74 iS Newcomb on observing the coming Transits of Venus. 
the conditions of mechanical equivalency as defined by Joule. 
In fact, such exceptional cases as the expansion of water in 
freezing seem to favor such a view, though the range of that 
phenomenon is very limited. One way of forming a mechani- 
cal representation of such a constitution would be by associat- 
ing molecular motion with the ae ake oma —- 
‘or without molecular attraction or rep 
Art. X.—On the aes of observing the sg Transits of Venus ; 
y Simon NEwcoms 
Transits of Venus over the disk of the sun have more than 
any other celestial phenomena occupied the attention and called 
forth the energies of the astronomical world. In the last cen- 
tury they furnished the only means known of learning the dis- 
tance of the sun with an approach to accuracy, and were there- 
fore looked for with an interest corresponding to the importance 
of this element. Although other methods of gs at this 
knowledge with about equal accuracy are now known, the rarity 
of the phenomenon in question insures for it an amount of 
attention which no other system of observation can comman 
As the rival method, that of observations of Mars at favorable 
times, requires, equally wi with this, the genera. codperation of 
astronomers, the power of securing this co 
wet give the ene of Venus an sageoaae they would not 
erwise 
1G Meee the a transit does not occur for four years, the 
pre. ents for its observation are already being 
made by the A TERN and scientific organizations of Kurope. 
It is not likely that our government will be backward in fur- 
i the means to enable its astronomers to take part in this 
The principal dangers are, I apprehened, shove of 
ee out with eatin nt preparation, wi with unma‘ plans 
* Read before the National Academy of Sciences, April 13, 1870. 
SNC oct Dt Oe eee 
