TRANSACTIONS OF SECTION A. 533 



stationary state or moving but slowly, and a quicker current catches a portion of it, 

 larger or smaller filaments that may be prominent are drawn out in what are 

 popularly called 'mares-tails: ' but this applies more to lower and denser clouds; 

 indeed, it will be observed that the upper stratum of clouds generally is modified 

 by conditions which do not sensibly affect the lower and denser clouds ; their attenu- 

 ated state shows the effects of vertical action as affecting their forms in a striking 

 manner, as well as the influence of varying velocity in horizontal motion. 



The principal clouds in this latitude come from the S. W., W., and N.W., and the 

 air from these points moves most rapidly ; the S.W. is much the fastest as well as 

 the most prevalent. It is a particularly rough and giisty stream, derived from the 

 heated tropical air which rises and is piled up as it were to a great elevation. This 

 as it flows away has to push that which is in advance of it until it has established 

 a stream ; the supply of air being more rapid than the outflow or diffusion causes 

 the elastic air to move in irregular gusts or masses, and hence the peculiar violence 

 <ind lulls alternating in the manner so well known. This stream descends to the 

 surface of the earth in the temperate zone, the latitude varying with the seasons. 



The more northerly and easterly currents are smooth, flowing to fill up the 

 deficiencies caused by the upward movement of the heated air, or by condensation 

 of vapour, by the fall of rain, or by eddies that carry the lower air into the higher 

 regions of the atmosphere, and by cyclones of various dimensions. 



The principles here set forth are only applied to two stratifications of clouds, an 

 upper and a lower one, but of course the same results are obtained in a lai-ger 

 number of strata ; it is only for simplicity of description and illustration that no 

 more than two are mentioned.* 



12. On a new Sunshine Recorder. By "W. E. "Wilson. 



The instrument consists of two parts, one of which, the indicator, is affected 

 by the sunshine, and the other of which registers the indications. The former, or 

 indicating apparatus, is a differential metallic thermometer, made of a spiral of two 

 metals (zinc and steel) soldered together. Half of the spiral is a right-handed one, 

 and the other half is left-handed. The complete spiral is fixed at its upper end. 

 At its lower end a lever or pointer is attached. The upper half or right-handed 

 portion of the spiral projects through the roof of a ventilated box, and is exposed 

 to the sunshine. The lower or left-handed half is in the shade. Any change in 

 the temperature of the air does not cause the lever or pointer to move, as the upper 

 half of the spiral tends to move it as much in one direction as the lower half tends 

 to move it in the other. "WTien the sun shines on the exposed upper half the lever 

 moves over and completes an electric circuit, which passes through the recording 

 part of the apparatus. 



The recorder consists of a drum driven by a clock. The drum revolves once 

 in twenty-four hours, and is mounted on an axis with a screw of ten threads to an 

 inch which turns in a nut. This gives the drum a longitudinal motion of jg" as well 

 as its motion of rotation. The clock makes an electric contact once every minute, 

 and the electric circuit is led through an electro-magnet which causes a pricker to 

 strike the drum when the circuit is complete. The circuit, as previously mentioned, 

 is led through the lever of the bimetallic indicator, and the circuit is only closed 

 when the sunshine causes the lever to do so. When the circuit is complete the 

 electro-magnet pricks off dots every minute, which represent so many minutes of 

 sunshine. The drum is of such a length that it holds the daily record of sunshine 

 for three months. 



The instrument also is made to give the total time during which the sun shone 

 in the day. The hands of the clock are not driven by it, but by an electro-magnet 

 which works a ratchet. The electric current is led through this magnet as well as 

 the one that works the pricker, so that as the pricker records minutes of sunshine 

 on the drum, the hands of the clock move forward in intervals of minutes. At the 



' The nimbus, or rain cloud, and all other varieties and combinations, or those 

 under electrical agencies, also eddies or whirl-storms of all sizes, are outside the 

 object of this paper. 



