Thomas Graham to Faraday   3 December 1849

4 Gordon Square | December 3, 1849.

Dear Faraday

Your understanding of the passage in my paper¹ to which you refer in your note of Dec. 1² is perfectly right. So also is the illustration you give afterwards speaking of weight instead of volume. The numbers, however, which you give in illustration of the last, 2172 of time & 5292 of time are, I find, from a table of results (page 384) which illustrate a divergence from the law, owing to the capillary being too short to give resistance enough. The one number should be double of the other, according to the law.

Air or coal gas of 4 atmos. density should pass through a long tube, such as Perkins³ 1 inch drawn-iron tubes⁴, with 4 times the velocity of the same gas of 1 atmos. density, for equal volumes; & consequently with 16 times the velocity for equal “weights”, the propulsive force being the same in both experiments. Hence a great facility in conveying coal gas in a dense state, by these pipes for long distances - such as many miles.

In the table head of page 385, would you be so good as to alter with your pen the first row of figures under 1 atmosphere from

1095 . 1096 . 1095.5 . 1095.5

into

1105 . 1107 . 1106 . 1106.

Your calling my attention to the point has led me to discover that I had calculated the last column of that table from a set of observations on air of 1 atmos. made last, & which I have omitted to give in the paper, & not from those observations for 1 atmos which are actually given.

It has been probably this which put you out. With the change, the numerical relations of the calculated times appear at once - 553 being 1/2 of 1106, &c.

Very faithfully yours | Tho. Graham

M. Faraday Esq.