1816.] Letter to Dr. Wells respecting Dew. ~ 108 
of the commencement of the disease shall be found to be inva- 
riable, it will present one of the most curious, amongst the many, 
provisions of nature hitherto noticed, for preventing the too great 
accumulation of a particular species. I may also remark that the 
particular situation in which the teenia solida is found in the gaste- 
rosteus aculeatus may perhaps in some degree tend to support Dr. 
Chisholm’s theory of the propagation of some worms hinted at in 
his paper on the malis dracunculus, or Guinea worm, in the 42d 
number of the Edinburgh Medical and Surgical Journal. 
T remain, Sir, your obedient humble servant, 
Taomas LaupEer Dicx, 
ArticLe VI, 
Leiter to Dr. Wells respecting Dew, from Professor Prevost, of 
Geneva. 
SIR, 
I wave but just had an opportunity of perusing your letter to Dr. 
Thomson, published in the April number of the Annals of Philo- 
sophy foy 1815, which relates entirely to Dr. Young’s criticism on 
your work on Dew. I had some time before read your letter in the 
November number of the same journal for 1815, in which you 
answer the request which I had made you for an explanation relative 
to the experiments of M. Benedict Prevost on the water deposited 
from the air. 
1. On reading this last letter, and before I had seen the pre- 
ceding one, I thought I saw very clearly that you considered it as 
impossible for water to be deposited on the outside of panes of glass 
when the external air is colder than the internal; but as M. Bene- 
dict Prevost affirms very positively that water is very frequently de- 
posited under such circumstances, I immediately requested of this 
philosopher his proofs, conceiving myself unconnected with the 
dispute, because it can be decided only by experiment, theory 
being unable to affirm any thing, either on one side or the other. 
But after reading your letter in which you discuss the criticisms of 
Dr. T, Young, I thought I perceived an agreement between us, not 
only respecting the facts, but likewise the explanation of them. 
Repectiog the Facts.—M. Benedict Prevost says that “ dew is 
often deposited on the outside of glass panes when the external air 
is colder than the internal.” Dr. Wells does not seem to deny this 
fact. He appears only to say “ that the body on which the dew is 
deposited is colder than the air from which it is deposited,” *—an 
assertion that may be reconciled with that of M. Benedict Prevost. 
* In the letter on dew in the Annals of Philosophy for April, Dr. Welle quotes 
