220 ANNUAL OF SCIENTIFIC DISCOVERY. 



the whole of the external part of the instrument, including the measurer of 

 velocity, the direction vane, and a rain gauge. This support is so made that 

 it can be easily adapted to the roof of any building upon which it may be 

 necessary to mount it. All the rotary parts of the anemometer run upon 

 friction balls. The shaft of the apparatus for measuring the movement of the 

 wind, by means of a diminishing train of wheels, is made to turn a cylinder 

 upon which is wrapped a sheet of paper of the kind used for " metallic memo- 

 randum books," this paper having the property of receiving a trace from 

 a style of brass. The sheet of paper is divided into two sections, upon one of 

 which is recorded the motion of the wind and upon the other the direction. 

 As the cylinder is being turned by the action of the wind a clock carries a 

 pencil along the cylinder at a uniform rate of 12 inches in the 24 hours. To 

 the lower end of the direction shaft is attached a spiral of such a figure that 

 equal angles correspond to equal increments of radius ; the edge of this 

 spiral consists of a thin slip of brass which touches the paper and records the 

 direction of the wind on a rectilinear scale. When the sheet of paper is 

 unwrapped from the cylinder after 24 hours, the motion of the wind and the 

 direction are both found projected in rectangular co-ordinates. 



TO ASCERTAIN THE DIRECTION OF THE WIND. 



Mr. T. Stevenson has communicated to the Edinburgh Neiv Pliilosopliical 

 Journal, the following accurate and easily applied method of ascertaining the 

 direction of the wind, by observing the reflected image of the clouds : 



In making some experiments, in which it was necessary to know accu- 

 rately the direction of the wind, Mr. Stevenson was much annoyed by the 

 insufficiency of vanes and all ordinary methods employed for that purpose. 

 The under currents of air are so numerous and conflicting, more especially in 

 towns, where the houses are lofty, that the author has seen it proclaimed to 

 be due east at one end of the street, while at the other it seemed with equal 

 certainty to be coming in a westerly direction. 



In this dilemma, it occurred to him that a more accurate conclusion might 

 be arrived at, by observing the direction of the drifting clouds when reflected 

 in a mirror. At first, Mr. Stevenson used a common mirror, placed horizon- 

 tally, so as to have the sky reflected in it ; and having fixed upon a cloud, he 

 watched its progress in the mirror, taking care to keep the eye steadily in 

 one position, and carefully marking the track of the cloud upon the glass with 

 a pencil of soap. When this was done, it was easy, by placing a compass 

 on the mirror, to ascertain the direction of the wind from that of the cloud's 

 path traced on the glass. A more convenient and portable instrument has 

 since been constructed, consisting of an ordinary compass having a silvered 

 disc in the centre of its covering glass of such a size as to allow the points of 

 the needle and the graduated circle of the compass to be seen beyond it. The 

 glass has cross lines cut upon it, passing through the centre, and drawn so as 

 to correspond with the cardinal points marked on the divided circle. The 

 whole compass can be made to revolve in the horizontal plane, upon a point 

 projecting from the bottom of the outer case. When the cloud which is to be 

 observed has been selected, as near the zenith of the observer as possible, the 



