MECHANICS AND USEFUL ARTS. 93 



each other in the same direction, and to maintain a continuous stream of 

 water, without the use of cranks. The slide valves were pressed up against 

 the ports of the cylinder, by means of springs, in order to retain them in 

 contact with the faces when the metre was at rest. They were free to re- 

 main stationary during the greater portion of the progress of the pistons ; but 

 just as the piston of one cylinder was completing its stroke, one of a pair of 

 tappets on the valve frame, carried by the piston rod of that cylinder, would 

 strike against the valve which that valve frame carried, and altered its posi- 

 tion over the ports of the other cylinder, whereby the direction of the flow 

 of water into that measuring cylinder was reversed. For transmitting the 

 reciprocating motion of the piston to the index, one of the valve frames was 

 furnished at the back with two ribs, or feathers set parallel to each other, 

 but one in advance of the other. These feathers acted as teeth, and in slid- 

 ing backwards and forwards with the piston rod, they entered alternately 

 the teeth of an escape-wheel, and so drove it round tooth by tooth. The 

 arbor of this wheel led through the water case to the counting apparatus ; 

 and thus motion was communicated directly to it, without the aid of pawls 

 and ratchet-wheels. 



By this arrangement the metre became a very simple and inexpensive 

 machine, not liable to derangement ; or if injured, it was easily repaired, as 

 the only moving parts were the two valves and two pistons. There was an 

 entire absence of concussion ; the pressure of the head was preserved, and 

 being similar within and without the cylinder, there was no friction upon 

 the pistons ; and the water-tight external case or chamber served as a de- 

 posit for sand or other extraneous matter. 



NEW SYSTEM OF NATURE PRINTING. 



The following communication on the above subject has been recently pre- 

 sented to the London Society of Arts by Mr. C. Dresser. 



The art of nature printing has been defined as " a method of producing 

 impressions of plants and other natural objects in a manner so truthful that 

 only a close inspection reveals the fact of their being copies ; " but this is 

 rather the result of its greatest achievement to us it merely implies print- 

 ing from nature, and in this light it will now be regarded. 



As this printing from nature, or " nature printing," is only in one sense 

 new, its history may prove interesting and useful, as this, and this alone, can 

 enable us to understand to what extent it is new, and the nature of any sup- 

 posed improvements or alterations in the art which may be offered. As far 

 back as about 250 years a simple mode of producing impressions of plants 

 upon paper was employed by naturalists. The plant, after being dried, was 

 held over the smoke of a candle or oil-lamp, when it became blackened by 

 a deposit of soot, after which it Avas placed between two sheets of paper and 

 rubbed with a smoothing-bone, which caused the soot to leave the promi- 

 nences of the leaf and adhere to the paper. In this way an impression of the 

 plant was produced. This method of procurring impressions was employed 

 as early as the year A.D. 1650. 



