338 PHILOSOPHICAL TRANSACTIONS. [aNNO 1759. 



May 2, he saw the comet again very distinctly with the naked eye ; but being 

 then in London, without either globe or planisphere, he did not pretend to 

 settle its place. The cloudiness of the evenings prevented his seeing the comet 

 any more till the 5th and 6th of May : and on these days partly thin clouds, 

 and partly the increasing light of the moon, rendered it much less easily discern- 

 ible, both by the naked eye and in the telescope. 



XVII. A Catalogue of the Fifty Plants from Chelsea Garden, presented to the 

 Royal Society by the Company of Apothecaries, for the Year 1758, pursuant 

 to the Direction of Sir Hans Sloane, Baronet^ Med. Reg. & Soc. Reg. nuper 

 ProeseSy by John fFilmer, M.D. clariss. Societatis Pharmaceut. bond. Socius, 

 Hort. Chelsean. Prafectus & Prcrlector Botanic, p. 96. 



This is the 37th presentation of this kind, completing to the number of 

 1850 different plants. 



XVIII. An Experimental Enquiry concerning the Natural Powers of fVater and 

 Wind to turn Mills, and other Machines, depending on a Circular Motion. 

 By Mr. J. Smeaton, F. R. S. p. 100. 



What Mr. S. communicates on this subject was originally deduced from 

 experiments made on working models, which he thinks the best means of ob- 

 taining the outlines in mechanical inquiries. But in this case it is very necessary 

 to distinguish the circumstances in which a model differs from a machine in 

 large; otherwise a model is more apt to lead us from the truth than towards it. 

 Hence the common observation, that a thing may do very well in a model, that 

 will not answer in large. And indeed, though the utmost circumspection be 

 used in this way, the best structure of machines cannot be fully ascertained, but 

 by making trials with them, when made of their proper size. It is for this 

 reason, that, though the models referred to, and the greatest part of the follow- 

 ing experiments were made in the years 1752 and 1753, yet he deferred offering 

 them to the Society, till he had an opportunity of putting the deductions from 

 them in real practice, in a variety of cases, and for various purposes ; so as to be 

 able to assure the Society, that he had found them to answer. 



Part I. — Concerning Undershot fVater- Wheels. 



Plate 13, fig. 9, is a perspective view of the machine for experiments on 

 water-wheels ; wherein abcd is the lower cistern, or magazine, for receiving the 

 water after it has quitted the wheel ; and for supplying de the upper cistern or 

 head ; where the water being raised to any height required, by a pump, that 

 height is shown by fg, a small rod, divided into inches and parts ; with a float 

 at the bottom, to move the rod up and down, as the surface of the water rises 



