(after stating the facts as above). The errors assigned in the trial of this cause are 22 in number, extending to rulings of the court upon matters of evidence and to the charge to the jury, the substance of which, when classified, is ¿.s follows:

(1) Admissibility of letters between the parties .prior to the execution of the contract.

*725(2) Proof by the plaintiff of the performance of its part of the contract.

(3) Admissibility of evidence of the tests of lamps made by the method employed during the months of January and February.

(4) The charge of the court under the prayers of both parties as to the tests of lamps made by the method used in the months of March, April, May, and June.

(5) The prejudicial effect of the use of the terms “penalty” and' “penalize” in the charge of the court.

[1] 1. When the Welsbach Company was invited by the city, through its department of public works, to submit proposals for lighting its streets by incandescent lamps, there accompanied the invitation “specifications” of the proposed contract and “instructions to bidders.” The former contained in the usual form the particulars and details of the matter contemplated by the contract into which the company was invited to enter, and the latter contained directions and instructions, intended to guide or .instruct bidders with relation to the subject-matter to which their bids were invited. Among the instructions in the two instruments are the following:

“Should a bidder find discrepancies in or omissions from the specifications and accompanying papers, or should he he in douht as to their memmff, he should at once notify the director, who will at once send a written instruction to all bidders. The city will not be responsible for any oral instructions.” Instructions to Bidders, section 3.

“Any doubt as to the meaning of the specifications or any obscurity as to the wording of them, will be explained by the director, and any directions which may be required to complete any of the provisions of the specifications will be given by the director.” Specifications, section 13.

Entertaining a doubt as to several phases of the specifications, especially concerning the method of test prescribed, the Welsbach Company, before submitting its proposals, or in other words before making its bids, obeyed the instructions adverted to, and wrote to the director of public works asking his interpretation thereof. To this inquiry the director made a written reply, the substance of which was:

(a) That in selecting 25 lamps from any district for test, his selection should be of lamps showing average conditions throughout the district.

(b) That the contractor should have timely notice of the time and place when lamps should be selected and tested, and that the representative of the company would be permitted to check the standards, .adjustments, and readings.

(c) That any deterioration of the mantle, or change in the adjustment of the burner, occurring during transportation of the lamp from the street to the place of test, should be corrected before the test is made, so that the test should he of the lamp in the same condition as when in operation on the street, but suggested that 2 or 3 extra lamp tops, in addition to the 25, be taken to the place of test as substitutes, in case one or more of the 25 became disarranged.

These letters were offered and admitted in evidence over the objection of the city that in effect they altered and varied the terms of the-written contract afterwards entered into and now in' suit.

*726There must be eliminated from the consideration of the question raised any idea that in this correspondence there existed or was attempted anything in the nature of private negotiations between a bidder and a municipal officer, thereby removing the question from the law and the cases pertaining to such a transaction. This correspondence was inaugurated, conducted, and concluded upon the invitation and within the purpose of formal instructions to bidders, prescribed therefor by the city itself. Its subject-matter did not contemplate nor did it effect a change or alteration of the terms of the specifications as made. The correspondence extended merely to an interpretation of terms concerning which the bidder had a doubt, which, when existing, it was the desire of the city to remove by the method suggested and pursued, thereby obviating misunderstandings between parties, and avoiding subsequent litigation.

The reply of the director of public works, in giving his interpretation of that part of the method of test concerning which the company had a doubt, did not describe or establish any precise test, either by reciting the test contemplated by the specification or by suggesting another. It contained, however, an implication that the lamp, with the mantle and the burner, would be removed from the post and transported from the street to the place of test, and the place of test would be at the “city’s photometric station or elsewhere,” at the director’s discretion. As to the location of the place of test, whether proximately or remotely distant fiom the. lamp in place, the correspondence added nothing to nor withdrew anything from the specifications. There is, however, in the letter of the director an implication that before test the lamp would De moved to the place of test. The implication that the lamps would be removed before tests acquired an importance, in view of the fact that the January and February tests were made with the lamps in place; but the importance of this implication was lost in contemplation of the other fact that in the January and February tests the city clearly violated an undisputed provision of the contract by making tests without removing the globes. In other words, the city committed a breach of its contract in January and February, when it made tests of lamps without removing the globes, an act to which the correspondence in question did not extend, making unimportant the question raised by the correspondence whether the tests should be made with the lamps in place or should only be made after the lamps were removed. The remaining representation of the director that, when a lamp was transferred from the post to the station, allowance would be made in the test for deterioration in thfe mantle or change in the adjustment of the burner, occurring upon removal, was nothing more than an interpretation of which the contract itself was susceptible without the aid of the director’s interpretation; the sole object of tests being to ascertain the candle power of the lamps, not when tested, but when in operation upon the streets. It occurs to us that the interpretation made by the director of the specifications to which the company addressed its inquiries has a very slight probative bearing, if any, upon the issues as they subsequently developed. Nevertheless they were admitted in evidence, and their admissibility has been challenged.

