484 Mr. W. Baily on an 



If, now, we were to add a mechanical counter to each of the 

 wheels W, the increase of the readings of these counters would 

 give the integral of the resolved parts of the wind towards the 

 North and Bast ; and inasmuch as the wheels W revolve in the 

 contrary direction when they have passed across the centre of 

 the disk X, South winds would act on the counters as nega- 

 tive North, and West winds as negative East. Two counters 

 would be sufficient, as a preponderance of South wind would 

 be indicated by a decrease in the reading of the North counter, 

 and a preponderance of West wind by a decrease in that of 

 the East counter. 



It was found, however, much more convenient to make 

 each wheel W complete an electric circuit at each revolution, 

 and to use an electric counter; but inasmuch as such a counter 

 will not distinguish in which direction the wheel is turning, 

 it was necessary to have separate counters for North and 

 South, and to shunt the circuit from one counter to the other 

 as the wheel W crossed the centre of the disk X, and to make 

 a similar arrangement for East and West. 



I will now describe the electric system adopted: — Parallel 

 with the tram-lines M (fig. 2) are insulated rails R, one of 

 which is seen in section in fig. 4. Each rail, R, is connected 

 by an msulated wire with an insulated binding-screw er. Each 

 truck, H, bears a thin insulated plate of brass, P, seen in face 

 in fig. 2 and edgeways in fig. 3; and each end of the plate P 

 carries a small wheel, Q. At any moment, except when the 

 truck is passing the centre of the cross, one of the wheels 

 Q rests on a rail R, and the other is in the air. This is 

 clearly seen in fig. 3. At the moment when one of the 

 trucks H (say the North and South one) passes, say, from 

 North to South across the centre, the wheel Q on the North 

 rail R runs off it, and the other wheel Q runs on to the South 

 rail R, so that the plate P of the truck is always in electrical 

 connexion with the binding-screw a belonging to the cardinal 

 point towards which the truck lies. The rails R have for their 

 upper edge a thin rail of platinum. This was added to the 

 broad rail of brass, as it was found that points of dust or corro- 

 sion sometimes made accidental breaks in the electrical contact 

 between R and Q. In the truck K which runs below H are 

 two parallel tongues Y, Z, very near one another (see fig. 3, 

 in which the full length of Y is seen, but only the end of Z). 

 Both are fixed to J ; but Z is also insulated from J, and is con- 

 nected by an insulated wire, which passes through the slit S 

 (fig. 4) and through a hole in H, with the insulated plate P. 

 The tongue Y is electrically connected with K, and so through 

 various points of contact with the table 0. There is a pro- 



