138 Hon. R. Abercrqmby. On the Relation between [Feb. 24, 



and when dV \&x = 0, 



bga?= 10sH-4s ' 



This will be satisfied by making x somewhat less than 1*9, so that 

 in this case the ratio of the diameters of the drums would be a little 

 less and very close to the ratio found for the pulleys. 



In order to illustrate the foregoing problems a model of a train of 

 pulleys and another of a train of drums made of brass were con- 

 structed by Mr. Yates. In the train of pulleys all the large ones are 

 1*9 inches in diameter, and all the small are 1 inch. Each of the former 

 weighs 2' 61 oz., and each of the latter 1*058 oz. ; as there are five 

 pairs their total weight is 18*340 oz., while they give a velocity ratio 

 of (1-9) 5 = 24-761, or a little more than 24|. 



The train of drums consists of large ones with diameters of 2*55 

 inches and small of 1 inch, the hoops are in all 0'5 inch in breadth, 

 and the spokes are half the volume of a complete disk. The weights 

 of the large drums are each 3*386 oz., of the small 0*811 oz. 



There are four pairs of drums, and their total weight is 16*788 oz., 

 or little more than 1 lb. 



The velocity ratio of this train is (51/20) 4 = 42*2825, or a little more 

 than 42£. 



II. " On the Relation between Tropical and Extra-Tropical 

 Cyclones." By Hon. Ralph Abercromby, F.R. Met. Soc. 

 Communicated by R. H. Scott, M.A., F.R.S. Received 

 February 7, 1887. 



(Abstract.) 



The conclusions as to the relation of tropical to extra-tropical 

 cyclones which the author has derived from the researches of which 

 this paper gives an account, may be stated thus : — 



All cyclones have a tendency to assume an oval form ; the longer 

 diameter may lie in any direction, but has a decided tendency to range 

 itself nearly in a line with the direction of propagation. 



The centre of the cyclone is almost invariably pressed toward one or 

 other end of the longer diameter, but the displacement may vary 

 during the course of the same depression. 



Tropical hurricanes are usually of much smaller dimensions than 

 extra-tropical cyclones ; but the central depression is much steeper, 

 and more pronounced in the former than in the latter. 



Tropical cyclones have less tendency to split into two, or to develope 

 secondaries, than those in higher latitudes. 



A typhoon which has come from the tropics can combine with a 