*727The letter of the company contained inquiries made upon invitation by the city. That invitation was embraced within the instructions to bidders and specifications, and the instructions to bidders, as well as the specifications, were embraced within and attached to the contract, and made a part thereof. The reply by the director to the inquiries made was an interpretation which the city required the director to make, under the method which the city had adopted to instruct bidders as to what it meant by its specifications, and having adopted this method of giving interpretations to doubtful expressions in specifications, before' those specifications became a contract, the interpretation by the city’s officer became an interpretation by the city itself, the meaning of which it is estopped to deny, and of the effect of which it cannot complain. '

We do not hold that every interpretation by a municipal officer binds the city, even under circumstances where the city has volunteered the interpretation of its employé; but under the circumstances of this particular case, to which the expression of our opinion is restricted, which involve a change by the city from a method of test prescribed by the contract, first, to a method plainly different from that contemplated, and then, second, to a method the similarity of which to the one prescribed is challenged, we believe the city is bound by the act of its director, and the correspondence objected to was properly admitted in evidence.

[2] 2. By the terms of the contract the plaintiff agreed to furnish about 18,000 lamps and to maintain the same at 60-candle power. In proof of performance, the defendant introduced evidence that, before each lamp of that considerable number was supplied and installed in place, it was tested and proven to possess the required candle power. It is contended on the part of the city, however, that while this may be proof that the lamps of the required candle power were furnished, it is not proof that the required candle power was maintained. We are of opinion that by the testimony offered the company prima facie has supported the burden placed upon it. This view is aided by an unmistakable inference from the contract, that when it entered into' the contract the city recognized the difficulty, if not the impossibility, of the company testing each one of the 18,000 lamps at different periods of each month in order to establish its right to receive or recover payment for maintaining the same at 60-candle power. Therefore the city and the company, in their contract, agreed that, for the purpose of ascertaining the quality and illuminating power maintained in the lamps, the city should select and test 25 lamps from the total in actual use in each district in each month, and that, if the average candle power of the 25 lamps selected and tested fell below the minimum candle power required by the specifications, then the company should receive as compensation for its lighting service in that district an amount decreased in proportion to the deficiency disclosed by the tests. Upon the plan thus agreed upon, knowledge whether the plaintiff had maintained at the required candle power all the lamps in each district for the whole oí a month was within the possession of the defendant, and only upon that knowledge, as derived from its tests, was the city required to make *728payments. We are of opinion that the plaintiff prima facie established its right to recover.

[3] 3. At the trial of this cause the defendant offered evidence of tests made during the months of January and February, holding that the same were in accordance with the specifications of the contract. Without reference to the interpretation placed upon the specifications by the director, previously considered, the specifications provide for a test of the lamps furnished under the contract, by selecting 25 lamps from those in actual use in each district in each month, and their illuminating power, determined photometrically by "any method sanctioned by standard practice at the city’s photometric stations or elsewhere,” at the director’s discretion. The specifications provide, further, that “the average candle power of lamps so tested shall constitute the basis for calculating the light .furnished during the month,” and that the term “lamp” means “the burner, complete with mantle and chimney, * * * as set up for' use in the street, and the tests * * * shall be made with the clear glass globe which incloses the lamp on the street removed, but otherwise under the same conditions as exist when the lamp is taken down; the object of the test being to determine that the actual illuminating power, as contracted for, is given on the street.”

