Theory:

     Not far from his grandmother's residence there was a windmill, which operated on a new plan. Isaac was in the habit of going there frequently, and would spend hours in examining its various parts. While the mill was at rest, he pried into its internal machinery. When its broad sails were set in motion by the wind, he watched the process by which the mill-stones were made to revolve, and crush the grain that was put into the hopper. After gaining a thorough knowledge of its construction, he was observed to be unusually busy with his tools.

     It was not long before his grandmother, and all the neighborhood, knew what Isaac had been about. He had constructed a model of the windmill. Though not so large, I suppose as one of the box-traps which boys set to catch squirrels, yet every part of the mill and its machinery was complete. Its little sails were neatly made of linen, and whirled round very swiftly when the mill was placed in a draught of air. Even a puff of wind from Isaac's mouth, or from a pair of bellows, was sufficient to set the sails in motion. And what was most curious if a handful of grains of wheat were put into the little hopper, they would soon be converted into snow-white flour.
Explanation:
 
There was a windmill near Isaac's house, which operated on a new mechanism. Isaac had the habit of going there often, and he spent a long time studying the various parts. When the windmill was not operated, he meddled with its internal machine parts. When the sails of the mill were set going by the wind, he saw the process by which the attached stones started spinning and crushed the grain that was put into the hopper. Hoppers are funnel-shaped bins for feeding grains into the mills. Once he understood the specific mechanism of the windmill, it was seen that he was very busy with his tools - he started making one by himself!
 
grinding-wind mill.jpg
A grinding stone, which is usually placed inside a windmill - used for grinding flour!
 
Soon, his grandmother and the neighbourhood came to know why Isaac had been busy. He had constructed a model of the windmill that he had seen, all by himself. It was smaller than the original - only the size of trap boxes which boys made to catch squirrels. But every part was intact, and the set was complete. He made the sails out of linen which moved in rounds in a rapid speed when the windmill was exposed to air currents in a closed space. Even when Isaac blew a puff of air or used bellows, the sails started moving beautifully. (Bellow is a device with an airbag that emits a stream of air when squeezed together - usually used to kindle fire). Not only did the sails work perfectly, when he put a handful of grains, they were soon converted into pure white flour! The grinding stones had also been properly fixed, and the machinery he created was perfect.
 
Meanings of difficult words:
 
No.
Words
Meanings
1
whirledmoved rapidly round and round
2
draughta current of cool air in a room or other confined space
3
bellowsa device with an air bag that emits a stream of air when squeezed together
4
hoppersfunnel shaped bins for feeding grains into mills
Reference:
State Council of Educational Research and Training (2019). Term-2 English Standard-8. Sir Isaac Newton- The Ingenious Scientist (pp. 84-97). Published by the Tamil Nadu Textbook and Educational Services Corporation