In the middle of the plate was a hole, and the air was pumped out from below through this hole. Air-pump experiments could thus be made with a great number of different objects, one after the other, by simply lifting off the shade and replacing it after putting the object upon the plate. This method was easier than screwing off the cap of the bottle used by Huygens. [Illustration: Fig. 42.--Papin’s Mining Pump. Water flowing over the water wheel Q turns the shaft P, provided with a double crank. This drives the double air-pump OO, which sends air through the tube R alternately out of the cylinders H and L at each turn of the cock S. The pistons wind and unwind the ropes E and F, and draw up water in the bucket C attached to the wheel B.] Papin described these improvements in the year 1674 in a little book called _Experiments on Empty Space, with a Description of the Machines by which it is obtained_. The little book was dedicated to his master Huygens, to whom he wrote as follows:--“These experiments belong to you. I have made nearly all of them according to your instructions. But I know that they are only a recreation to you. You would never have written them down, and certainly you would never have published them. And so, I am sure, you will not mind my doing it for you.” Ten years after his book was published, Papin was appointed by the Royal Society of London a curator of experiments, his duty being to devise and show experiments to the members of the Society. Papin was greatly interested in mining, and he was always puzzling about some way of pumping up the water out of a mine. At that time the pumping had all to be done by hand, and it was done by a string of buckets, such as are used in wells. Papin had the idea of making a water-mill--driven by a brook—pump the water out of the mine. But this method was difficult unless a brook was close to the mine. However, Papin designed a system by which the water-power could be transported easily to the shaft of the mine. The water-mill drove the pistons in two cylinders connected with a long pipe. The pistons compressed the air and drove it along the pipe till it got to the mine. At the mine there was a big wheel, or pulley, bearing a rope with buckets attached to it. Other ropes were attached to the shaft of the pulley, and were fixed to two other pistons moving in two cylinders. These cylinders were connected with the pipe containing the compressed air. First one cylinder was connected, and the buckets were lowered; then a tap was turned which connected the other cylinder instead, and this raised the buckets full of water.