Mr R. Adie's Description of a new Anemometer. 311 



out of the water, and the measurement of the force with which 

 it acts is accomplished in the following manner : To the axis 

 having the wheel on it, over which the cord suspending the cy- 

 linder passes, is fixed a spiral of variable diameter, in the form 

 of the fuse of a watch ; from this spiral is suspended a weight 

 which, when acting on the largest radius, counterbalances the 

 weight of the cylinder suspended in the water when no compres- 

 sion of air is allowed to exist within it ; this point is marked zero 

 on the scale or dial-plate, placed at one end of the axis. The 

 dial-plate is divided into parts, shewing the force with which 

 the wind is pressing up the cylinder. The scale is formed from 

 actual experiment, by applying weights to the cord passing over 

 the variable lever, the area of the top of the cylinder being ac- 

 curately known. Any length of scale may be given to the in- 

 strument by making the change in the radii of the spiral in- 

 crease more or less rapidly, and it may be made to mark the 

 force of the lightest air or of the most violent gale. 



The instrument is also made to register the maximum and 

 minimum force in the absence of the observer. This is done by 

 means of two light pointers, attached to the centre of the dial- 

 plate, and requiring a very small force to move them : one is 

 placed on each side of the pointer attached to the axes, and by 

 it pushed both ways, the one to the greatest, the other to the 

 least pressure that has occurred. 



Having now described the instrument and its mode of action, 

 I may notice some experiments that were made, for the purpose 

 of determining what form of orifice it was best to apply to the 

 end of the tube for receiving the current of air, in order to as- 

 certain whether large or small, or having different forms of ori- 

 fice, would cause any difference in the results. Three kinds 

 were tried ; one, a piece of tube the same as the long tube com- 

 municating with the cylinder, another with an oblong opening, 

 and a third a funnel. These were made so that they might be 

 shipped off and on without loss of time, and the morning being 

 fine, with a gentle and steady breeze, I could discern no differ- 

 ence in the amount of pressure with the different mouths, but I 

 thought the funnel kept its head most steadily to the wind, and 

 consequently the hand did not vibrate so much. 



