WCP1379

[1]

Allegheny Observatory

Allegheny, Pennsylvania

M[ar]ch. 24^th, 1887

Prof. Langley,

Dear Sir:

Your letter dated "Cosmos Club" M[ar]ch. 21^st is received.

Mr. Ericsson's writings are rather deficient in details of experimental verification.

I quote from page 77 chap. IV of "Contributions to the Centennial Exhibition". He says

"Fully convinced, however, that the difference of polar intensity resulting from the variation of distance between the sun and the earth was the true cause of the irregularity and breaks in the curve which I had constructed according to the observed temperatures. I availed myself of every [2] favorable opportunity to ascertain the maximum intensity produced during the summer and winter solstices. It will be well to state, for the information of those who have not paid ^special attention to the subject, that mean results of observation are inadmissible in records intended to establish solar energy. The superior intensity ascertained at a single observation will set aside the result of previous observations continued for many years."

Thus Ericsson's determination of the ratio between solar radiation at perihelion and at aphelion rests upon only two measurements; and if a mistake has been made in only one of these, the remarkable agreement with theory, upon [3] which he felicitates himself, falls to the ground.

On page 81, he says "It should be observed that the assumed temperature of 49°-9 F., at a zenith distance of 68 deg. in the middle of June, is not imaginary, having frequently been observed during my investigations of solar energy. Again, temperatures exceeding 54° F. have been observed during mid-winter at a zenith distance of 68 deg."

This approaches about as near to an account of the experimental verification of the theory as anything that can be found in the chapter (IV) on "periodic variation of solar radiation."

Ericsson's table covers six [4] pages and as it is the result of interpolation from a curve, I presume it will be sufficient for me to give you the first line. It gives intensity of radⁿ on Ecliptic

[]

Date	Maximum Radn			Increment from Apheln
Jan. 1	71°.94	F=	37°.97C.	4°.66	F.=	2°.59C.
Feb. 1	71.59		39.77	4.31		2.39
Mar. 1	70.79		39.33	3.51		1.95
Apr. 1	69.60		38.67	2.32		1.29
May 1	68.45		38.03	1.17		0.65
June 1	67.58		37.54	0.30		0.16
July 1	67.28		37.38	0.00		0.00
Aug. 1	67.58		37.54	0.30		0.16
Sept. 1	68.45		38.03	1.17		0.65
Oct. 1	69.60		38.67	2.32		1.29
Nov. 1	70.79		39.33	3.51		1.95
Dec. 1	71.63		39.79	4.35		2.41

Waterston's writings are in Monthly Not. R.A.S., XVII,205; XX,197; XXII,60 869.

I have never examined our own work in connection with this point.

Yours truly | V. H. W. Very [signature]