502 PHILOSOPHICAL TRANSACTIONS. [aNNO 1774. 



the explosion much stronger and louder, at its striking the ball, than at its 

 striking the point. 



It may here be observed, that a point cannot possibly be placed in circumstances 

 more unfavourable than these, to its operation as a point: the body of electric 

 fluid falling on it almost instantaneously, with the stroke from b to e, so that it 

 had scarcely any measurable time, wherein to act as a point, in diminishing the 

 quantity, before the whole fell on it as a conductor. In the use of points to 

 eceive and conduct lightning, they generally act on the electrical atmosphere of 

 a cloud, while the cloud is yet at a distance, diminishing gradually that atmos- 

 phere, before the cloud approaches near enough to give the stroke, and thus 

 diminishing the stroke, if not quite preventing it. If the small conductor c 

 be placed so as to be in contact with the large conductor a, instead of being 4 

 inches distant, as before, the electrical fire will not strike to the point at any 

 distance whatever; but the point will carry oft' silently all the electrical fire from 

 the conductors, as fast as the cylinder supplies them, even if the point is placed 

 at the distance of 10 inches or more. 



To this machine there was another large conductor, 12 inches diameter, and 

 5 feet long, which being applied with its points to the back of the cushion, the 

 machine was either negative or positive, only by hanging a chain on either 

 conductor. 



VIll. On the Noxious Quality of the Effluvia of Putrid Marshes. By the 

 Rev. Dr. Priestley to Sir John Pringle. p. QO. 



" Since the publication of my papers, (says Dr. P.), I have read 2 treatises, 

 written by Dr. Alexander, of Edinburgh, and am exceedingly pleased with the 

 spirit of philosophical inquiry which they discover. They appear to contain 

 many new, curious, and valuable observations; but one of the conclusions, 

 which he draws from his experiments, I am satisfied, from my own observations, 

 is ill founded, and from the nature of it, must be dangerous. I mean his 

 maintaining, that there is nothing to be apprehended from the neighbourhood of 

 putrid marshes. I was particularly surprized to meet with such an opinion as 

 this, in a book inscribed to yourself, who have so clearly explained the great 

 mischief of such a situation, in your excellent treatise on the diseases of the 

 army. On this account I have thought it not improper, to address to you the 

 following observations and experiments, which I think clearly demonstrate the 

 fallacy of Dr. Alexander's reasoning, indisputably establish your doctrine, and 

 indeed justify the apprehensions of all mankind in this case. 



I think it probable enough, that putrid matter, as Dr. Alexander has endea- 

 voured to prove, will preserve otlier substances from putrefaction ; because, being 