During the months of January and February of the year of the contract the city made its tests without taking the lamp down from the post or removing it to a testing station, and without removing the glass globe. In other words, the method pursued during the months of January and February was to test the lamp in place with the globe in place. Recognizing that a lamp which gave a light of 60-candle power without the globe would show appreciably less candle power with the globe in place, due to the absorption of light by glass, the city made a test of 120 lights to determine the average percentage of light absorption by the globes, and ascertained the same to be 13.4 per cent. To the candle power ascertained by test of a lamp in place inclosed by a globe, the city added the estimated percentage of light absorption by the globe, and the two together were computed to represent the candle power of the lamp, upon which the city based its calculations, and made and withheld its payments to the company. Evidence of tests of this character was offered, upon the claim that tests with globes removed, as provided by the contract, were impossible, and therefore tests with .the globes in place, being the only method found possible, were permissible. Such did not appear to be the fact, and the trial court, in our opinion, properly excluded testimony of tests'by the method followed, upon the ground that they were made by a method in complete conflict with and wholly different from the one agreed upon in the contract.

4. The method of the tests employed by the city during the months of March, April, May, and June differed radically from the method employed during the months of1 January and February. In these months the city used a portable photometer, which was moved or conveyed to the lamp. It consisted of a photometric apparatus inclosed in curtains. During this period, tests were made in three ways: First, *729with the same apparatus used in January and February with the globe in place; second, the lamp was taken down from the post and placed in the portable photometer and. tested with the globe in place; and, third, the lamp was tested in'the portable photometer, with the globe removed.

The resxtlts of these tests demonstrate to a certainty the inaccuracy either of the method pursued in January and February or of the one pursued in the months following. In the latter months, tests were made of the same lamp by both methods. Among the reports of these tests, as shown by the testimony of the city, appears attest of one damp made first by the early method with the globe on, yielding 35.4-candle power, to which was added the 13.4 per cent, candle power estimated to have been absorbed by the globe, and the two together were reported as showing 40.9-candle power. But when the same lamp was tested by the portable photometer under the method of the latter months, with the globe removed, instead of yielding 40.9-candle power, or precisely the same as the total of the two items that comprise this figure, it yielded but 16.02-candle power. This variation was extreme. The variations in the scores of tests of other lamps were not so wide as in this one, but variations existed in all, and in none did the test with the globe removed correspond in candle power with'the test made with the globe on plus the estimated light absorbed by the globe.

Evidence of the tests made in January and February having been excluded by the court, the city offered, and the court admitted, evidence of its tests made by the method employed in March, April, May,- and June. The company contended that according to the specifications the tests were not only to be made with the globes removed, but were to be made under conditions equivalent to those which prevailed in a laboratory, maintaining that a very slight difference in temperature of a burner, caused by a draft of air, would change its light-giving properties; that the testing apparatus of the portable photometer was protected only by loose curtains; that it was productive of drafts and currents of air, which seriously impaired the efficiency of the naptha burner while under tests with the globe removed; and that the apparatus employed was one not contemplated by the contract, that it was not a standard method, and was merely an experiment in photometry. On the other hand, the city contended it was not restricted to making tests in photometric stations, but under the terms of the specifications could make them at “the city’s photometric stations or elsewhere?’; that the photometer used, though portable, was of a character contemplated by the contract; that jt produced accurate results, and the method followed was sanctioned by standard practice. As the contract required the tests to be made by “any method sanctioned by standard practice,” “at the city’s photometric stations, or elsewhere,” the trial court submitted to the jury the question whether the method followed was the one contemplated by the contract, and also whether the tests made upon the device employed were pursuant to a method sanctioned b.y standard practice. We find ■no error committed by the court in submitting these questions to the *730jury, either in what its charge contained or omitted. The issue was clearly and fully submitted to the jury, and by the jury was decided.

[4] 5. In the charge to the jury the court alluded to the deductions made by the city from the bills presented by the company as “penalties.”' The word appears in several places in the charge. Strictly speaking, the deductions made by the city were not penalties, and the effect of making the deductions was not to “penalize” the company. If there existed in the case nothing to indicate why the court used these worcfe, we might think the minds of the jurors might have been influenced and prejudiced by them. But in charging the jury the court was not using technical terms with their legal meanings. It was, employing terms which had been introduced into the case by the city itself, and which had first been used by the city in rendering its monthly statements to the company of the deductions made, to which deductions it referred as “penalties.” Each notice or letter conveying information of the amounts deducted contained a table, which was described as “a statement of penalties by districts.” Around these statements and the methods by which they were reached revolved the whole controversy in the case, and very naturally the court drifted into using the descriptive words employed by the city. With these words the jury was quite as familiar as the court, and when they were used by the court the jury knew what they meant and to what they alluded, and therefore could not have been prejudiced by them.

We find no error in the trial below, and affirm the judgment.