x 
136 D. P. Todd—The Transit of Venus, 1882. 
Storing of the Photographic Plates, ete.—After the quality of 
the photographs had been noted in detail, they were carefully 
packed in boxes of the ordinary pattern, and these latter stored 
in the upper part of the vault forming the interior of the brick 
pier, which supports the twelve-inch equatoreal. Tests for 
absence of moisture were applied to this vault, and as it has 
the means of pretty thorough ventilation, it is difficult to see 
how the photographs could be in a more secure place. 
Four of the photographs were brought by Captain Floyd to 
San Francisco and placed in the vaults of the Safe Deposit 
Company of that city. Within a few days I have sent him the 
numbers of twenty-four additional plates which he intends 
bringing down at an early day for safe keeping in the same 
place. These twenty-eight plates are so selected that in the 
event of destruction of all those remaining on the mountain, 
they will of themselves give a very satisfactory record of the 
transit as seen from Mount Hamilton. 
Before I left the summit for San Francisco, there were also 
stored in the Observatory vault the following parts of the pho- 
toheliograph, the constants of which have yet to be investigated: 
I. The measuring-rod (in five sections). 
Il. The jaw-micrometer. 
Ill. The Chesterman steel tape (50 feet). 
IV. The heliostat mirror. 
The photographic objective. 
The reticle-plate. 
mS 
NL. 
All these were so marked that no doubt can ever arise in re- 
gard to the station at which they were used. In addition to 
the five-section rod used in determining the focal length of the 
photographic objective, there is on the mountain another rod, 
of similar pipe-material, which has been carefully compared 
with the principal rod. This additional rod was left in post- 
tion over the tube of the photographic telescope. It is made up 
of three lengths of pipe, put together in the ordinary plumber- 
fashion, the joints being so marked that the lengths may be 
brought up always to the same relative position to each other. 
In the possible event of loss or destruction of the five-section 
rod, this additional rod will at any time give the focal length 
of the photographic telescope with nearly equal accuracy. 
n conclusion, it is proper that I should remark the full gen- 
erosity with which the entire outfit for our work on the moun- 
tain was provided by the Trustees, and which, while it was in no 
sense lavish, contributed very largely to the success so gratify- 
ing to us all. 
Lawrence Observatory, Amherst, Mass., 
anuary 16, 1883. 
