to the difficulty of transportation 

 to and fi'om the city, a distance of five miles, and placing it in a suitable 

 position on the end of the pier. The services of a watchman were also 

 procured to protect the instruments and to prevent them from freezing. 



Method of making the Measurements and accuracy of the Readings. 



The method of making the measurements consisted, as in the pre- 

 vious experiments, of immersing the two stems of the thermometer in a 

 carefully prepared mixture of snow and water contained in the shanty, and 



