1881.] H. F. Blanford — Van Ryssellerglie Meteorograph. 67 



cylinder and bears a number of deep grooves on its surface corresponding 

 to the graduation of the instruments. A spring which presses on the grooved 

 surface and conveys the current, suffers an interruption of contact when 

 passing over the grooves, and thus produces a minute break in th e 

 engraved line, and, when the burin is not engraving, allows it to mark a 

 series of dots corresponding to the graduation. 



At the commencement of each revolution and before the registra- 

 tion begins, an endless screw which carries the burin is made to revolve 

 through a sufficient distance to bring the burin about -^ of an inch, 

 lower on the plate ; and at the end of every two hours through a somewhat 

 greater distance, leaving a broader interval between the engraved lines and 

 furnishing a time-scale. 



The first instument which gives its trace is the dry bulb thermometer. 

 The thermometers are open at top, and the tubes give admission to two 

 probes of platinum wire, which, by means of the mechanism before noticed 

 are made to descend slowly until they form contact with the mercurial 

 columns. As the dry bulb thermometer always (except in an absolutely 

 saturated atmosphere) stands at a higher temperature than the wet bulb 

 its contact is first made, completing the regulating circuit. This acts on 

 an electro-magnet, which, by attracting an armature, completes the burin 

 current and draws the burin back from the cylinder ; and, at the same time, 

 causes a catch to start forward and arrest the further descent of the platinum 

 probe. That of the wet bulb continues to descend, until, on making contact 

 with the mercury, the regulating current acts on another electro-magnet, 

 which interrupts the burin current, releasing the burin, which again presses 

 on and engraves the plate till the regulating current is cut off by the action 

 of the commutator. 



Next comes the trace of the rain gauge. The rain gauge is now 

 situated on the roof of the office and communicates with the registering 

 apparatus by two wires, which form a part of the circuit of the regulating 

 current. The regulating current (or a branch of it) can always pass through 

 this circuit, when contact is made in the rain gauge. When rain is falling 

 it passes from the collecting funnel into a tip bucket, and this when full tips 

 and empties itself, at the same time making a metallic contact which allows 

 a momentary current to pass. This current acts on an electro-magnet in 

 the registering apparatus below, the armature of which acting on a ratchet 

 wheel makes a small brass cylinder revolve through a single tooth. One 

 half of this cylinder is of greater diameter than the other, the two 

 surfaces being separated by a helical margin, such as might be produced 

 by applying a wedge-shaped strip of brass plate to the surface of a cylinder, 

 with the two rectangular edges of the wedge respectively parallel to the axis 

 and base of the cylinder, while the hypotenuse forms the helical margin. 



